3.7.2 -  Prospective short-circuit current (PSC)

The current which is likely to flow in a circuit if line and neutral cables are short circuited is called the prospective short circuit current (PSC). It is the largest current which can flow in the system. and protective devices must be capable of breaking it safely. The breaking capacity of a fuse or of a circuit breaker is one of the factors which need to be considered in its selection. Consumer units to BS EN 60439-4 and BS 88 (HBC) fuses are capable of breaking any probable prospective short-circuit current, but before using other equipment the installer must make sure that their breaking capacity exceeds the PSC at the point at which they are to he installed.

The effective breaking capacity of overcurrent devices varies widely with their construction. Semi-enclosed fuses are capable of breaking currents of 1 kA to 4 kA depending on their type. whilst cartridge fuses to BS 1361 will safely break at 16.5 kA for type 1 or 33 kA for type II. BS 88 fuses are capable of breaking any possible short-circuit current. Miniature circuit breakers to BS EN60898 have their rated breaking capacity marked on their cases in amperes (not kA) although above 10000 A the MCB may be damaged and lower breaking currents (75% for 10000 A and 50% above that level) must be used for design purposes.

Prospective short circuit current is driven by the e.m.f. of he secondary winding of the supply transformer through an impedance made up of the secondary winding and the cables from the transformer to the fault {Fig 3.21}. The impedance of the cables will depend on their size and length, so the PSC value will vary throughout the installation, becoming smaller as the distance from the intake position increases. (313-01-01] requires the PSC to be 'assessed' by 'calculation, measurement, enquiry or inspection'. In practice, this can be difficult because it depends to some extent on impedance's which are not only outside the installation in the supply system, but are also live. If the impedance of the supply system can be found, a straightforward calculation using the formula of {Fig 3.21} can be used, but this is seldom the case. An alternative is to ask the local Electricity Company. The problem here is that they are likely to protect themselves by giving a figure which is usually at least 16 kA in excess of the true value. The problem with using this figure is that the higher the breaking capacity of fuses and circuit breakers are (and this must never be less than the PSC for the point at which they are installed), the higher will be their cost.. {Table 3.6} gives a method of arriving at PSC if the type and length of the service cable is known,

 

Table 3.6 - Estimation of PSC at the intake position
Length of supply cable (m)	PSC (kA) up to 25mm2 AI,	PSC (kA) over 35mm2 AI,
-	16mm2 Cu supply cable	25mm2 Cu supply cable
5	10.0	12.0
10	7.8	9.3
15	6.0	7.4
20	4.9	6.2
25	4.1	5.3
30	3.5	4.6
40	2.7	3.6
50	2	3.0

The table is not applicable in London, where the density of the distribution system means that higher values may apply. In this case it will be necessary to consult London Electricity.

There are two methods for measuring the value of PSC, but these can only be used when the supply has already been connected. By then, the fuses and circuit breakers will already be installed.

The first method is to measure the impedance of the supply by determining its voltage regulation, that is, the amount by which the voltage falls with an increase in current. For example, consider an installation with a no-load terminal voltage of 240 V. If, when a current of 40 A flows, the voltage falls to 238 V, the volt drop will be due to the impedance of the supply.

Thus Zs	= systems volt drop =	240 – 238 W	= 2 W =	0.05 W
	current	40	40

Then PSC	= Uo =	240 A	= 4800 A or 4.8 kA
	Zs	0.05

A second measurement method is to use a loop impedance tester see { 5.3 and 8.6.2 } connected to phase and neutral (instead of phase and earth) to measure supply impedance. This can then be used with the supply voltage as above to calculate PSC. Some manufacturers modify their earth-loop testers so that this connection is made by selecting 'PSC' with a switch. The instrument measures supply voltage, and calculates, then displays, PSC.

A possible difficulty in measuring PSC, and thus being able to use fuses or circuit breakers with a lower breaking capacity than that suggested by the Supply Company, is that the supply may be reinforced. More load may result in extra or different transformers and cables being installed, which may reduce supply impedance and increase PSC.