Testing pfc

Prospective fault current is required by BS 7671 to be determined at relevant points, which includes switchgear and protective devices (and hence each distribution board).

However, that doesn't mean a measurement has to be taken, and in some cases there are significant risks involved in taking a measurement. See Appendix 16 of BS 7671. In cases where the prospective fault current is high, say much above 10 kA, the resolution of standard loop test equipment can be too high to be relied upon.

Excellent thanks and what if the pfc exceeds the breaking capacity of the protective devices on the distribution boards? 

That depends on whether the appropriate backup protection has been provided, and/or whether there is a conditional short-circuit rating applied for the board with the relevant protective devices.

For example, UK 'consumer units' for domestic use, typically have 6 kA protective devices, sometimes 10 kA with RCDs, but are required to have a conditional fault current rating of 16 kA as per the tests of the National Annex to BS EN IEC 61439-3. The 16 kA conditional fault rating is dependent on having a maximum 100 A BS 88-3 or BS 1361 protective device upstream of the consumer unit

Ha! then it may be unsafe as the breakers may weld up or catch light but not break.

But before rushing off to condemn half the dis-boards in the land, there are a few fairly common ways to address this. Firstly ohms law is your friend, and you don't need that many tens of milliohms of wire between the origin and the breaker in question to tame the wilder currents.

If you were to measure down the line, the PSSC falls due to the wire resistances.

As an aside, I've mentioned it on here a few times but I have a 'rule of 16' that is a handy way to work out if things are close to needing a proper set of calcs. A 1m length of  1sq mm cross section of copper is 16  milliohms. And the same a 25m length of 25mm2 or a 10m length of 10mm2 etc.  This is not quite right, when wires are warm its nearer 18 or 19, but it allows you to pace things out while looking up at the cable sizes and do a quick sum to decide if 'that will need its calculations checked' or 'no worries' for both voltage drops and PSSC. And (nothing to do with it, but) 1.6mm is very close to 1/16 of an inch, and the thickness of a 16SWG plate.

Now the other trick is to accept that a really high PSSC is unlikely and if it occurs it might damage the breaker but arrange so it will blow something else further back that limits the destructive energy available. Much as the fuse in the domestic cut-out by the meter gives 'top cover' to the usually 6KA rated breakers in the box beside it - so even if the house is next door to the substation, so PSSC is 15kA or something,  and there is a silver nail perfect fault and within a few metres of the CU, maybe the circuit breaker gets damaged , but the situation is perfectly safe.
The use of up front fuses as a 'let-through energy' limiter in this way allows smaller ACBs and busbars in some factory settings. If they blow then other things will need inspecting and maybe replacement too, but that is sometimes a perfectly acceptable design decision. 

Mike.

sorry i was clearly composing while Graham was typing- we are really looking at the same thing and agreeing, even if it does not read like it.
Mike.

IMHO, it depends ...

If the boards are adjacent to the origin, it makes no difference; but if they are remote, Lee is correct. However, if they are remote, the PFC is less likely to be scarily high.