What's the problem in

grounding systems used in

buildings ?
NOTE: This documents tries to give you general information about electrical
wiring inside building. This documents is only written to give a general
understanding of some of the most common electrical wiring systems. This
document is not ment to be an accurate description of all wiring systems. Don't
do any electrical work based on information provided here. Leave the work to
professionals who know all this and the local regulations.

Building grounding
In the construction of most commercial buildings, one ground is usually run throughout
the building to keep the impedance as low as possible. Low impedance in the ground is
needed to makse sure that the the fuse blows when something gets short circuited to
ground wire (for example cable insution breaks inside the equipment and touches the
grounded metal case). The grounding system is primarily designed for electrical safety
in mind. The Protective Earth connection should be able to carry a heavy current to
protect the user from live-to-chassis faults by ensuring that the fuse or circuit breaker
will operate so the requirement is that the Protective Earth conductors can carry a 25A
fault current for at least 1 minute. The grounding system in the building electricity
distribution has only effect on the electromagnetic environment inside the building
which you must understand.

Unfortunately all building have big electrical equipment such as air conditioning units,
refrigerators, washers/dryers and other high current devices connected to the building
ground (the same ground you use for your AV system). Computers and other
equipments which use stiched power supplies generate harmonics to the electrical
power which usually end up being noise in the gounding system. Light dimmers are also
a storng source of interference and some of it typically ends to the ground wires also.
Thus the chances of getting a clean ground in a typical audio visual installation is slim,
especially in large commercial buildings, hotels, hospitals or convention centers.

Grounding practices in buildings

Electric power distribution throughout Finland and many parts of Europe is made by
230/400Vac, 3 phase, four wire, Multiple Earth Neutral (MEN). One, two or three phases
are brought into the customer's premises depending on the maximum demand. This
applies to both residential and business premises.

For those not aware of the MEN system, the neutral bar is connected to an earthing
stake driven into the ground as near as possible to the customer's switchboard. All earth
wiring from power points, etc, is connected to the neutral bar. In the UK the same
practice is called Protective Multiple Earthing (PME). With P.M.E. the neutral and earth
conductors of the supply are combined. The supply company connects the neutral
solidly to earth frequently throughout the distribution network. At the customer's
connection point the company supplies an 'earth' (which is actually connected to the
neutral) to which all the installation earths and equipotential bonding are connected.
Another approach to bring grounding to the building is to bring it through armouring of
the supply cable. If the electricity company cannot easily supply or guarantee an
adequate earth conductor (for example supply comes on a pair of overhead wires), the
ser is generally responsible for the adequacy of the earth electrode. The method of
earthing can normally be found out by tracing the wiring from the meter/consumer unit.
It is usually fairly obvious.

How Good Should the Earth Be ?

This is a difficult question to answer; in general the impedance of the earth connection
must be low enough to ensure that sufficient current can flow through the protective
device so that it disconnects the supply quickly (<0.4 second) and that voltage on the
earth connection does not rise more than 50V. Measuring the resistance of an earth
electrode is not easy and should be left to professionals.

Safety if the neutral in the feeding is cut

How safe or unsafe MEN or PME is depends on the rules which cover its application, and
the record of the supply utility in avoiding neutrals going open circuit.

The key word in the titles is MULTIPLE. The exact situation will vary dramatically
depending on where the break in the neutral occurs. If it is just outside the substation,
then the neutral conductor will be replaced by all of the multiple earths in parallel, and
providing the load is balanced over the three phases, the voltage disturbance will not be
too serious.

When the supply company neutral goes open, the neutral return is via the earth stake.
Soil types here range from sand to loam to clay to rock, so the ground stake can range
from a good to a very poor earth. The voltage from each active to the "neutral" will
depend on the loads on each phase and the ground resistance.

The worse the balancing of your load over the three phases, the worse the voltage
disturbance will be. If we assume that the earthing spike has a resistance of 100 ohm, it
is pretty clear that your equipment is not going to work, but your neutral and earthed
metal work is going to rise to something near to phase voltage. This sound horrific, but
is actually not dangerous provided that all earthed metal work is nicely bonded, and
there are no unbonded earthed objects around that are better earths than your earth
spike.

How the ground connections are made in the main

distribution board
The three phase power power comes from power company using just four wires L1, L2,
L3 and PEN (protective earth and neutral). L1, L2 and L3 are just connected to the
power bars in the main distribution panel. The PEN wire is connected to the PE
(protective earth) bar which is connected to the central grounding bar. Neutral bar is
connected to the PE bar in the main distribution panel and nowhere else in the building
which has 5 wire 3 phase wiring.
The central grounding bar works as the central point for whole building grounding
system and every grounded system in the building is connected to it. The central
grounding bar is just a metal bar which connected the grounding wires from the mains
power, telephone equipments, antenna wiring, lightning protectors, metal plumbing,
water pipes, building steel structures and building grounding electrodes together.
As mentioned elsewhere, a fault current flowing in the earth wiring will cause the
voltage on that wiring to rise relative to true earth potential. This could cause a shock to
someone touching, for instance, the case of a faulty washing machine and a water tap
at the same time. In order to minimise this risk, an 'equipotential zone' is created by
connecting the services to the main earthing point. Such services are metal pipes (gas,
water, etc.), central heating, metallic ventilation trunking, exposed parts of building
structure, lighting conductor and any other metallic service.
The equipotential bonding reduces the voltage difference which could exist between the
metalwork of these services if an earth fault occurred to any one of them.

How the ground is wired to electrical outlets

The best way would be to run all the grounds separately back to a single block of copper
at the central grounding bar. Unfortunately this is often difficult to achieve in practice.
The practical way is to arrange your grounds as a strict 'tree' structure, with equipment
only connecting to the leaves of the tree.

For ground wire routing the electrical installation regulations worldwide generally state
that the ground wires should be routed on the same route than the mains curren
carrying wires going to the same outlet or distribution panel. This is the most often used
practice. Usually the safety ground is a separate yellow/green wire in the cable and
sometimes it is a separate wire in the same cable bundle (for example for 3 phase
distribution you might see sometimes a 4 or 5 wire cable and a separate safety ground
cable bundled on the side of the cable). In some countries (for example in USA) in some
case the metallic piping used to protect the mains carrying wire inside the walls can be
used as safety ground conductor (not usually very good or reliable in practice I think,
but is allowed in some cases).

How power is delivered to the house

Typical one phase feeding to building
One phase distribution is typically used in small residental building. The power company
feed live wire and neutral+gound wire to your house. The power in the power company
system is typically three phase power and the power company then feed one phase to
your house (you neighbour can have their power from other phase for even distribution
of load).

Single phase power in North America

ANSI C84.1 "Electric Power Systems and Equipment - Voltage Ratings (60 Hz) sets the
preferred nominal voltage at 120V and allows a range of 114 - 126V (240V nominal,
range 228 - 252V). Equivalent Canadian spec is CAN3-C235.
Voltage at a 120 volt nominal single phase receptacle should be 110 to 125V under
normal conditions.

However, the California Public Utilities Commission has specified that the service voltage
shall be kept in the range 114-120V, with some exceptions. This was done because
some studies showed a reduction in energy consumption at the lower voltages.
Information on NEMA plug configurations is available in NEMA Configuration Chart, Form
No. H4513. For availability check http://www.hubbell-wiring.com/.

Single phase power in Europe

The nominal European voltage is now 230V 50 Hz (formerly 240V in UK, 220V in the
rest of Europe) but this does not mean there has been a real change in the supply.
Instead, the new "harmonised voltage limits" in Europe are now:
 • 230V -10% +6% (i.e. 207.0 - 243.8V) in most of Europe (the former 220V
nominal countries)
• 230V -6% +10% (i.e. 216.2 - 253.0V) in UK (former 240V nominal)

his is really a fudge and means there is no real change of supply voltage, only a change
in the "label", with no incentive for electricity supply companies to actually change the
supply voltage.
To cope with both sets of limits an equipment will therefore need to cover 230V +/-10%
i.e. 207-253V. This will actually become the official limit for the whole of the EU in
2003.

Single phase power in rest of the world

A listing of nominal voltage/frequency and plug/socket types used in many countries is
given at http://kropla.com/electric2.htm.

Three phase distribution

The 3-wire system that the user sees is typically derived from three phase distribution,
which uses a 5-wire system. In the 5-wire system, there are 3 hot wires, 1 neutral wire,
and 1 grounding wire. The common 3-wire receptacle uses only one of the 3 hot wires.
This 5 wire wiring system is basically good and it is used in most buildings and places
where ground loops are expected to be a problem.

This three phase power system is called THREE-PHASE STAR; FOUR-WIRE; EARTHED
NEUTRAL system. This is the most common way used in European wiring systems (and
used almost everywhere in Finland), but note that three-phase distribution circuits come
in several flavours. There is a distinct difference between those in the US and those in
Europe. They are classified as follows:

• TN: Transformer star point earthed. Protective Earth and Neutral share the
"ground" conductor (PEN) and are separated at the fuse panel. This circuit is
also referred to as TN-C (C for common PE and N). In UK this is called TN-C-S
(i.e. combined in supply and separate in the installation), and it is also referred
to as Protective Muliple Earthing (PME - as the PEN supply conductor is grounded
at regular intervals along the supply).
• TN-S: As above, but PE and N are brought separatley all the way from the
earthed transformer and never allowed to get into contact with each other
elsewhere. The idea is that PE shall never carry any current (it shall
consequently not carry any potential and is supposed to be very "clean". All
return currents go through the N conductor all the way to the transformer star
point. This system has become very popular in new installations in Europe and
has been a standard in hospitals for a long time.
 • IT: The transformer is not erthed at all. The star point floats. Mostly used in
heavy and process industry where continued operation - even if there is an earth
fault - is required. The more common (european) voltages in these systems are
500 V and 690 V. In this case housing of the objects are connected to local
grouns.
• TT: Transformer and objects have separate grounds. Common in US.

Three phase in Europe

In Europe most use 230/400V where the 230V can be found between any of the 3
phases and neutral and the 400V can be found between two of the three phases. Phase
difference between phases is 120 degrees. Three phase power is normally available in
at least Finland, Sweden and Germany being used for ovens, electric stoves, large
motors and dryers. Three phase power is also typically available in places where large
sound and light systems are used (around stages etc.).

Typically there are 4 wires routed to every house for 3 phase feed. Those are typically
them are labeled R, S, and T, the fourth being ground. The phase shift between R and S
are 120 deg., the same phase shift exists between S and T and between T and R. The
voltage difference between the live phases is 400 V, the voltage difference between any
live phase and ground is around 235 V. The usual household power outlet connection
uses one phase and ground. Three phase is usally used only on some permanetly wired
high power loads (typically ovens and electric stoves in normal household). A typical
rating for mains fuse in typical household in Finland which has three phase power is
3x25A (25A per phase).

If three phase connector is avaible some heavy equipments (in places where heavy
machinery is used), then the most common one available is 3x16A connection.

Other possibilities for power distribution

Some smaller electrical installations (small houses) only use one phase feed. In those
cases the power company only brings one of the three phases to the house. In those
cases the wiring from power company is implemented using two wires:
"neutral+ground" and "phase". The frequency in USA is 60 Hz and nominal voltage in
USA is defined in the following manner:

• 120 volts is the voltage at the transformer

• 115 volts is the voltage at the panel (voltage drop losses in the cable from the
transformer to the panel)
• 110 volts is the voltage at the receptacle (voltage drop losses in the cable from
the panel to the receptacle)

So if the outlest are very lightly loaded, you will get nearly 120V and if wiring hevily
loaded, the voltage drops to around 110V. In Europe the frequency is 50 Hz and voltage
on the outlet is nowadays is 230V (the real voltage typically is between 220V and
240V).

In USA the domestical service has typically 3 wires: 2 hots and a neutral. The voltage
between the 2 hots is 240 and the voltage from either hot to the neutral is 120 (half).
Normal electrical outlets are connected between the neutral and one hot wire. Some
heavy loads (like air conditioners) are connected between those two hot wires and
receive the full 240V load.

House wiring details

What does a typical power outlet look like ?
A typical office wall outlet has three electrical connections, which are the "hot",
"neutral", and "grounding" wires.

All office equipment requires only the hot and neutral wires to function. The third or
grounding wire is connected to exposed metal parts on the equipment. Within the
building, the grounding connections of all electrical receptacles are wired to one another
and are connected to the water piping. This ensures that all electrical equipment with
exposed metal parts has these parts electrically connected to each other and to exposed
metal fixtures in the building such as water fixtures.

The hot and neutral wires are interchangeable as far as the equipment is concerned (be
warned that there are some exceptions in some countries). Both are power carrying
wires. One of the power carrying wires is grounded for reasons of safety. In many parts
of Europe (nordic counties, Germany etc), the normal 3-wire receptacle is symmetrical
so that the neutral and hot wire connections can be swapped by simply rotating the
plug.

Earthing of Electrical Installation

Each circuit requires an earth conductor to accompany (but kept separate from) the line
and neutral conductors throughout the distribution. All metal boxes should be connected
to the earth.

What are the wire colors used in wiring

House wiring colors used in USA

Green body color Grounding Conductor

White body color Grounded Conductor (Neutral)

ANY other Body Color Figure that it's HOT

Wiring colors used in equipment cables

GREEN with YELLOW stripes Ground
BLUE Neutral
BROWN Live

Typical colors used in house wiring in Europe

Information of this is from regulations in use in Finland.

GREEN with YELLOW stripes Ground

BLUE Neutral
BROWN or BLACK Live

Grounding (Green or green/yellow) means that it's there to tie all of the stray metal
parts together so that (hopefully) none of them can get to where they'll make a hazard.
A far better term for this wire is that it is the "Bonding" conductor. Grounding wire
should NEVER be asked to carry current.

Do not thrust the color coding unless you know under which standard the wiring is
done. There some some other color codes also in use. Inside of any electronic
equipment, it is dangerous to trust any color codes unless you know which "Standard"
that unit was built under.

House wiring problems

Problematic old wiring
The most problematic are those builing which are wired using 4 wire 3 phase wiring,
where neutral and ground share the same conductor at some places of wiring. This is a
bad thing because in this situation there will be always current flowing in the same wire
which should distribute the same ground potential to different places. If your building
has four wire 3 phase wiring you can expect quite noticable ground potential differences
of the power taken from different distribution panels.

The practice where safery ground is connected using the same conductor as neutral is
called PEN (TN-C) and practice where there is separate ground wire in whole system is
called PE (TN-S).

Two wire 1 phase grounded outlet

And worst of all is a 2 wire 1 phase jack wiring where neutral and ground share a
common wire. This practice is very often used in older buildings in Finland and causes
terrible ground loop problems even between nearby power outlets. If your are planning
to install any dedicated equipments (computer connected to LAN, interconnected audio
or video equipment etc.) to building which has this kind wiring system is advicable to
get a lincensed electrician to rewire the room with proper outlets. This wiring has also
some other problems and that's why it is not allowed anymore in new installations in
Finland.

Circuit breaker boxes: The main breaker box to the building is the single location where
the neutral and the ground wires come together. The electrical service will be grounded
at this point. IN ALL DOWNSTREAM BREAKER BOXES BOTH THE NEUTRAL AND GROUND
WIRES MUST BE KEPT APART FROM ONE ANOTHER. Otherwise you will have neutral
currents flowing on the ground wire. This is extremely important and is a major safety
and signal issue.

These simple rules apply to ALL cabling including CATV, Video, AC and signal. One
exception is the ethernet. Ground the computer LAN one end (preferably to the same
point as your audio system) and make sure that the thin ethernet connector metal parts
do not touch any parts of computer case (there are nice plastic isolation cases available
for them). I would recommend to use 10 Base-T ethernet which used twisted pair wiring
because it does not need any grounding and does not cause ground loops in any case.

What are isolated ground receptables ?

NOTE: The following description describes isolated ground as defined in USA wiring
system. In other countries "isolated ground" can mean different things (for example real
ground but not connected to the power ground bar).

Many new buildings in USA are equipped with "Isolated ground" receptacles. These are
normally recognizable because they are bright orange and have a triangle marked on
the face. Basically, these receptacles have a separate "green wire" equipment ground,
and the wire goes back directly to the circuit breaker panel, without being connected to
anything else. Isolated ground receptacles are installed in the hope that electrical noise
generated in the building, or by other pieces of equipment, will not disturb the operation
of delicate computer equipment plugged into them.

As far as what the NEC allows, an isolated ground is a grounding connection which is
grounded only at the separately derived system from which the circuit is supplied. It is
permitted to pass through panelboards, junction boxes, etc. without being bonded to
the equipment grounding conductor which serves those devices, thus minimizing
electromagnetic interference. It must be used in conjunction with an isolated grounding
receptacle to be effective. More details of isolated ground can be found at NEC 250-74
Exc #4.

My understanding on using a term "isolated ground" is mostly for marketing purposes. I

feel that their purpose is a marketing response to inadequate design by some electronic
equipment manufacturers and to inadequate grounding practices by some electricians.
The ground terminal is isolated from the mounting yoke; in conventional receptacles the
yoke and the ground terminal are connected. The "idea" is that the electrician connects
a special "clean" ground to the ground terminal, while the yoke and all other non-
energized metallic parts are connected to the "dirty" equipment ground.

How to avoid ground loop problems

Most electronic equipment is sensitive to ground loops and ground-induced noise. A
proper earth ground at the building services entrance is the first step to avoiding such
problems. In many cases, a proper earth ground is provided by a connection to the steel
rebar in the building's foundation.

All outside service grounds must be solidly connected to this ground point, including
power, telephone and cable television. For lightning protection, any antenna masts
should be grounded here as well. Ground connection points from the telephone system
controller, security alarm panel, audio equipment and other electronics gear should be
connected to this ground buss. All distribution of 3 phase voltage inside of building
should be done using 5 wire system. Distribution of 1 phase power should be done using
3 wire system. The safety ground wires should be interconnected in star or tree like
fashion. For more information check Residential Wiring and Grounding Guidelines from
Power Clinic.
If possible, all electronics and computer equipment should have a separate isolated
electrical subpanel with isolated ground receptacles provided at all locations remote
from the main. Isolated ground means that the ground wiring is otherwise isolated form
all other wiring except that it is connected to the main grounding bar for one single
point. This practice will ensure that all electronic equipment grounds are at the exact
same electrical potential and avoid the "minute differences" in grounds that cause
ground loops. These differences are reflected in signal-carrying conductors or shields
between the components and may be amplified to audible or visible levels.

Components that cannot have "equal-potential" grounds should have signals that are
isolated from each other. This can be expensive and difficult to achieve. It is much
easier to prevent the problems in the first place when designing the electrical
distribution. More information on that is available from Equitech articles: Power
Management in the Studio, Audio Wiring and Grounding, 1996 National Electrical Code
Technical Support Bulletin and Installing a Technical Grounding System. Those articles
provide you understanding how to make good grounding system for studio.

Do not try to modify your electrical wiring yourself. When you know what needs to be
done call professionals to do the job properly (you might need a special consultant to do
the plans for modifications because standard electricians don't usually know all the
special requirements audio studio has). When you have proper groundung system in
your studio then you can start doing the the audio wiring in a right way. You can easily
easily make your system very sensitive to power system noise if you do not do the
wiring properly. Rane application note Sound System Interconnections gives you good
undertanding how the audio connections should be done.

The problem is that in many cases you don't have possiblity to change the electrical
distrubution system already in the place, because it will come hard to do and expensive.
Then you have to live with what you get and try to solve those problems with suitable
isolation devices.

Improper grounding can create a lethal hazard. Even if you advert danger,
ground loops are the most common cause of AC line frequency hum in sound
systems. So it pays to learn about grounding, and use what you learn.

What is a ground loop?

A ground loop occurs when there is more than one ground connection path
between two pieces od equipment. The duplicate ground paths form the
equivalent of a loop antenna which very efficiently picks up interference
currents. Lead resistance transforms these currents into voltage fluctuations. As
a consequence of ground loop induced voltages, the ground reference in the
system is no longer a stable potential, so signals ride on the noise.The noise
becomes part of the program signal.

Can ground loops be eliminated?

Sometimes, in poorly designed sound equipment, ground loops occur inside the chassis.
Even thought the equipment has balanced inputs and outputs. In this instance, little can
be done to get rid of hum.
You should avoid unbalanced equipment in your system. The exception to the rule
would be with equipment that will be very close together, connected to the same leg of
the AC service.
Figure 1 illustrates a typical ground loop situation. Two interconnected pieces of
equipment are plugged into grounded AC outlets at separate locations, and the
signal ground is connected to earth in each of them. The earth ground path and
duplicate signal ground path form a loop which can pick up interference. If the
equipment is not properly built, these circulating ground loop noise currents
(which act like signals) travel along paths not intended to carry signals. The
currents, in turn, modulate the potential of the signal carrying wires, producing
hum and noise voltages that cannot easily be separated from the program
signals of the affected equipment. The noise is then amplified along with the
program signal.

What can you do to avoid ground loops?

There are four basic approaches to dealing with grounds within an audio system: single
point, multiple point, floating, and telescoping shield. Each has specific advantages in
different types of systems.

Figure 2 illustrates single-point grounding. Chassis ground in each individual

component is connected to earth. The signal ground is carried between components and
connected to earth at one central point. This configuration is very effective in
eliminating line frequency hum, but is easier to use in permanently installed systems.
Single point grounding is how TSC normally wires studios and is common practice in
most other studios as well. It is also effective in wiring individual racks for theatre and
church installs.
Multiple point grounding (figure 3)is what you find with unbalanced equipment in
whcih chassis connect to signal ground. It is very simple in practice, but is not
very reliable ... particularly is the system configuration is changed frequently. A
good example of multiple grounding would be a guitar rack. One advantage to
the rack system is that the chassis are on the same rack rails which in turn
referenced the whole rack as a single point ground.

Multiple point ground systems that employ balanced circuits with properly designed
equipment present no special noise problems.

Figure 4 shows the floating ground principle. Note that the ground in completely
isolated from the earth. This system is useful when the earth ground carries significant
noise. It does, however, rely on the equipment input stage to reject interference induced
in cable shields, so the input amp better be good.

Figure 5 illsutrates the principle of telescoping shields.

This scheme is very effective in eliminating ground loops. When noise enters a
shield connection only to earth, that noise can't enter the signal path.

Implementing this approach requires balanced lines and transformers since ground is
not carried between components. There is still debate about the use of transformers in
modern audio equipment though.

Grounding Safety
The main reason we ground a sound system is for safety. Proper grounding can prevent
lethal shocks. The next reason for grounding a system that includes AC powered
equipment is that proper grounding may reduce external noise pickup.

The AC power cord ground (the green wire and the third pin on the AC plug) connects
the chassis of electronic equipment to a wire in the wall power service that leads to an
earth ground. The earth ground, required by electrical codes everywhere, can contribute
to ground loops.

Don't break the AC ground!!!

With just one path to ground, there can't be a ground loop. Can there be a
ground loop with one audio cable joining a console to a power amplifier? Yes! A
ground connection through the AC cables and the chassis of the two units
campletes the second ground connection.

One way to break this ground loop is to lift the AC ground on one piece of
equipment, typically the power amplifier. This removes the safety AC ground.
The system now relies upon the audio cable to provide the ground ... a practice
that is hazardous!!! You also put at risk your multi-pair snake, console, post rack
equipment, and most important the client. I do not endorse the use of AC
ground lifts for any system ... anywhere. Don't do it.

In certain situations you can lift the shield at one end (usually the output) of an
audio cable and eliminate the most likely path that carries ground loop currents.

This is the way all TSC amp racks are wired and is seen as standard for most
tour type rigs. This method takes into account that the shield doesn't carry audio
signals. It does protect against static and radio noise. With one end lifted
however, it continues to reject static and other interference into the audio path.
Note: don't cut the shield of a mic cable that carries phantom power or you'll cut
the power to the mic.

Maximize safety while avoiding ground loops

Try not to lift the safety ground on any piece of equipment. NEVER defeat the
AC safety ground on your console or any piece of equipment connected directly
to your mics. Mics take priority in grounding safety because the performers
holding them may touch other grounded items, including a wet stage ... and
then .... !!!

Always try and plug your equipment into the same AC service leg. This includes
FOH, amp land, monitor land, and band back-line. This not only reduces the
potenial of a ground loop, but also reduces the danger of electrical shock.
Always connect lighting, air conditioning, rigging motor, and so on to a
completely different phase or leg of the main power distribution.
Remember to plan ahead and always think safety.

Using Grounding to Control EMI

Electromagnetic compatibility is an important consideration in the design and
operation of today's sophisticated medical electronic equipment, particularly as
portable systems proliferate. Electronic devices can both emit and be damaged by
electromagnetic interference (EMI) and must be protected from its harmful effects.
Issues of patient and operator safety must also be addressed. Previous articles
have covered such means of achieving EMI control as filtering, cable shields, and
enclosure shielding (MD&DI, February, July, and November 1995, respectively).
This article focuses on grounding.

Perhaps no topic in electronics is as misunderstood as grounding, which usually

evokes an image of a long braid snaking off to a ground post set into a concrete
floor. As the following discussion makes clear, an earth ground is not essential to
EMI control and is almost never needed. In the overwhelming majority of medical
electronic applications, good grounding involves achieving a sufficiently low-
impedance return path for the highest interference frequency of interest. If it were
possible to achieve zero impedance, all other grounding issues would become
meaningless. Since it isn't, device designers need to seek ways of maximizing the
effectiveness of the grounds that can be implemented.

WHAT IS A GROUND?

Succinctly put, a ground is a return path for current. Its purpose is to close the
current loop, not to lead it into the earth. If an interference current is diverted
successfully into earth ground, it will simply come out elsewhere in order to return
to its source. The only time earth ground is necessary is for lightning.

Confusion arises because the term ground is used for a variety of applications and
means different things to different people. Facility engineers, for example, look at a
ground as a return for lightning strikes. In this application, the ground needs to be
able to handle currents up to 100,000 A for a few milliseconds. Because the
approximately 1-microsecond rise time produces significant Fourier frequency
components up to about 300 kHz, inductance can become an important concern. In
contrast, electricians look at a ground as being a return path for fault currents,
which may involve tens or hundreds of amperes at 50 or 60 Hz. At this frequency
level, inductance is not important, so a length of 4/0 wire connected to the nearest
building steel works just fine--an earth ground may be present, but is not needed
for electrical safety.

These two cases are the most commonly known uses of grounding, but the
grounding requirements for EMI control in medical device applications are vastly
different. EMI can cover a very wide range: currents from microamperes to
amperes and frequencies from direct current to daylight. The duration of an event
can range from nanoseconds, in the case of a transient, to years, in the case of a
continuous wave. For the specific case of electrostatic discharge (ESD), transients
are measured in nanoseconds (giving Fourier frequency components up to 300
MHz), and currents range to 10 A or even higher. The edge rates and current
magnitudes are such that significant voltage bounce will occur across even the
smallest length of wire or circuit-board trace. Whatever the condition, however,
device designers must provide a way for the interfering current to return to its
source, and that rarely involves earth ground.

GROUND LOOPS AND SINGLE-POINT GROUNDS

Whenever grounding is an issue, design engineers inevitably turn to ground loops

and single-point grounds. What do these terms mean and when are the techniques
appropriate?

A ground loop exists whenever there is more than one conductive path between
two points. This condition allows interference currents to mix with signal currents,
which may lead to ground interference. Figure 1(a) shows the effects of a ground
loop when stray interference currents divide and flow through signal ground. This
problem can be eliminated by having a zero-impedance ground. Lacking such a
ground, separate ground paths can be provided. As shown in Figure 1(b), by
breaking the ground loop, the device designer has created a single-point ground.

The need for a single-point ground originated in telephony, where it was almost
impossible to get impedances low enough to prevent power line frequencies from
intruding as a hum, and the technique is still useful in a number of low-level, low-
frequency analog applications.

However, a single-point ground is not suitable for handling the higher frequencies
encountered in modern computing devices. Figure 2 shows the effect of a standing
wave on a cable shield that has been grounded to its enclosure at a single point. If
the shield were exposed to an incident interference of 150 MHz (a popular land
mobile radio frequency) with a wavelength of 2 m, the cable, which is represented
here as being a 1/4 wavelength of the interfering frequency, or 0.5 m, would act as
an efficient antenna, with standing wave voltage on the shield as indicated in the
figure. In the immediate proximity of the ground connection, the shield voltage is
near zero, but at the unterminated end, the voltage is at a maximum, and with
stray capacitance, there is ample coupling to the signal lines.

The fundamental assumption behind the principle of single-point grounding is that

the velocity of light is infinite. Any time designers need to consider the velocity of
light, notably at computer speeds, the single-point ground technique doesn't work.
A useful rule of thumb is that a single-point ground is appropriate if the longest
dimension of interest is less than a 1/20 wavelength of the highest-frequency
threat. Thus, single-point grounds are appropriate for handling EMI with audio
frequencies in most cases but inappropriate and unachievable for radio frequencies
used in digital electronics.

Consider, for example, the case of a designer who wanted to use a single-point
ground for two freestanding cabinets located about 10 ft apart. Based on the
common assumption that the inductance of a wire is 20 nH/in., the minimum
inductance for the single-point ground path would be about 2.5 µH. Using the
formula for impedance

Z = 2¼fL

where f is frequency in megahertz, L is inductance in microhenries, and Z is in

ohms, the impedance at 100 MHz would be 1600 ‡, which is hardly a short circuit.
Using the rule of thumb that capacitance between freestanding equipment and
ground is ~100 pF and the formula
where C is capacitance in microfarads, the impedance with two 100-pF capacitors in
series with a ground plane is 30 ‡. This is not a short circuit either, but is certainly
a lot lower than that of the intended single-point ground path.

ACHIEVING GOOD GROUNDS

Achieving a low-impedance ground for a medical electronic device is easy in

concept-- use a ground plane. At 50/60 Hz, the impedance of a grounding wire will
be primarily resistive, but above audio frequencies inductance begins to dominate
and at radio frequencies the inductive impedance of even a short wire or circuit-
board trace is enough to cause problems. To determine the requirements of a
particular application, the designer needs to know what voltage the device can
tolerate, the magnitude and frequency of the anticipated interference current, and
the impedance of the path. Given these data, Ohm's law can be applied to find out
when problems will occur.

A lightning strike, for example, might result in 10,000 A flowing in an I-beam with
10-V transients across even short lengths. Two interconnected devices grounded to
that I-beam at different points may easily experience upset. Or suppose a 1-in.
length of wire or circuit-board trace were subjected to a 10-A ESD event. Assuming
an inductance of about 20 nH, the voltage drop across the wire or trace could be
calculated using the equation

where E is voltage drop across wire, L is inductance in nanohenries, di is magnitude

of current transient (assumed to be 10 A), and dt is rise time (assumed to be 1
nanosecond). For these typical conditions, E = 200 V. Thus, it can be seen that a
length of wire as short as 1 in. makes a poor ground for ESD purposes.

Because ordinary wire is not a satisfactory ground in many circumstances, the

common wisdom is to use a flat strap instead. This approach is indeed appropriate,
but the rationale behind it is widely misunderstood. To achieve low inductance, the
key factor is not the strap's flatness but its length-to-width ratio. To ensure that the
inductance of a ground strap is sufficiently low, its width must be at least one-fifth
or, better yet, one-third of its length. If a designer cannot achieve this ratio, there
will not be a satisfactory high-frequency current return path.

Circuit-Board Grounds. It is almost impossible to get good low-impedance grounds

on two-sided circuit boards, so it is critical to keep ESD currents and high-level
radio-frequency interference off such boards. On the other hand, it is easy to
achieve low impedances with the ground plane underneath the traces on multilayer
boards. Circuits built immediately above the ground plane are well protected,
regardless of the threat. Our observation is that EMI control is always problematic
with double-sided boards, while electronic devices with multilayer boards are rarely
harmed. If a manufacturer is adamant about using double-sided boards, the
product development budget should include additional funds and three months
should be added to the schedule for test and redesign. Even then, there will be a
high probability that EMI control will not be achieved.

Probably nowhere in electronics do designers face such a difficult challenge as that

posed by sensitive analog input circuits. The circuits can be fairly well protected on
an isolated ground plane; the problem involves interconnections to an unisolated
ground or to the wires and cables that connect the sensor to other equipment. For
an isolated ground, it is important to minimize the amount of external EMI currents
that reach the ground plane. Once the sensitive input signal has been captured and
amplified, or perhaps digitized, crossing the boundary to unisolated circuits is the
remaining design problem. Any interference currents that are diverted to the
isolated ground become common-mode interference and must be handled by an
isolator component, of whatever type. Although some fairly effective isolators are
available, they have their limits, so it pays to minimize the common-mode currents
in the first place.

Interconnect Grounding. Once the designer has coped with the circuit-board
ground, the next consideration is the interconnects within the equipment, such as
the connections between the mother and daughter boards and the ribbon cables
between modules. EMI problems are frequently the result of high-impedance
interconnects. Again, designers need to keep the ground impedance low, either by
connecting the circuit boards or modules to a common ground plane or by providing
a very-low-impedance ground interconnect via the cable, usually by allocating as
many connector pins to grounds as possible. Although the connector space is an
important concern, so is functionality. For high-speed (100-MHz) interconnects,
there should be one ground line for each signal line. For lower speeds (~10 MHz),
one ground line for each five signal lines may be sufficient. Anything less is inviting
trouble.

External Grounding. Finally, designers need to consider the interconnections

between various pieces of equipment. If a low- impedance ground plane can be
implemented between enclosures, and multipoint grounds are used for cable
shields, problems should be minimal. However, if cables run long distances or if
sensitive low-frequency analog signals are being transmitted, audio-frequency
interference may be a concern. In such cases a single-point ground may be needed
as well as the multipoint ground required to control high-frequency interference. A
hybrid ground with a capacitor termination at one end, typically 0.010.1 µF, and a
hard termination at the other end can provide an open circuit at audio frequencies
and a short circuit at radio frequencies, thus combining the best of both worlds.

CONCLUSION

Medical electronics designers can base their decisions on how to implement

grounding for EMI control on three principles:
* An earth ground is not necessary for EMI control (although it may be
needed for safety). What is needed is a low-impedance current return path, usually
a conductive plane or a shield.
* Single-point grounds are usually appropriate only for handling audio-
frequency interference and are not achievable at radio frequencies. The 1/20-
wavelength criterion can be applied to determine if a single-point ground is
acceptable.
* Ground impedance must be kept acceptably low at the current frequency
of the anticipated interference event. At high frequencies, inductance gives rise to
high impedances, so use of ground wires is generally not acceptable. A wide ground
strap or plane can be used to reduce impedances.
Figure 1. Schematics showing ground loop currents: (a) unbroken and (b) broken
(thereby providing a single-point ground).

Figure 2. Effects of a standing wave on a single-point-grounded cable shield.

Why Ground?

There are a number of good reasons to ground but primary among

them is to ensure personnel safety. The following agencies and
organizations all have recommendations and or standards for
grounding, to ensure that personnel safety is being protected. The
organizations that provide guidelines/rules for grounding are; The
National Electrical Code (NEC), Underwriters Laboratories (UL),
National Fire Protection Association (NFPA), American National
Standards Institute (ANSI), Mine Safety Health Administration
(MSHA), Occupational Safety Health Administration (OSHA),
Telecommunications Industry Standard (TIA) and others. Good
grounding is not only for the safety of personnel but to provide for
the protection of plants and equipment. A good ground system will
improve the reliability of equipment and reduce the likelihood of
damage as a result of lightning or fault currents.

What is a ground and what does it do?

The NEC, National Electrical Code defines a ground as: "a conducting
connection, whether intentional or accidental between an electrical
circuit or equipment and the earth, or to some conducting body that
serves in place of the earth." When talking about grounding it is
actually two different subjects, earth grounding and equipment
grounding. Earth grounding is an intentional connection from a
circuit conductor usually the neutral to a ground electrode placed in
the earth. Equipment grounding is to ensure that operating
equipment within a structure is properly grounded. These two
grounding systems are required to be kept separate except for a
connection between the two systems to prevent differences in
potential from a possible flashover from a lightning strike. The
purpose of a ground besides the protection of people plants and
equipment is to provide a safe path for the dissipation of Fault
Currents, Lightning Strikes, Static Discharges, EMI and RFI signals
and Interference.

Ground resistance values

There is a good deal of confusion as to what constitutes a good ground and
what the ground resistance value needs to be. Ideally a ground should be of
zero ohms resistance. The NEC has stated that "A single electrode consisting of
a rod, pipe, or plate which does not have a resistance to ground of 25 ohms or
less shall be augmented by one additional electrode..." Once you have added
the supplemental ground you have met the requirement for the NEC. This does
not mean that the value of the ground now has to be 25 ohms or less. The
ground resistance values objectives vary from industry to industry
telecommunications industry has often used 5 ohms or less as there value for
grounding and bonding. The goal in ground resistance values is to achieve the
lowest ground resistance value possible that makes sense economically and
physically.

Ground Electrodes consist of three basic components;

1) a ground conductor,
2) the connection/bonding of the conductor to the ground electrode,
and

3) the ground electrode itself.

The resistance of a ground electrode has 3 basic components:

A) The resistance of the ground electrode itself and the connections

to the electrode.

B) The contact resistance of the surrounding earth to the electrode.

C) The resistance of the surrounding body of earth around the ground

electrode.
Multiple ground electrodes
A) The resistance of the ground electrode and it's connection is generally very low,
ground rods are generally made of highly conductive/low resistance material such
as copper of copper clad.

B) The contact resistance of the earth to the electrode: The Bureau of Standards
has shown this resistance to be almost negligible providing that the ground
electrode is free form paint, grease etc. and that the ground electrode is in firm
contact with the earth.

C) The resistance of the surrounding earth: The ground electrode is surrounded by

earth which is made up of concentric shells all having the same thickness. Those
shells closest to the ground electrode have the smallest amount of area resulting in
the greatest degree of resistance. Each subsequent shell incorporates a greater
area resulting in lower resistance. This finally reaches a point where the additional
shells offer little resistance to the ground surrounding the ground electrode.

The NEC specifies that the ground electrode shall be installed so that it is at least 8
ft. in length and in contact with the soil. There are 3 variables that affect the
resistance of a ground electrode.

1. The ground Itself

2. The length/depth of the ground electrode

3. Diameter of the ground electrode.

Types of Ground Systems

There are two types of grounding systems, simple and complex. Simple consists of
a single ground electrode driven into the ground. The use of a single ground
electrode is the most common form of grounding and can be found outside your
home or place of business. Complex grounding systems consist of multiple ground
rods connected, mesh or grid networks, ground plates and ground loops. These
systems are typically installed at power generating substations, central offices and
cellsites.

Ground Resistance Testing

Why measure soil resistivity?

The reason for measuring soil resistivity when selecting a location for a sub-station
or central office is to find a location that has the lowest possible resistance. Once a
site has been selected, measuring the soil resistivity will give you the information
necessary to design and build a ground field that will meet your ground resistance
requirements.

There are a number of factors affecting soil resistivity, soil composition being one of
them. Soil is rarely homogenous and the resistivity of the soil will vary
geographically and at different depths. The second factor affecting soil resistivity
is moisture or the amount of water in the ground. Moisture content changes
seasonally, varies according to the nature of the sub layers of earth and the depth
of the permanent water table. The chart below shows two differing types of soil and
the affects that moisture has on their resistivity.

Since soil resistivity is so closely related to moisture and moisture is present in the
soil we can logically assume that as moisture increases resistivity will decrease and
vice versa. As shown in the chart below you there can be a change in resistivity
from top to bottom by a factor of 50 fold.

Soil resistivity consists of, soil composition, moisture and

temperature. It stands to reason that soil resistivity will vary through
out the year in those areas of the country where seasonal changes
bring about a change in the moisture and temperature content of the
soil. For a grounding system to be effective it should be designed to
withstand the worst possible conditions.

Since soil and water are generally more stable at deeper strata it is
recommended that the ground rods be placed as deep as possible
into the earth, reaching the water table if possible. ground rods
should also be installed where there is a stable temperature i.e.
below the frost line.

Caution! Soil that is low in resistivity is often highly corrosive

because of the presence of water and salts, and this soil can eat
away at ground rods and their connections. That is why it is highly
recommended that grounds and ground fields be checked at least
annually. Although resistance to ground will change seasonally and
over time any increase to resistance >20% or more should be
investigated and corrective action taken to lower the resistance.

Soil Resistivity
Type of Soil Soil resistivity
RE Earthing Resistance (½)
Earthing rod m depth Earthing strip m
½m 3 6 10 5 10 20

Moist humus soil,

moor soil, swamp 30 10 5 3 12 6 3
Farming soil,
loamy and clay
soils 100 33 17 10 40 20 10
Sandy clay soil 150 50 25 15 60 30 15
Moist sandy soil 300 66 33 20 80 40 20
Dry sand soil 1000 330 165 100 400 200 100
Concrete 1:5 400 - - - 160 80 40
Moist gravel 500 160 80 48 200 100 50
Dry gravel 1000 330 165 100 400 200 100
Stoney soil 30,000 1000 500 300 1200 600 300
Rock 10 7 - - - - - -

To test soil Resistivity connect the ground

tester as indicated

Measuring Soil Resistivity 4 - Pole Method

The measuring procedure described below uses the universally accepted Wenner
method developed by Dr. Frank Wenner of the US Bureau of Standards in 1915. (F.
Wenner, A Method of Measuring Earth Resistivity; Bull, National Bureau of
Standards, Bull 12(4) 258, s 478-496; 1915/16.)

The formula is as follows:

p = 2 AR
Where: p = the average soil resistivity to depth
in ohm - cm
¹ = is the constant 3.1416
A = the distance between the electrodes
in cm
R = the measured resistance value in ohms from
the test instrument

The calculation of this measurement can be simplified by converting distance in cm

to distance in feet giving you the following equation:

p = 191.5AR
Where: p = the average soil resistivity to depth
in ohm - cm
A = the distance between electrodes in feet
R = the measured resistance value in ohms
from the test instrument

Note: Divide ohm - centimeters by 100 to convert to meter - ohms.

For example, you have decided to install 10' ground rods as part of your grounding
system. To measure the soil resistivity at a depth of 10' requires that the spacing
between the test electrodes is 10'. The depth that the test electrodes is to be driven
is A/20. To measure the soil resistivity start the GEO and read the resistance value
in ohms. Now if your resistance reading is 100 ohms the soil resistivity for one
cubic meter would be:

p = 191.5 x 10 x 100
p = 191500 ohms per centimeter
p = 191500 divide by 100
p = 1915 ohms per cubic meter.

The ground stakes are positioned in a straight line equidistant form one another
and at a distance between one another reflecting the depth to be measured. The
ground stakes should be screwed in no deeper than 1/3 the distance from one
another. A known fixed current is generated by the GEO between the two outer
ground stakes and a drop in potential (which is a result of the resistance) is then
measured automatically between the two inner ground stakes. The GEO then
display this resistance value in ohms.
Because measurement results are often distorted and invalidated by underground
pieces of metal, underground aquifers etc. additional measurements in which the
stakes axis is turned 90 degrees is always recommended. By changing the depth
and distance several times a profile is produced that can determine a suitable
ground resistance system.

Soil resistivity measurements are often corrupted and or prevented by the

existence of ground currents and their harmonics. To prevent this from occurring
the GEO uses an Automatic Frequency Control System (AFC), that automatically
selects the testing frequency with the least amount of noise enabling you to get a
clear reading.

The 3 - pole fall of potential method is used to measure the dissipation capability a
single ground electrode, ground grids, foundation grounds and other grounding
systems.

The potential difference between rod under test attached to terminals E and S is
measured with a voltmeter and the current flow between rod under test attached to
terminal E and H is measured by an ammeter. These functions are done internally
by the GEO.

Other manufacturers of ground testers may use the letters X,Y, and Z or C1, P2,
and C2 as connection descriptions. Terminals marked X or C1 are terminal E on the
GEO, terminals marked Y or P2 are terminal S on the GEO, and terminals marked Z
or C2 are terminal H on the GEO.

Using Ohm's law:

R = E/I we can calculate R

For example: If the voltage between E and S is 30 volts. The current between E and
H is 2 amps. We can calculate the following.

R = E/I
E = 30 and I = 2
R = 30/2
RE= 15 ohms

Connect the ground tester as shown in the picture below. Push start, and read out
the RE, (resistance) value. This is the actual value of the ground electrode under
test. If this ground electrode is in parallel or series with other ground rods the RE
value is the total value of all resistances.
 Stake Setting
To achieve the highest degree of accuracy when performing a 3 - Pole
ground resistance it is essential that the auxiliary ground electrode be placed
outside the sphere of influence of the ground electrode under test and the current
probe.

If you do not get outside the sphere of influence the effective areas of resistance
will overlap and invalidate any measurements that you are taking.
The following chart can be used as a guideline when setting of auxiliary (S) and
current (H) ground stakes.

Approximate distance to auxiliary Probes

using the 62% method
Depth of
Electrode under
test E Distance to
probe S Distance to
Auxiliary probe
H
6 ft 45 ft 72 ft
8 ft 50 ft 80 ft
10 ft 55 ft 88 ft
12 ft 60 ft 96 ft
18 ft 71 ft 115 ft
20 ft 74 t 120 ft
30 ft 86 ft 140 ft

To test the accuracy of the results and to ensure that the ground stake are outside
the 'spheres of influence' reposition ground stake S 3 ft in either direction and take
a fresh measurement. If the measured value remains fairly constant the distance
between the ground stakes is sufficient. If there is a significant change in the
reading (30%) you need to increase the distance between the ground rod under
test and S and H until the measured value remains fairly constant when
repositioning the S ground stake 3 ft. or so.

3 - Pole Fall of Potential Testing

for Ground Grids, Loops and Multiple
Ground Systems
Often time the driving of a single ground rod into the earth does not result in
a resistance reading low enough or desired, this is especially true in substations
and central offices where resistance’s of 3 ohms are required. In these case the
guidelines for setting your auxiliary ground stakes may not get you outside the
sphere of the influence of the ground field. The rule now is to take the maximum
distance of the ground field either straight line or diagonal and the spacing for the
first reference stake to be twice the distance. The chart below can be utilized as a
guide for setting reference stakes for complex ground systems.

Ground resistance measurements are often corrupted and or prevented by the

existence of ground currents and their harmonics. To prevent this from occurring
the GEO uses an Automatic Frequency Control System (AFC), that automatically
selects the testing frequency with the least amount of noise enabling you to get a
clear and accurate reading.

Ground Resistance Testing

Existing
Systems 'Selective' Clamp-on
By measuring the current flow
This unique through a single ground electrode
exclusive LEM using a specialized clamp-on
method has Diagonal
Distance Distance
current transformer the effects of
been created Measurement
to Probe to Probe
parallel resistances are
of Ground eliminated from the measuring
to measure Grid, or Field
S H
process and therefore do not
resistances of distort the measuring results. A
50 100 160
individual special rectification method is
75 150 240
ground used to isolate or in effect
100 200 320 'digitally filter' out other currents
electrodes in to significantly increase accuracy.
all types of 150 300 500 As with the standard 3-Pole Fall
grounded 200 400 650 of Potential testing the
rules/guidelines for the setting of
systems
ground stakes apply for both
including simple and complex grounds.
ground grids
and wired
meshes as are
common in
substations,
high voltage
pylons with
ground
cabling, and
commercial
settings with
multiple
grounds.
The Ground Under
Test Does Not Have
To Be Disconnected

Connect the ground tester

as shown in the picture at
the left. The blue clip lead
from GEOxS terminal E
must be above the clamp
but BELOW any parallel
ground connections
(ground interconnections or
building/structure steel).
The clamp must be in the
dirt, with no interconnection
bellow the clamp. Push
start, and read out the RE,
(resistance) value of that
individual ground
resistance path. This is the actual value of this one individual ground electrode in a multiple
ground system. The GEOxS is the only ground tester that can accurately measure each
ground path in a multiple ground system WITHOUT DISCONNECTING EACH GROUND
DEVICE.
To test the
accuracy of the
results and to
ensure that the
ground stake are
outside the
'spheres of
influence'
reposition ground
stake S 3 ft in
either direction
and take a fresh
measurement. If
the measured
value remains
fairly constant the
distance between
the ground stakes
is sufficient. If
there is a
significant change
In the case diagrammed above, remember that the total
in the reading
resistance of an individual tower is the parallel sum of all
(30%) you need
grounds. In the above diagram the tower has 4 individual
to increase the
grounds you must measure all 4, generating the individual
distance between
resistance and then calculate for the tower ground as follows.
the ground rod
under test and S
and H until the
measured value
remains fairly
constant when
repositioning the
S ground stake 3
ft. or so. Ground
resistance
measurements
are often
corrupted and or
prevented by the
existence of
ground currents
and their
harmonics. To
prevent this from
occurring the GEO
uses an
Automatic
Frequency Control
System (AFC),
that automatically
selects the testing
frequency with
the least amount
of noise enabling
you to get a clear
and accurate
reading.

'Selective'
Measuring of
High Voltage
Transmission
Towers

Testing individual
ground electrode
resistances of
high voltage
transmission
towers with
overhead ground
or static wire
requires that
these overhead
ground wires be
disconnected. If a
tower has more
than one ground
at it’s base, these
must also be
disconnected one
by one and
tested. The GEO
X with LEM
INSTRUMENTS
INC's 12'
diameter clamp-
on current
transformer can
measure the
individual
resistance’s of
each leg without
disconnecting any
ground leads or
overhead
static/ground
wires.

Ground Resistance Testing

Existing Systems
"Stakeless"
The GEO Xs measures
individual ground
resistance's in multi-
grounded systems using
two current clamp-on
current transformers,
eliminating the
dangerous and time
consuming activity of
disconnecting parallel
grounds as well as the
process of finding
suitable locations for
auxiliary ground stakes.
How it Works... The GEO
Xs works on the principle
that in a parallel
/multigrounded systems
the net resistance of all
ground paths will be
extremely low as
compared to any single
path (the one under
test).

Since the net resistance

of all the parallel return
path resistance's
(R1...Rn) is effectively
zero. It is reasonable to
assume any resistance
measured must be
associated with the
individual path to ground
the clamp is around
(Rx).

The first current transformer induces voltage in the

circuit while the second current transformer measures
the actual current flowing allowing the GEO Xs to
calculate the resistance of the ground path after
synchronous rectification of current and voltage.

Clamp-on technology only measures individual ground

rod resistance's in parallel to earth grounding systems. If
the ground system is not a straight parallel to earth then
you will either have an open circuit or be measuring
ground loop resistance.

Ground Resistance Testing 2 - Pole

 In situations where the driving of ground stakes is
neither practical or possible the GEO does give you the
ability to do 2 - pole ground resistance/continuity
measurements.

To perform this test

requires a good known
ground such as an all
metal water pipe. The
water pipe should be
extensive enough and be
metallic throughout
without any insulating
couplings or flanges.
Unlike many testers the
GEO performs this test
with relatively high
voltage AC with up to
250 mA of current.

Ground Impedance Impedance of auxiliary

Measurements ground stake is higher
than 100 times the
When attempting to impedance under test.
calculate possible short
circuit currents in power Power utilities testing high
plants and other high voltage transmission lines
voltage/current situations, are interested in two things
determining the complex The ground resistance
grounding impedance is under test in case of a
important since both lightning strike and the
inductivity and resistivity impedance of the entire
are present. Because system in case of a short
inductivity and resistivity circuit on a specific point in
are known in most cases the line. Short circuit in this
actual impedance can be case means an active wire
determined using a complex breaks loose and touches
computation. Since the metal grid of a tower.
impedance is frequency
dependent, GEO’s use a
55Hz signal for this
calculation to be close to
mains as possible without
corrupting the
measurement. Accurate
direct measurements of
grounding impedance are
possible with all GEOs under
the following conditions.
Phase angle at 50/60 Hz:
30...60 inductive
Measuring Ground
Resistance at
Substations

There are 3 separate

types of ground
measurements that are
necessary to conduct
when doing a grounding
audit of a substation.
When conducting a
grounding audit at a
substation, first
determine the nature of
the ground system, i.e.
mat, rods, water system,
combination etc. Once
the grounding system
has been determined the
audit process can
proceed. Substations
generally consist of high
voltage transmission
towers and transformers
that are connected and
grounded to a ground
grid.
The first measurement to be taken at a substation is a
"Stakeless" measurement. Use the GEO Xs to clamp
around all grounding connections.

The purpose of a "Stakeless" measurement in a ground

grid is to verify that there is an electrical connection
(bonding to the ground) and that it is capable of passing
current. These measurements should be recorded either
on paper or through the use of the GEO Xsi interface The
"Stakeless" measurements of grounds in a substation
grid are loop or continuity measurements. You are not
measuring actual ground resistance. To measure ground
resistance "Stakelessly" requires a straight parallel to
earth path.

The second measurement to be taken at a substation for

a ground audit is the 3-pole fall of potential of the entire
ground system. Connect to the grid as below.
Remember to follow the rules/guidelines for stake setting
and to ensure that the measurement is accurate and has
not been influenced by the effects of the grid, reposition
P2/S a yard or so and take a fresh measurement. If
there is a significant change in the measurement >30%
of measured value, reposition both P2 and C2 further
from the ground under test and repeat the above. This
measurement should then be recorded. To ensure that
the ground grid is in good working order these
measurements should be repeated at least annually to
detect any change within the ground grid (See chart
below).

Approximate distance to auxiliary Probes using the

62% method
Distance to
Depth of electrode Distance to
auxiliary probe
under test C1/E probe C1/E
C2/H
6 ft 45 ft 72 ft
8 ft 50 ft 80 ft
10 ft 55 ft 88 ft
12 ft 60 ft 96 ft
18 ft 71 ft 115 ft
20 ft 74 ft 120 ft
30 ft 86 ft 140 ft

After having completed the 3-pole fall of potential test

for the entire grid we then proceed to measuring
individual ground rods and their connections in the grid
using the selective clamp-on method. The selective
clamp-on test measures each connection separately
 without having to disconnect. The purpose of the
selective clamp on is to ensure that the resistance within
the grid are fairly uniform. A measurement that showed
a great degree of variability in relation to the other
measurements is probably indicative of a problem that
should be investigated. To conduct a selective clamp-on
test connect the GEO X as indicated in the illustration
below, keeping in mind that the spacing requirements for
the reference stakes are the same as with a s standard
fall of potential test. Make sure that you leave enough
slack in your leads so you can move easily from
connection to connection. The results of this test should
be recorded and repeated at least annually. After we
have completed the Stakeless check of the tower and
building we will then want to measure the resistance of
the entire system via the 3-pole fall of potential method.
Keep in mind the rules for stake setting when doing your
3-pole test. This measurement should be recorded and
measurements should take place at least semi-annually

Measuring
Ground
Resistance
At Central
Offices
When conducting a
grounding audit of a
central office there are
3 or 4 different
measurements that you
will want to make. First
locate the MGB (Master
Ground Bar) within the
central office to
determine what type of
ground system they
have in place. Generally
speaking the MGB will
have a ground lead
going to the MGN
(Multi-Grounded
Neutral) or incoming
service, a separate
ground lead from the
MGB to the ground field
for the central office,
another ground lead
from the MGB
connected to the water
pipe and a separate
ground lead connected
to structural or building
steel.
 The first measurement to take is stakeless measurement
of all the individual grounds coming off of the MGB. The
purpose is to ensure that all the grounds are connected
especially the MGN. It is important to note that you are
not measuring the individual resistance rather the loop
resistance of what you are clamped around. Connect the
GEO Xs as shown below and measure the loop resistance
of the MGN, the ground field the water pipe and the
building steel.
The second
measurement to be
taken at a central office
for a ground audit is the
3-pole fall of potential
of the entire ground
system. Connect to the
MGB as illustrated
below, keeping in mind
the requirements for
the setting of the
reference ground
stakes. To get to
remote earth many
phone companies have
been known to utilize
unused cable pairs
sometimes going out by
as much as a mile.
Once the fall of potential test has been completed, record
the measurement and this test should be repeated at
least annually.

After completing the 3-pole fall of potential test we now

want to measure the individual resistance’s of the
groundsystem using the Selective clamp-on capability of
the GEO X.Connect the GEO X as shown below. Start out
measuring the resistance of the MGN, the measurement
you generate is the resistance of that particular leg of
the MGB. Them move on and measure the ground field,
this resistance reading is actual resistance value of the
Central Office Ground Field, now move on to the water
pipe and than repeat again for the resistance of the
building steel. You can easily verify the accuracy of these
measurements through ohms law. The resistance of the
individual legs when calculated should equal the
resistance of the entire system given

This is the most accurate way to measure a central office

in that it gives you the individual resistance’s and there
actual behavior in a series ground. If you were to
disconnect the various legs of the MGB and measure the
resistance measurements would be accurate but would
not show how the system behaves or reacts as a
network, because of course in real life in the event of a
lightning strike or fault current everything is connected.
To prove this out you can measure each leg separately
 disconnected via the 3-pole fall of potential method and
record each measurement. Now using ohms law again
these measurements should = the resistance of the
entire system. Once you have completed the calculations
you will see that you are anywhere from 20 -30 % off
the total RE value.

The final way to measure the resistance’s of the various

legs of the MGB is the 'Selective Stakeless Method'. This
method works on the same theory as the Stakeless
Method but it differs in that we use the two separate
CT's. We clamp the inducing voltage CT around the cable
going to the MGB, and since the MGB is connected to the
incoming power which is straight parallel to earth system
we have achieved that requirement.

We then take the sensing CT and clamp it around the

ground cable leading out to the ground field (See
illustration below). Now when we measure the resistance
this is the actual resistance of the ground field plus the
parallel path of the MGB which because it should be very
low ohmically should have no real effect on the
measured reading. This process can be repeated for the
other legs of the Ground Bar i.e. water pipe and
structural steel.

To measure the MGB via the Stakeless Selective method

clamp the inducing CT around the lead going to the
water pipe as the water pipe should have very low
resistance, your reading will be for the MGN only.

Measuring Ground Resistance at Cellular Sites/Mic

There are 3 ground resistance measurements that are required when conducting an audit at a cell sig
individually grounded. These grounds are then connected with a # 2 gauge bare copper cable. Next t
building there is a halo ground and a MGB. The halo ground is connected to the MGB. The cell site bu
corners are also interconnected via #2 gauge copper wire. There is also a connection between the bu
The first measurement to be taken at a cell site is a Stakeless measurement of the individual legs of
because of the network ground. This is mainly a continuity test to verify that we are grounded and do

After we have completed the "Stakeless" check of the building we will then want to measure the resis
stake setting when doing your 3-pole test. This measurement should be recorded and measurements

Measuring Ground Resistance at Remote Switching Sites

There are 3 key measurements when conducting tests at remote switching sites also
digital line concentrators and probably more. The remote site is generally grounded
and then will have a series of ground stakes around the cabinet connected by #2 ga

The second measurement to be taken at a remote site for a ground audit is the 3-po
entire ground system. Connect to any of the grounds as illustrated here. Keep in mi
setting of the reference ground stakes. This measurement should be recorded and m
place at least semi-annually.

When you have finished the 3-pole fall of potential you will now want to measure the
through the selective clamp-on method. This will verify the integrity of the individua
connections and that the grounding potential is fairly uniform throughout. Do a selec
both ends of the remote site .
Measuring Ground Resistance for
Lightning Protection
Commercial/Industrial

There are 3 ground resistance measurements required when conducting an audit o

protection system. Most lightning fault current protection systems follow the design
the building grounded and these are usually connected via a copper cable. Depend
building and the resistance value that was tried to be obtained number of ground r

The first measurement to be taken at a

commercial/industrial lightning protection
site is a Stakeless measurement of the
building (shown here). This is not a true
ground resistance measurement because
of the network ground. This is mainly a
continuity test to verify that we are
grounded and do have an electrical
connection and can pass current.

After we have completed the "Stakeless"

check of the building we will then want to
measure the resistance of the entire
system via the 3-pole fall of potential
method. Keep in mind the rules for stake
setting when doing your 3-pole test. This
measurement should be recorded and
measurements should take place at least
semi-annually.
When you have finished the 3-pole fall of
potential you will now want to measure the
individual grounds through the selective
clamp-on method. This will verify the
integrity of the individual grounds and
their connections and that the grounding
potential is fairly uniform throughout.
Measure the resistance of each leg of the
tower and all 4 corners of the building. If
any of the measurements show a greater
degree of variability than any of the others
it should be investigated.

Ground Testing Instruments

UNILAP GEO

Universal ground testing instruments for testing installations and 3-pole

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UNILAP GEO X

Ground tester fo
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single grounds o
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Software for PC
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Small, handy ground tester 3-pole ground measurement 2-pole remote control,
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Custom carrying cases and water proof hard shell field cases

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