If you intend to use the main switch, it is good practise to turn all the mcbs etc off first
Changing light switch - how to confirm live/dead circuit
- Thread starter jonnyw
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Why turn all the mcbs etc off first (i.e. is order important)?
(Incidentally, I've now replaced the switch, by switching EVERYTHING in the consumer unit off first - though I started with the mains switch first).
(Incidentally, I've now replaced the switch, by switching EVERYTHING in the consumer unit off first - though I started with the mains switch first).
No, order is not important. It's theoretically unnecessary, but it's good practice, since it means that the main switch doesn't have to break any current - hence no 'stress' on it.
That's obviously almost 'fail safe' - but, as I said, you do still have to confirm that the circuit you are working on really is dead if you want to be certain that you're safe. A few years ago, a friend of mine got 'caught out' in a house he had recently moved into. He had turned everything off at the main (he thought only) CU, but found that the circuit he was going to work on was not dead. What he didn't know/realise was that there was a small 'secondary' CU that supplied that circuit!!
Kind Regards, John
As john says above, its only to reduce the load.
Not sure if anyone else does it, but its what i do as well as put the mcbs etc back ON after the main switch.
I feel the switching back on is slightly more important, though I do commercial work where the main switch could be doing 72 ways across 3 phases.
Your main switch is a SWiTCH so likely made to switch in excess of 80 amps, which in reality is unlikely.
I was always taught SWITCHES can operate under load and ISOLATORS to be used only with no Load or as an emergency off, though nowadays i think a lot of isolators seem to be switches
In commercial where a main switch may operate several smaller switches feeding numerous Distribution boards, it is sometimes found the smaller switches have timed reinstatement, so in a power cut, they do not switch the whole site on together and perhaps take out the supply
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Same here, but really just out of habit - since, as you go on to say, main switches should be more than happy to switch 'high loads' without suffering any harm.
Where a similar consideration is important is if one is going to pull upstream fuses (e.g. cutout fuses). One should eliminate downstream loads before doing that, since 'pulling' a fuse which is carrying a high current can be a hazardous business!
Yes, I have always understood that to be the defined difference - but, as you say, I think that, these days, most things called isolators are also able to break currents up to their rating. Indeed, if they couldn't (without being damaged), they would not really be usable as "an emergency off".
Kind Regards, John
Yeah, but.....
This is jonny's house. What kinds of active loads could there possibly be if he's happy to be turning everything off?
If one knows that it's only going to be a very short-term 'turning everything off' it could be all sorts of things - cookers, immersions, fan heaters etc. However, as I said, it's not really necessary when (as we do) we have main switches which can happily break their full rated current.
Kind Regards, John
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Well, a separate RCD can only break 63a or so?,I wonder what happens when there's a short to earth? The mcb has 0.4 seconds to react and can easily break your fault current but the rcd will be quicker to try. Will there be enough time to damage it? I guess the real breaking capacity is much higher than the rated current, is the 100a rating of a main switch the breaking current or the continuous current?
They (and, I presume) the relevant Standards) have thought of that. Taking MK Sentry RCDs as an example ....
Kind Regards, John
Really?
Really?
You'd turn the power off to change a light switch while someone was cooking?
I think you know the answer to that - if I wanted to change a light switch, I probably wouldn't "turn all the power off" (whether someone was cooking or not), even if I 'should'.Really? Really? You'd turn the power off to change a light switch while someone was cooking?
Kind Regards, John
Seriously. For the sake of the OP and any other diyer, there is NO circumstance where you should contemplate pulling the fuse so you can work on a circuit.
1) It's a criminal offence
2) A mistake can kill you
I'm surprised nobody picked this one up
Is it really a criminal offence?
I'm sure we can agree that no DIYer (and, indeed, most electricians - unless 'given permission') should pull cutout fuses, so maybe I should not have given that as an example. My point was the more general one (and of potential relevance to DIYers), that one should not remove any fuse under load - and that includes the countless 3036s still in CUs all over the place.
Kind Regards, John
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