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.
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.
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
- 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.