Unit 11 Elctrical Hazards Control
Unit 11 Elctrical Hazards Control
Unit 11 Elctrical Hazards Control
Electrical
hazards and
control
Aims: understand:
Specific outcome.
Be able to:
Reference:
Basic Circuitry
1.3
Principles of electricity
When the electrons are poised in that static condition (just like water
sitting still, high in a reservoir), the energy stored there is called potential
energy, because it has the possibility (potential) of release that has not been
fully realized yet.
When you scuff your rubber-soled shoes against a fabric carpet on a dry
day, you create an imbalance of electric charge between yourself and the carpet.
The action of scuffing your feet stores energy in the form of an imbalance of
electrons forced from their original locations.
1.3.4
In other types of materials such as glass, the atoms' electrons have very
little freedom to move around. While external forces such as physical rubbing
can force some of these electrons to leave their respective atoms and transfer to
the atoms of another material, they do not move between atoms within that
material very easily.
This relative mobility of electrons within a material is known as electric
conductivity. Conductivity is determined by the types of atoms in a material (the
number of protons in each atom's nucleus, determining its chemical identity)
and how the atoms are linked together with one another.
Materials with high electron mobility (many free electrons) are called
conductors, while materials with low electron mobility (few or no free
electrons) are called insulators.
Conductors:
• Silver.
• Copper.
• Gold.
• Aluminium.
• Iron.
• Steel.
• Brass.
• Bronze.
• Mercury.
• Graphite.
• Dirty water.
• Concrete.
Insulators:
• Glass.
• Rubber.
• Oil.
• Asphalt.
• Fibere Glass.
• Porcelain.
• Ceramic.
• Quartz.
• (Dry) cotton.
• (Dry) paper.
• (Dry) wood.
• Plastic.
• Air.
• Diamond.
• Pure water.
1.4 Hazards of electricity The Mechanism of
Electric Shock.
The respiratory control signal is passed from the brain to the diaphragm. The
diaphragm comprises a large flat muscle situated immediately below the base of
the lung and this muscle initiates the breathing cycle.
The mechanisms which control the body's circulatory and respiratory functions
are electro-chemical systems, situated in the upper torso. The most dangerous
path for an electrical current to take is through the body's upper trunk. This
could be as a result of an electric shock resulting from hand to hand or hand to
foot contact.
The severity of an electric shock will depend on the magnitude and duration of
the current which flows.
Much will depend upon the electrical resistance of the body. Most of the human
body's resistance to the flow of electric current is provided by the skin. The
actual value of this resistance is dependent on the skin's thickness, its moisture
content and the applied voltage. All of these are personal, climatic and
environmental variables.
The resistance figures quoted below represent average values and are given to
emphasise the uncertain degree of low voltage hazards.
At mains voltage (240 V), the body's resistance allows a current of 240 milli-
Amperes (mA) to flow. This level of current would prove fatal if the contact
occurred for longer than a few milliseconds.
At 90 V (the voltage likely to be present on, for example, the faulty metal case
of a piece of portable electrical equipment, with a phase to earth short circuit
and before fuse failure), the body's resistance increases carrying a potentially
fatal current of 45mA.
A further small increase in current will cause the respiratory muscles and heart
muscles to be affected.
Relatively small amounts of current flowing through the body will cause serious
damage. This current is related to the applied voltage, the current path through
the body and the resistance of this path is also time-related.
The fundamental aim of electric shock prevention measures is to ensure that the
human body is subjected to the minimum voltage and current for the shortest
period of time.
In summary: the factors affecting the severity
of the shock are
• duration;
• path of current;
• size of current;
• voltage;
• frequency;
• personal susceptibility;
• environment;
• possible proctection afforded by PPE.
1.4.1
First-
First-aid treatment for
electric shock
Burns.
Secondary effects.
Fires.
Sparks.
A spark arises from a sudden discharge through the air between two
conductors, or from one conductor to earth. The current produced is usually
small so that serious fires are unlikely unless explosive gases or vapors are
present, or highly flammable material is in contact with the conductor.
Arcs.
A n arc is a much larger and brighter discharge where the current flow
may be hundreds of amps. It usually arises when a circuit is broken or when a
conductor melts or fractures leaving a gap, across which current continues to
flow. When an arc is established, the air in the vicinity becomes ionized and
forms a conductor which may allow current to flow to a nearby metal
framework. Any combustible material in the vicinity could therefore lead to a
fire.
Short circuits.
A short circuit is formed when the current finds a path from the
outward conductor wire to the return wire other than through the equipment to
which it is connected. The current flow may be large because of the low
resistance of the leads, and arcing often occurs at the contact between the
conductors. Insulation may therefore be burned and set fire to adjacent
flammable material. Batteries have a low internal resistance and can give rise to
very large currents under short circuit conditions, causing a large arc from
which molten metal may be splashed.
The insulation of wiring which has been in use for a number of years
tends to become brittle and, where alterations and additions are required, the
cable must always be checked by a competent electrician and replaced
completely if there are indications of failure of the insulation.
Portable appliances.
One high risk situation is the use of Pressure water cleaner outside,
powered by 240 volt electrical supply, with the cable trailing on the
ground where it can be damaged by vehicles and other equipment, and
where surface water is present.
Damage to the cable or other parts is likely to result in the operator or
others receiving an electric shock.
Lower risks result from floor cleaners or kettles, which are generally
used in a less hazardous environment, e.g. offices and hotels, but can be
subject to intensive use and wear. This can eventually lead to faults
which can also result in shock, burns or a fire. Other common accident
causes are:
Overload.
This occurs in a healthy circuit where equipment has been
mechanically overloaded or an excessive number of appliances
have been added to a system. The consequences of overload
usually involve overheating and, if uncontrolled, fire.
Most electric shocks that people receive are between a live conductor and earth;
these shocks would not be possible on an isolated (unearthed) system. For this
reason, isolated systems with special transformers are used locally in very
hazardous surroundings such as laboratories or workshops where electronic
equipment with earthed cases is opened up for repair. This is a specialised
condition and the isolation feature is continuously monitored.
One such occurrence alone would not be noticed or cause a direct problem;
however, the inherent safety of an isolated system would no longer exist.
Inevitably, a second earth fault would arise and an uncontrolled current would
circulate through earth via the faulty connections. This situation would probably
cause a fire.
Public supply systems are therefore earthed and it is now common practice to
improve the earth/neutral bond by creating multiple connections throughout the
supply network. This is known as Protective Multiple Earthing (PME).
Interconnection of earth and neutral paths provides the lowest possible fault
impedance. The consequence of a PME connection is that both fault and load
current are shared between earth and neutral in proportion to their respective
resistances.
With every electrical system, provision must be made for switching off
the supply.
It is essential to provide suitable means first for cutting off the supply and
secondly for isolation. This is the cornerstone for a safe system of work based
on de-energised plant.
The need to ensure that, if at all possible, the circuit is dead when being
worked upon leads us on to situations where this is not possible, and live
working must be undertaken.
1.6.6
Live working
working
The first precaution is to ensure that live working is indeed unavoidable -
it is not enough for management to say that they did not want to stop
production. There must be absolutely compelling reasons why live working has
to be undertaken.
... w ork on or near live electrical equipment shall only take place if it is
unreasonable for the equipment in all circumstances to be dead, reasonable
in all circumstances for work to be carried out on or near the equipment when
it is live and suitable precautions have been taken.
Once this live working need has been established, then the engineers
working on the live system must be protected by an appropriate system of
work:
1.6.7
Special 110 V appliances are used which operate from 55-0-55 V centre-
tap earthed transformers. These appliances may be Class 1 or Class 2
construction.
1.6.7
Residual current devices as control measures
The primary protection against contact with live parts must be by way of
insulation and appropriate mechanical protection. Supplementary shock
protection may then be added with an RCD which will disconnect 30 mA in
200 mS (milli-seconds) equivalent to 0.2 seconds and 150 mA in 40 mS (0.04
seconds).
Higher rating may be used to give protection against fire or large earth
faults in circumstances where there is an inherent earth leakage associated with
equipment. Over-sensitive operation is not desirable. In some cases rapid, low
fault-current disconnection may be inconvenient or even introduce
consequential dangers.
Competence to test
The PAT will conduct each test using the correct test voltages and
currents in a sequence that will ensure that if a failure occurs, there will
be no danger to the test person.
Extreme care must be taken with high voltage flash testing. This
may be unnecessary unless an appliance has been completely overhauled
and full manufacturers' test procedures are necessary. Flash testing is
hazardous and may cause damage to sensitive equipment.
1 . The unit under test should never be touched during the
test process.
If any faults are found, the equipment should be withdrawn from service for
repairs followed by a full test.
It may therefore be argued that the "type approval" tests conducted by the
manufacturer could be used as a reference for the routine periodic safety tests.
Record Keeping.
With electrical fixed plant, the duty holder should keep an inventory of
equipment to be tested and a repair history.
The records may be kept in the form of a paper copy if the quantity of
equipment to be tested is small. Each test result should be recorded as the tests
progress and care should be taken to reproduce the test figures accurately.
The test person should fix a label to equipment that has been successfully
tested, giving the following information:
Such a label will allow the duty holder to install a management system
whereby no electrical item may be used outside of the two dates shown on the
safety test label. Items that fail the safety tests should be immediately
withdrawn from service for repair.
The on-site location may not be under the direct control of the test person
and thus may present extra hazards compared with a purpose-built workshop.
Temporary installations.
Particular concern should be given to temporary
installations. No relaxation of safety rules or protection is
permissible. Temporary installations must be designed to at least
the same standard as permanent installations and must be inspected
and tested more frequently (i.e. every three months).
Utilisation.
Construction of buildings.
Levels of combustibility must be considered together with
life expectancy of the installation and maintainability.
1. ? True
2. ? False
The most important and relevant test of electrical equipment is via a ......
Identify the hazards and evaluate the consequential risks from the use of electricity in
the workplace:-
Hazards of Electricity
• Electric Shock.
• Fires
• Sparks
• Arcs
• Short circuits
• Overloading and old wiring
• Explosion Hazard
• Static Electricity
Advise on the control measures that should be taken when working with electrical
systems or using electrical equipment.
The main techniques of controlling and minimising risks associated with electricity are the
correct selection, installation and maintenance of equipment, the insulation of live parts and
the retention of the electric current in the correct place at the correctly rated value.
o user checks;
o formal inspection and tests;
o frequency of inspection and testing;
o records of inspection and testing;
o inspection and testing of Portable Appliance
Testing (PAT).