MCB kA Rating 3 phase Board?

[happysteve]

Assuming same size neutral cable as lines cables... my understanding was that a line-line fault current would be sqrt(3) times the L-N PSCC; but a symmetrical fault would be 2 times the L-N PSCC. However I've never seen a convincing set of maths to verify this. There are hints of it in this link... but "Z" is ill-defined there, and there seems to be a factor of 2 out.

Question: would a single phase circuit breaker in a 3-phase board ever have to deal with a symmetric fault? A 3-phase breaker, sure (perhaps a broken star-delta starter?). Because I've seen situations where sqrt(3) x PSCC is on the "ok" side of the line, whereas 2 x PSCC is more on the "explody fireball of doom" side...

 

[davesparks]

Whatever. It doesn't help that the installation test sheets refer to Ze at a consumer unit as Zs. So excuse my ambiguity. My meter was connected to Line, Neutral and Earth. I tested each phase for worse case. I understand that if I take the single phase value and double it, this gives me worst case scenario.
Pete

It's not whatever, it's an important technical detail. This is a technical trade and anyone practicing it should be capable of using a reasonable degree of technical accuracy in their communication.

It's no good just saying that your meter was connected to three points, especially when you should be carrying out a test which only needs two test leads, so that in itself suggests that you have carried out a three wire low current test instead of a high current test so your results are quite possible wrong as a result of this.
So what were you measuring? L-E or L-N?

There is no such thing as Ze at a consumer unit, Ze is a measurement of the external loop and is only at the supply terminals, before the consumer unit.





Sent from my iPad using Tapatalk

 

[davesparks]

What is the pollution category of the devices?

I know where you are going with this, but I think there are somewhat bigger concerns here!


Sent from my iPad using Tapatalk

 

[davesparks]

I do as well but the problem is not all MFT's can do L-L tests.

Then perhaps one should consider whether such a meter is suitable for testing a three phase supply of it cannot perform all of the tests?


Sent from my iPad using Tapatalk