Is it OK(ish) to treat garage MCBs as switches?

[JohnW2]

Mind you, the note relates essentially to on-load switching, and therefore does not necessarily relate to essentially off-load switching such as skotl was talking about.

That's true I guess, I was confused by the reference to light switching earlier (and in my memory) when reviewing the OP's intentions.

The OP told us was that light switching was the one thing that he did not use an MCB for, and that his switching off the MCBs of sockets circuits was just a precaution 'in case he had forgotten to switch something off' - so nearly always would be off-load switching.

I'm glad it's in there, as the cynic in me has always thought DPCs were just the IET telling us there will be changes whether you like it or not, but we need to go through this process to keep people happy.

I'm not sure I understand your point. This change was in the DPC and persists, I think unchanged, in the BYB. As I recently said, I could not find any obvious difference between what is in the BYB and the proposals in the corresponding DPC - and I recall that the same was true of Amd1. Hence, if you wanted to be cynical in the way you suggest, you could argue that they had just 'gone through the motions' and not made any changes to their proposals as a result of the consultation process. I do not doubt for a moment that the committees 'give serious consideration' to all comments they receive during the consultation process, but I don't recall having seen any obvious examples of this 'consideration' resulting in anything changing.

Kind Regards, John

 

[EFLImpudence]

Just a small point -

Why do they deem it necessary to add the '(3)' - suitable for on-load isolation - in the 'Isolation' column but not in the 'Emergency switching' or 'Functional switching' column?
'Isolation' purposes being the only one where it is not required.

Surely it should be next to the device number.
Just another mistake???



On a wider point about the regulations -

I'm beginning to think that instead of Amendments they should more honestly be called Erratum.

 

[JohnW2]

Just a small point - Why do they deem it necessary to add the '(3)' - suitable for on-load isolation - in the 'Isolation' column but not in the 'Emergency switching' or 'Functional switching' column?
'Isolation' purposes being the only one where it is not required. Surely it should be next to the device number. Just another mistake???

I'm not so sure about that. Essentially by definition, 'Emergency Switching' and 'Functional Switching' usually involve switching off in an 'on-load' situation. Hence, I would say, saying something is suitable for emergency or functional switching implies that it is considered suitable for switching of an 'on' load. That is not necessarily the case with isolation, so it seems appropriate, and correct, that the 'suitable for on-load isolation' indicator is only applied to 'Yes' entries in the 'isolation' column for those devices considered suitable for on-load isolation.

On a wider point about the regulations - I'm beginning to think that instead of Amendments they should more honestly be called Erratum.

That would imply the correction of errors. Whilst that is undoubtedly the case with some things which are changed by an Amendment, many of the changes we see introduced by Amendments result from changes in thinking (or technical innovations) and do not mean, or imply, that previous versions were 'incorrect'.

Kind Regards, John

 

[skotl]

Cool - my very own diynot/electrics debate

Soooo.... we think that on balance it *is* ok to use the MCB as an off-load isolator switch?

As John said, this isn't for the lights (I wouldn't want anyone to have to fiddle with a CU / MCBs just to turn the lights on), it's for the welder and the sockets.

Of course, the whole question goes away if I/we conclude that I'm being unreasonable cautious and should simply leave the MCBs on, turn out the lights and lock the garage.

 

[JohnW2]

Soooo.... we think that on balance it *is* ok to use the MCB as an off-load isolator switch?

For reasons I gave at the very start (and as now more-or-less echoed in Note 5 to Table 53.4), I personally would not do it routinely. MCBs are not designed for frequent manual operation (and therefore, for example, are probably not tested in terms of 'number of operations' etc.).

Of course, the whole question goes away if I/we conclude that I'm being unreasonable cautious and should simply leave the MCBs on, turn out the lights and lock the garage.

It would! However, if you felt that you would be more comfortable with all circuits in the garage (other than lighting) being isolated when you left it, there would be ways of engineering that which would not involve using MCBs for that purpose.

Kind Regards, John

 

[skotl]

Thanks John (and all).

I'll just stop doing it - that seems the most prudent solution!

 

[ban-all-sheds]

However, if you felt that you would be more comfortable with all circuits in the garage (other than lighting) being isolated when you left it, there would be ways of engineering that which would not involve using MCBs for that purpose.

You could even padlock it when off, if you wanted.

 

[JohnW2]

<pic of rotary isolator>

Indeed. He might even consider using the main switch of the CU (although I'm not sure they are really designed for frequent operation, either). However, either way, he would have to engineer things so that his lighting circuit (which he wants to stay live) bypassed that isolator/switch.

Kind Regards, John

 

[ban-all-sheds]

I envisaged 1 per final circuit, after the CU.

 

[JohnW2]

I envisaged 1 per final circuit, after the CU.

Oh, fair enough . I was thinking more in terms of a neater/more convenient 'single isolator' solution (for all the non-lighting circuits at once) - which would be do-able.

Kind Regards, John

 

[bernardgreen]

Switches tend to switch things on and off therefore the contacts open and close with the load current,

Isolators tend to isolate after the load(s) have been turned OFF by the switch(es). Therefor the contacts operate with out a current flowing through them.

Some isolators have to operated under load. These are most often emergency stop devices.

The contacts and mechanisms are designed for these different functions.

A set of contacts that are not designed to switch under load should not be repeatedly used to switch under load.

 

[JohnW2]

Switches tend to switch things on and off therefore the contacts open and close with the load current, ... Isolators tend to isolate after the load(s) have been turned OFF by the switch(es). Therefor the contacts operate with out a current flowing through them. ... Some isolators have to operated under load. These are most often emergency stop devices. ... The contacts and mechanisms are designed for these different functions. ... A set of contacts that are not designed to switch under load should not be repeatedly used to switch under load.

Well, yes, I think we've discussed all of that. In context, the OP would be using the switch/isolator/whatever essentially for 'off-load' switching/isolation.

FWIW, as regards situations different from the OP's, BS7671 indicates that isolators to BS EN 60669-2-4 and BS EN 60947-3 are suitable for on-load isolation. Whether that means that they are designed, specified and tested to be used repeatedly for switching under load, I don't know.

Kind Regards, John

 

[skotl]

I've perhaps given the impression that my vast and luxurious "workshop" allows me to maintain 19th century steam locomotives as well as the odd light aeroplane.

In fact, it's a standard single garage so it won't take long just to walk round the eight sockets and flick their switches off. If I was to do anything at all then b-a-s' suggestion of an isolator per tool-carrying ring would be an idea.

But I probably won't

Thanks for the help, everyone.

 

[JohnW2]

In fact, it's a standard single garage so it won't take long just to walk round the eight sockets and flick their switches off. If I was to do anything at all then b-a-s' suggestion of an isolator per tool-carrying ring would be an idea. ... But I probably won't

Isolating a ring is 'interesting'. I suppose one can get SP isolation by using a standard DP isolator to 'isolate' just the two Ls. If one wanted DP isolation, I suppose one could use a 3P+N one to isolate all four live conductors.

Kind Regards, John

 

[skotl]

In fact, it's a standard single garage so it won't take long just to walk round the eight sockets and flick their switches off. If I was to do anything at all then b-a-s' suggestion of an isolator per tool-carrying ring would be an idea. ... But I probably won't

Isolating a ring is 'interesting'. I suppose one can get SP isolation by using a standard DP isolator to 'isolate' just the two Ls. If one wanted DP isolation, I suppose one could use a 3P+N one to isolate all four live conductors.

Kind Regards, John

Ah - great point. So an isolating switch in one direction would still leave the circuit live, but with diminished current carrying capacity! Definitely not doing that, then!