Quick question

[plugwash]

But that gets picked up via safe isolation procedures - a shared neutral doesn't

If by "shared line conductor" he means two circuits supplied by different breakers have their lives connected together and one of the breakers happens to already be off at the time the work begins "safe isolation" procedures won't nessacerally pick up that the circuit is powered by two different breakers which means it is very likely that only one of the two breakers will get locked off.

That is the problem with "safe isolation", it's possible to show with a high degree of confidence that a circuit is dead at the time of test but much harder to show that it will stay dead.

 

[JohnW2]

But that gets picked up via safe isolation procedures - a shared neutral doesn't

If by "shared line conductor" he means two circuits supplied by different breakers have their lives connected together and one of the breakers happens to already be off at the time the work begins "safe isolation" procedures won't nessacerally pick up that the circuit is powered by two different breakers which means it is very likely that only one of the two breakers will get locked off.

I had assumed that when BAS talked of 'safe isolation procedures', he was including 'testing for dead' - which would obviously pick up the situation of the part to be worked on still being live despite one of two breakers supplying it having been opened. (The last time I cam across that situation was with two ‘cross-connected’ ring finals - i.e. one end of each connected to one MCB, and the other ends of both connected to a different MCB.)

Kind Regards, John

 

[ban-all-sheds]

I was, but plug's point is that if one of the two was already off, turning off the one you thought was it would pass all the safe isolation tests, but then you'd only lock off the one you turned off.

Probably not a significant risk in a domestic dwelling with 1 CU and a handful of circuits. Guess the answer would be to lock off all MCBs already off.

 

[JohnW2]

I was, but plug's point is that if one of the two was already off, turning off the one you thought was it would pass all the safe isolation tests, but then you'd only lock off the one you turned off.

Yes, I realised that but, as you go on to say ...

Probably not a significant risk in a domestic dwelling with 1 CU and a handful of circuits. Guess the answer would be to lock off all MCBs already off.

True. I would think it is an extremely unlikely scenario in any domestic installation, although far more likely in complex non-domestic installations - when goodness knows where 'cross-connected electricity' may be coming from!

As plugwash says, ensuring that any circuit is 'dead' is quite straightforward, but (unless one is certain one knows everything about the installation) being certain that it will 'stay dead' is difficult, if not impossible. I suppose the most certain thing one could do (as I believe is done in some industries) is, having established that a circuit is 'dead', to install a heavy L-N link fairly close to where one was working (and then remember to remove it before re-energising ).

Kind Regards, John