High PSC’s…

I know this is nerdy stuff, but today I encountered the highest PSC (Prospective Short-Circuit Current) that I’ve ever encountered on an installation.

23kA was the figure I found, with an external loop impedance of just 0.01A

So, why you ask am I sharing this?

Let me briefly explain.

All breakers have a maximum “breaking capacity”. It’s a number, usually in a box, on the front of the device. Most domestic ones are 6kA, industrial tend to be 10, 12, 15 or even 25.

If a short circuit occurs which takes the fault current above the breaking capacity of the breaker then the breaker may not actually trip, or it might fail, or the it may trip but the spark will then “hop the gap”.

So, what do we do in this circumstance?

Well we could protect the installation with BS1361 fuses, however, that is not guaranteed to get your 0.4 second trip time.

My chosen method is to put an MCCB (Moulded case circuit breaker) with a 25kA breaking capacity into the meter tails. Not only do you then get your 0.4s trip time, but you get a guaranteed trip at high PSC’s.