Why is there a main switch as standard in a consumer unit? What is it for?

ericmark

ericmark

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The HSE rules say isolate else where, so we have an independent isolator outside the consumer unit so it can be isolated before the lid is removed, so why have a main switch in the consumer unit, is there some rule about double isolation?

Seem to remember with E-stops I had to have two contactors in series in case one sticks, but it seems a bit OTT with a consumer unit, there are internal guards so one can safely test for dead after removing the cover, so why do they have an isolator in the consumer unit?
 
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It enables the average home owner to quickly isolate the power to the property without having to locate and operate a remotely located isolator

It also provides a convenient way to connect the meter tails to the bus bars in the consumer unit.
 
Im no electrician, but imagine its a bit like at the gas meters, a handy quick shut off for emergencys for the householder.
 
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ericmark said:
The HSE rules say isolate else where, so we have an independent isolator outside the consumer unit so it can be isolated before the lid is removed, so why have a main switch in the consumer unit, is there some rule about double isolation?
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No rule about 'double isolation' of which I am aware, but seemingly definitely a requirement for a single means of isolating the entire installation ...
BS7671:2018 said:
462.1 Each electrical installation shall have provisions for isolation from each supply.
462.1.201 A main linked switch or linked circuit-breaker shall be provided as near as practicable to the origin of every installation as a means of switching the supply on load and as a means of isolation. A main switch intended for operation by ordinary persons, e.g. of a household or similar installation, shall interrupt both live conductors of a single-phase supply.
Click to expand...

Kind Regards, John
 
JohnD said:
What is the relevance of HSE rules?
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To work in a consumer unit following HSE rules it must be isolated else where, yes I know we often don't, but rules are isolate else where, so either an external isolator should be used, or remove load and draw the fuse, which could be a problem as likely that would break DNO seals, which was why it was at one time down to DNO to provide an alternative isolation method.

But following the rules we should have an isolator before the consumer unit, and often in the consumer unit we also have RCD's, so it just seems OTT to have a DNO fuse, and external isolator, then isolator in the consumer unit, then two RCD which also isolate, it would be like requiring a RCD on the pitch for a caravan and then also one in the caravan, or same with narrow boat, yes there will be the on board generator with some, so there will be the odd time you do need the two, but it does seem in the main to be OTT.

So I could have an extra two RCBO's in my consumer unit if there was no isolator or it could be smaller, it just seems daft to have a isolator in the CU and one outside it.
 
Do you have a citation for the rules in question? when I did a quick search I couldn't seem to find anything relavent.
 
plugwash said:
the rules in question?
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Probably EAWR 1989, regulation 12.
However that doesn't require the use of another isolator before the consumer unit, only that there is some means to isolate it when required. That function is provided by the supplier cutout, or in some cases a separate red link cutout for situations where the fuse is located elsewhere.

The isolator in the consumer unit is for disconnecting the electrical installation for whatever reason, and can be used by anyone at any time.
The cutout is for isolating the consumer unit once every couple of decades when it requires replacement. Or when the meter is replaced at about the same intervals.

If people want to fit a separate isolator switch between the meter and the consumer unit which will never be used, then good luck with that. It certainly isn't required and never was.

Working in a consumer unit which has the main switch locked off isn't live working either, as the only live parts will be covered by insulation.

Some old Wylex and other fuseboxes did have live screws exposed with the switch off, particularly if the terminal covers were missing, but anything of that age should have been replaced years ago, and there are no circumstances where anyone should be adding circuits to such a thing.
 
What if you need to work on the isolator, it would need another isolator, then if you need to work on that one.........
 
flameport said:
.... The isolator in the consumer unit is for disconnecting the electrical installation for whatever reason, and can be used by anyone at any time. .... The cutout is for isolating the consumer unit once every couple of decades when it requires replacement. Or when the meter is replaced at about the same intervals. .... If people want to fit a separate isolator switch between the meter and the consumer unit which will never be used, then good luck with that. It certainly isn't required and never was.
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All true.

However, in answer to eric's question ("Why is there a main switch") I think the answer remains what I wrote in post #5 above - namely that 462.1.201 of BS7671 requires a single point of isolation for the whole installation (which I take to mean the installation downstream of the meter), which I assume refers to an means of isolation which is 'consumer-accessible (hence the DNO cutout would not 'count'). Is that not the case?

Kind Regards, John
 
462.1.201 did not exist in 17th Edition, but since we can have henley blocks and more than one consumer unit to have a single point of isolation would often need some thing before the consumer units, I would guess two reasons, one historic, distribution boards have always had them, and possible it was listed as required by type testing. And the second it would need some sort of terminal block to replace the main isolator, and at around £13 retail price is it worth have two versions of a consumer unit one with and one without an isolator?

The problem we have is getting a modern consumer unit to fit in the space where the old one was, with even 4 circuits the box is often 8 module wide, with two modules for the SPD and two for the Isolator. So to fit in the space have two 4 module units could be the answer, one with the SPU, and breaker for that if required and the isolator, and the other with RCBO's, allowing the second box to be small enough to replace the original fuse box with ease.

But I have never read 462.1.201 so not a clue what it says.
 
flameport said:
Some old Wylex and other fuseboxes did have live screws exposed with the switch off, particularly if the terminal covers were missing, but anything of that age should have been replaced years ago, and there are no circumstances where anyone should be adding circuits to such a thing.
Click to expand...

Apart from the older Wylex Standard, which had very exposed connectors, I have seen relatively modern CUs where the screws for the live incoming terminals are accessible.

I remember thinking the MEM ones were better, with sliding insulated colour-coded covers for the screwheads, retained, and not easily dislodged or lost.
 
I seem to remember Merlin Gerin had switches to each MCB so even the tags from bus bar dead, seem to remember you could have an isolator at bottom of bus bars, but this was optional.

However through history it is not the installation designer who decided what protection was installed, but the manufacturer, when I fitted two RCD's feeding the two Wylex fuse boxes in my old house, Wilex-board-with-RCD.jpg I could not have fitted a twin RCD consumer unit as simply not made.
 
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