06.06.2007
Ray Upton and Chris Smith of Eaton's MEM ask the rhetorical question to gain a real answer:
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| Busbar trunking & fuse-combination switch. |
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| Mempower busbar trunking in Bullring Centre. |
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| Mempower 6,300A busbar trunking. |
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Busbar trunking installations and other low voltage assemblies could frequently
be designed to a lower rating - with consequent savings in material and cost -
if principles similar to those used to design of cable installations in BS 7671,
The IEE Wiring Regulations, were also applied to busbar systems.
Busbar trunking, packaged substations, switchboards, distribution boards etc.
are normally assigned a short-circuit rating which is determined by the maximum
prospective fault level at the point where the assembly is connected to the
power supply. That short-circuit rating is applied for a specified time,
typically 0.2, 1 or 3 seconds.
However, if the system is protected by a fuse or circuit-breaker, that device
should operate well within the short-time value and so the system is unlikely to
face the full force of the short-circuit energy. Furthermore, such devices are
also energy limiting in their nature; they limit not only the duration of any
short-circuit which may occur but also the value of the peak current which flows
in to the short-circuit. Under short-circuit conditions all conductors, be they
cable or busbar, suffer from mechanical stresses which are related to the peak
current which flows and thermal stresses which result from the energy which
flows (energy being the square of the rms current multiplied by the time the
current flows for).
In designing busbars, manufacturers need to take into account not only the
normal load current for which the busbar is designed but also the short-time
withstand requirements. Often the short-time withstand rather than load current
considerations determine the size of copper used. Busbar trunking could be
assigned a "conditional short-circuit rating" which takes account of the
installation conditions and, in particular, the operating characteristics of the
upstream short-circuit protection devices. In many cases this could allow the
use of a smaller busbar trunking system with consequent savings in cost and
space. In extreme cases a reduction of 50% or more might be possible.
The International Electrotechnical Commission (IEC) has appointed a project
team to look into the terminology and methodology used in specifying
short-circuit ratings for low voltage assemblies. This follows the presentation
of a paper on the subject by the Engineered Systems Product Group of BEAMA
Installation.
Conductor ratings and protective devices:
BS 7671 offers cable rating tables giving cable ratings for different types
of protective device. This approach is well established for low voltage cable
installations in domestic and commercial premises.
When it comes to busbar trunking systems the factors are not so clearly
understood - hence the frequent use of short-circuit rating criteria that take
no account of the short-circuit protection devices.
Unfortunately, a variety of terms is used to define short-circuit parameters
in low voltage equipment. One of the responsibilities of the IEC Project Team
looking into the subject will be to produce unified definitions.
To determine the performance of equipment under short-circuit conditions, it
is necessary to know the prospective fault level at the point of installation.
Fault levels are defined in various ways (See Table 1):
- Prospective (available) short-circuit current.
- Peak short-circuit current (Ip).
- Symmetrical short-circuit breaking current (Ib).
- Steady-state short-circuit current (Ik).
A low-voltage assembly (busbar trunking, packaged substation, switchboard
etc.) will have a short-circuit rating that is defined in terms of the first of
these, the maximum prospective fault level at the point of connection to the
supply. This short-circuit rating may be:
- Rated short-time current (Icw).
- Rated peak withstand current (Ipk).
- Rated conditional short-circuit current (Icc).
An assembly may be assigned values of rated short-time current (Icw) and
rated peak withstand current (Ipk) but not one of these on its own.
Alternatively it may be assigned a value of rated conditional short-circuit
current (Icc) alone for specific circuit protective devices. A third possibility
is that all three parameters may be quoted.
The conditional rating (Icc) will generally be higher than the rated
short-time current (Icw). This means that the assembly can be used on a higher
prospective fault level, provided it is used in conjunction with the specified
short-circuit protective device. Manufacturers will normally test equipment with
a protective device - fuse or circuit-breaker - of their own manufacture, if
available, or from their preferred manufacturer. If the customer wishes to use a
short-circuit protective device of his own choice, the manufacturer should be
prepared to provide a conditional rating for that device.
Busbar trunking:
The foregoing comments relate to all low voltage assemblies.
As far as busbar trunking is concerned, if the rated short-time current (Icw)
is higher than the prospective fault current, the only requirement is to limit
the time for which any short-circuit could persist so that it does not exceed
the short-time value (0.2s, 1s or 3s). This will normally be achieved by the
natural operating characteristic of an HRC fuse or by suitable selection of
overcurrent release time on upstream circuit-breakers.
If the busbar trunking has a rated short-time current (Icw) lower than the
prospective current level but has a conditional rating (Icc) higher than the
steady-state short-circuit current (Ik), all that is necessary is to use the
specified short-circuit protective device upstream, or at the end-feed.
The short-circuit protection device may be a fuse or circuit-breaker. The
busbar manufacturer will determine its suitability by comparison of the cut-off
current and the Joule-integrated characteristics with the proof-tested
parameters supplied by the device manufacturer.
Fuses will normally operate well within the short-time value specified for
Icw. The operating characteristics of a circuit-breaker should also be
considered, for example a 40kA air circuit-breaker may have an instantaneous
override at fault current levels greater than 15kA.
In a typical application, a busbar trunking installation supplying a 600A
load requiring a 45kA fault rating may result in the selection of trunking rated
1,000A with 57kA Icw for 1s and 125kA Ipk. Application of a conditional rating
would result in selection of a 630A busbar trunking system with a conditional
rating of 80kA fused or 45kA circuit-breaker protected. The material costs of
the installation would be halved.
Conclusion:
Significant economies in installation costs, and space requirements, could
frequently be achieved, with no compromise in installation performance or
safety, by considering busbar trunking (or other low voltage assemblies) in the
context of the total installation and applying a conditional short-circuit
rating.
Table 1: Definitions
IEC Publication 60781:Application guide for calculation of short-circuit
currents in low-voltage radial systems defines four different short-circuit
rating parameters:
- Prospective (available) short-circuit current -
The current that would flow if the short-circuit were replaced by
an ideal connection of negligible impedance without any change of the supply.
- Peak short-circuit current (Ip)
- The maximum possible instantaneous
value of the prospective (available) short-circuit current.
- Symmetrical short-circuit breaking current (Ib) -
The RMS value of an integral cycle of the symmetrical a.c.
component of the prospective (available) short-circuit current at the instant of
contact separation of the first pole of a switching device.
- Steady-state short-circuit current (Ik)
- The RMS value of the short-circuit current which remains after
the decay of the transient phenomena
- unlimited.
- limited by an SCPD
(short-circuit protective device).
IEC60439-1: Low-voltage switchgear and controlgear assemblies - Part 1:
Type-tested and partially type-tested assemblies defines the following
terms:
- Rated short-time current withstand (ICW) -
The RMS value of short-time current that a circuit of an assembly
can carry without damage under specified test conditions, defined in terms of
current and time e.g. 20kA, 0.2s.
- Rated peak withstand current (Ipk) -
The value of peak current that a circuit can withstand
satisfactorily under specified test conditions.
- Rated conditional short-circuit current (ICC)
- The value of prospective short-circuit current that a circuit,
protected by a specified short-circuit protective device (SCPD), can withstand
satisfactorily for the opening time of that device, under specified test
conditions. The SCPD may form an integral part of the assembly or may be a
separate unit.
The pictures show: 1) Busbar trunking fed from a fuse-combination
switch fitted with BS88 fuses automatically has an 80kA fault rating; 2)
Mempower busbar trunking feeding risers in the Bullring Centre, Birmingham.
Special phase change units on top of the main switchboard provide correct
alignment of the conductors; and 3) Eaton's MEM has recently introduced 6,300A
low impedance busbar trunking to its Mempower range.
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