Four Very Important Precautions For The Installation of Cables and Busbar Trunking Systems
Four Very Important Precautions For The Installation of Cables and Busbar Trunking Systems
Four Very Important Precautions For The Installation of Cables and Busbar Trunking Systems
First, to be clear, there are dozens of concerns and precautions you should be aware of when we talk about
energy transport. Cables and busbar systems are the most common and reliable ways to do so, at least until
wireless energy transport is developed :) However, many potential issues need to be addressed. This article
deals with four significant precautions you should take – grouping conductors in parallel, short circuits,
magnetic effects, operating current, and voltage drop.
Four very important precautions for the installation of cables and busbar trunking systems
If you ask me, I will always prefer the prefabricated busbar trunking systems over cables, where possible, of
course. There is no rule for such a statement, but if you can choose and have the financial support – go for
prefabricated busbars. Pay more for reliable power transport and less headache.
Ok, let’s address these three critical precautions for the installation of cables and busbar trunking systems.
Table of contents:
Above a certain current (usually several hundred amperes), the use of several conductors in parallel allows their
cross-section to be limited and thus their handling made easier. This technique, very often used for the
conductors between the transformer and the main low voltage switchboard, is also used for high-power
outgoing connections.
The arrangement of conductors in a triangle (or in a trefoil) provides the best balance, but is generally limited to
two or even three conductors per phase. Above this, the overlapping of layers limits cooling and installation in a
bundle is preferable.
Basic Rules: If several conductors are arranged in parallel, they must be arranged in as many groups as there
are conductors in parallel, with each group containing one conductor from each phase. the groups of conductors
must themselves be installed close to each other.
This proximity rule also applies to single conductors (phases, neutral and protective conductor).
Figure – Careful layout of the cables complying with both the grouping rules and the precautions against fire
3-phase distribution via the conductors in parallel must comply with the strict geometrical layout rules. This
also supposes that all the conductors are of the same type, same cross-section and same length and that they do
not include any tap-off in their route and cannot be supplied individually.
In the event of failure to comply with any one of these conditions, the overall protection of the bundle of
parallel conductors by a single device would not be possible; one protection device per conductor would then be
necessary. It is recommended that the number of parallel conductors is limited as far as possible.
Above four cables, it is preferable to use prefabricated busbar systems, providing a better distribution of
currents.
Table 1 – Layout of the conductors in parallel and correction coefficient as per IEC 60364
Table 1 – Layout of the conductors in parallel and correction coefficient as per IEC 60364
With alternating current, electrical conductors have an impedance (expressed in Ohms) which is the complex
function of three factors:
It is considered that beyond 240 mm², the contribution made by reactance lω becomes the dominant factor in the
impedance. The conductor therefore behaves like a receiver, shifting the current and the voltage.
The illustration opposite is given for a phase shift of 45° (cosϕ = 0.5). Resistance and reactance equal. It should
be noted that, for these currents, the capacitance component can be ignored.
Shifting the current and the voltage for 45° (cosϕ = 0.5)
It determines the electromotive force “e” circulating in a conductor following the variation in magnetic flux
(Φ) surrounding the conductor. The conductor’s inductance depends on the material’s magnetic characteristics,
the medium and its geometry (length, number of turns):
e = −L × dΦ/dt
Mutual inductance
For a symmetrical link, the self-induction coefficient is perceptibly identical for each conductor, this is:
where: d is the average distance between the axes of the conductors, and r is the radius of the core of the
conductor.
In an asymmetrical arrangement, since the distances are different, the mutual inductances between conductors
will also be different. From this, it follows that the distribution of the current will be asymmetrical.
Symmetrical conductors
The equal distribution of currents in several identical conductors in parallel is uniquely linked to the equality of
the impedances in each of the conductors. With the inductance proportion becoming dominant with the increase
in section, the geometrical layout of the conductors will dominate (identical distances for each of them).
Three-phase layout
In a cable or bundle of conductors in 3-phase (with or without neutral), the vectoral sum of the currents is nil
and the resulting magnetic induction created by the conductors remains very low if they are grouped together
and arranged in a regular pattern. If this is not the case, the self-induction coefficient of the conductors will be
modified by the interaction of the magnetic field created.
Own and mutual inductances and the distribution of the currents will then be out of balance.
There are two destructive effects which can affect conductors in the event of a short circuit:
1. Thermal stress, protection against which is normally provided by the limiting power of the protection
devices (fuses, circuit breakers)
2. Electrodynamic stresses, whose forces between conductors can have destructive effects.
When a short circuit between two active conductors occurs (the most probable), the conductors suffering the
intense current of the short circuit will be repelled with a force proportional to the square of the intensity. If they
are poorly secured, they will start to whip and could tear out of their ties and touch another conductor or an
earth causing a new short circuit with a highly destructive arcing effect.
Aluminum cable tray ladder for building cabling projects
Multi-conductor cables are designed to withstand the forces that could be exerted by these conductors.
The indications given below, intended to draw attention to the importance of holding conductors securely,
cannot by themselves guarantee that short circuit conditions will be withstood. These will require test
simulation.
Even if there are few limitations in the use of prefabricated busbar trunking, it is still important to check that its
short circuit resistance characteristics are actually coordinated with its upstream protection devices.
The trunking must be able to withstand the thermal stress associated with the short circuit for the entire duration
of the fault, i.e. for the whole of the time necessary for the protection device (circuit breaker) to trip. Similarly,
the electrodynamic forces permitted by the busbar trunking must be compatible with the peak current limited by
the upstream protection.
The presumed peak value (Ipk), can be determined by reading devices’ limiting curves or in the absence of
data, by applying an asymmetry factor n (see Table 2 below) at the effective value of the short circuit current
(Isc).
Table 2 – Effective values of the short circuit and applied asymmetry factor
As with trunking made up of conductors and cables, presumed short circuit current calculations and the
determination of protection devices must be done prior to any installation.
Further reading…
Passing high currents through conductors induces magnetic effects in adjacent metallic masses, which can result
in the unacceptable heating of the materials.
1. To reduce the induction created, it is necessary to arrange the conductors so that the field is as weak as
possible. So far as is possible, conductors should be arranged in a trefoil to reduce induced fields (see
diagram for grouping conductors above).
2. To prevent significant heating in cable tray sections, it is advisable to remove the parts that create loops
around a conductor.
3. Breaking the magnetic loop by removing sections is also possible. In all cases, check that the mechanical
strength remains acceptable.
4. Cutting wire cable tray in order to prevent magnetic fields likely to cause heating.
Cutting wire cable tray in order to prevent magnetic fields likely to cause heating
Since the vectorial sum of the currents is nil, the one of the fields created is too.
All phases and neutral conductor must be positioned within the same metal compartments
The circulation of a current I in a conductor creates a proportional field H , the effect of which is to create
induction B in the surrounding medium. The value of B depends on the value of the field (therefore on the
current) but also on the magnetic characteristics of the medium or the material. It is the magnetic permeability
µ expressed in henries per metre (H/m).
The more the permeability of the material increases, the more the field lines are concentrated and the higher the
induction. Above a certain value there is saturation and heating.
Ferrous materials (steel) being magnetic by nature, are particularly likely to conduct fields but also to become
saturated if these fields are too high.
Circulation of a current in a conductor creates a proportional field and the effect of induction in the surrounding
medium
To calculate the actual current that will allow the choice of the busbar trunking, a certain number of data must
be known:
For a 3-phase supply, the actual operating current is determined by the formula:
where:
The trunking will be chosen using the rated current immediately above the calculated current. The rated current
applies to a specific orientation of the trunking. However, the influence of the orientation may be ignored for
short vertical sections in horizontal trunking (less than 3m long, for example).
Losses through the Joule effect are essentially due to the electrical resistance of the bars. lost energy is
transformed into heat and contributes to the heating of the trunking.
where:
For a precise calculation, the losses through the Joule effect must be calculated for each section between tap-
offs by taking into account the actual current circulating in it.
If the trunking is particularly long (≥ 100m), it is necessary to check the voltage drop. According to standard
IEC 61439-6, the voltage drop in 3-phase trunking may be calculated using the following formula:
u = k × √3 × (R × cosϕ + X × sinϕ) × IB × L
where:
To simplify the calculations, every manufacturer indicates in the tables the characteristics and unit voltage
drop K, according to the values of cos ϕ. The voltage drop at the end of the trunking can then be calculated
using the following formula:
u = b × K × L × IB × 10−6
where:
The use of cable tray systems for power distribution requires detailed knowledge of electrical installation
characteristics. For installations with long runs, it is particularly important to check voltage drops.
If the voltage drop is greater than the permitted limit, it will be necessary to increase the section of the
conductors until the voltage drop is less than the prescribed value.
When the main cables in the installation are longer than 100 m, the permissible limit values can be increased by
0.005 % per meter above 100 m, without this addition itself exceeding 0.5%. The value of the unit voltage
drop v (in volts per ampere and for 100m), can be read directly in the manufacturer’s tables.
4.3 Supplying motors
If the installation supplies motors, it is advisable to check the voltage drop under start-up conditions. To do this,
simply replace current IB in the formula opposite with the starting current of the motor and use the power factor
on starting. in the absence of more accurate data, the starting current can be taken as being 6 × In.
The voltage drop, taking into account all the motors that may start at the same time, must not exceed 15%.
apart from the fact that too high a voltage drop can hinder other users of the installation, it may also prevent the
motor starting.
Sources: Legrand,