BS7671 Help with some PFE questions

clanky · Jul 17, 2021

Apologies for the delay in replying. Ive been stuck on a brutal night shift for the last week so its been a bit of eat sleep and work...repeat. Plus i didnt receive any notifications for replys...

Anyway, huge thanks to everyone for chipping in and although im fairly confident ill pass, i just find these foibles (love that word) annoying.

clanky · Jul 17, 2021

timhoward said:
If you read the list in the regulation you mentioned, the key thing is that in all cases you can't possibly have both supplies on at once. A separate switch for each doesn't prevent you turning them both on. The point of the regulation is that you mustn't ever be able to do that.

Ooops. ive just reread the question and realised my mistake! Should pay more attention.

clanky · Jul 17, 2021

pc1966 said:
I don't have a copy of the questions, etc, but this is one odd aspect where common usage of 'live' differs from the definitions.

Live = Any current-carrying conductor under normal use (so all but the CPC)
Neutral = A live conductors that somewhere is connected/referenced to Earth.
Line = The live conductor(s) not referenced to Earth

Indeed it really is the referencing of a current-carrying conductor to Earth that defines neutral, and how/where that is done is of course on of the key aspects of the supply.

While an IT supply may not have any reference to Earth, most of any significant power level (e.g. excluding shaver supplies) do by some high impedance so they technically can have a neutral even though a single line-Earth fault has no immediate impact on operations as normally the earthing impedance is quite high and designed to sustain such a single fault. Used for situations where you really don't want an immediate interruption if at all possible (e.g. operating theatre supply, ship electrics, etc) and it is under skilled supervision.

Thanks for the detailed answer. I appreciate the effort as not being an electrician, it all adds to my knowledge. Again, ive just reread the question and i think ive made the same mistake of trying to be too quick in doing my answers. Deep breath needed when i get round to doing the exam for real.