Supply Main Isolation Switch

Postby Masterpop » Tue Nov 02, 2010 2:56 pm


I have a question-- I'd like to have a Main Isolation Switch fitted to my supply to sit between the end of the cable run coming out of the group, but before it gets to the meter.

I called my supplier (SSE) and they said the distributor installs it. So I called the distributor (EDF) and they said the supplier installs it.

Who should install it? And how much should it cost?

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Simply Build It

Postby sparx » Fri Nov 05, 2010 6:20 pm

hi, neither are obliged to fit an isolator switch although some are starting to do so.
A good local electrician should be able to supply and fit one.
I presume you have a number of sets of 'tails in a 'Henley' connector block connected to the meter tails.
Fitting a switch would give a single point of isolation as now required in the regs.
If this is the case I would supply and fit for around £80 plus tax.
regards Sparx
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Postby ericmark » Sat Nov 06, 2010 8:51 pm

The idea of a single point of isolation has been referred to many times however although 514.15.1 refers to the use of a dual supply there is no electrical regulation on single point of isolation. Only notices required where there is a dual supply. The Electricity Safety, Quality and Continuity Regulations 2002 and HSE rules are complex as they interact with each other and one of the HSE requirements is when working on a unit it should be isolated else where. With most domestic premises the only way to isolate else where is to remove the fuse. However the fuse should be sealed and only removed by DNO. So to comply with both regulations an isolator is required. However this could be for example a 100ma RCD as used with a TT installation or a plain isolator as found in a TN installation. This could be either the DNO's equipment or the consumers. Where it is the consumers then of course to change the RCD it would still require the fuse drawing. In an emergency of course one can draw the fuse although one should not replace it. And of course firemen do just this. But although the DNO is not to the letter allowed to permit electricians to remove and replace the fuse in many areas they do.

But having said that you state you want an isolator between head and meter an I have never seen that on a domestic. With commercial where the meter is a current transformer then often the main isolator is on the high voltage side but one needs very special permission to work on high voltage. With the Low voltage supplies I have never seen an isolator placed between head and meter.

One has to ask why is it wanted? Normally the DNO do not want the user to be able to isolate their meter as this would allow people to steal power with more ease. And with modern smart meters it would stop the DNO getting information from the meter.
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Postby Masterpop » Fri Nov 12, 2010 10:40 am

Thank you chaps.

The rational for fitting one was simply that my house needs quite a lot of electrical work which will occur over a long period of time (say 1 year, during which time I would need to isolate the main supply 2x I reckon, potentially just the once with some planning), so rather than rely on the DNO to come take out the main fuse and re-seal as needed I thought it would make sense to fit a main isolation switch. However, when I spoke to DNO, they said they were happy with the electrician to take out the fuse and asked that they be contacted when work is finished so they can come and re-seal it. at which point i thought maybe an isolation switch is not that necessary.

Your point on where to place it is interesting, you're right, after the meter, before the CU makes more sense. the way my installation is, physically between head and meter is more convenient.

Thank you
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Postby sparx » Sun Nov 21, 2010 3:44 pm

only just seen this since I have been sick so late reply but,
Single point of isolation IS required not by labeling reg you quote, but by reg. 537.1.3 and 537.1.4 "'A' main linked switch.......'SHALL' be provided...."
'A' being singular & 'shall' meaning must be.
the OP is not talking about multiple supplies as refered to in 537.1.5/6.
I should have quoted the reg number in my original reply, sorry
regards, Sparx
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Postby ericmark » Sun Nov 21, 2010 11:02 pm

I have taken this regulation to an English lecturer who has a degree in English and asked the question as to singular and plural.

He pointed out car must have a brake but nothing stops it from having two. So use of "a" means minimum of one and there is nothing to stop one have multiple switches either in series or in parallel.

If one looks at other regulations for example:- 533.1.1.3 A fuse shall preferably be of the cartridge type. Where a semi-enclosed fuse is selected, it shall be fitted with an element in accordance with the manufacturer's instructions. if any. In the absence of such instructions, it shall be fitted with a single element of tinned copper wire of the appropriate diameter specified in Table 53.1.

Where it means "single" is says "single" not just "a" so this confirms the interpretation I have given is correct and there is no regulation to say one needs a single point of isolation.

However having said that I personally feel with a domestic premises there should be a clearly marked isolator for use by ordinary persons which will isolate the whole supply. However we all know it is common for Henley Blocks to be used to supply secondary consumer units especially for the supply of items like a chair lift who's failure could prevent an occupant from vacating the premises.

I understand amendment 1 to BS7671:2008 is to have some special requirements for hospitals and the like so this may become more plain? It will also it seems allow sockets to be added to existing circuits without RCD protection without protecting the new cable and the use of SELV circuits without RCD or permitted encapsulating earths which of course makes sense as of course impossible to protect the cable to a bell push using either RCD or Ali-tube cable as by very nature it is separated from the earth.

In the old regulations it use to have a phrase "Common sense should prevail" although I can see people would try to use it to excuse non compliance I feel it should not have been removed as it seems people regard the big red book as a bible not a guide to acceptable practice.

With electricians who are members of a scheme I suppose it is. As they seem to fear the removal of their ability to self certify so follow it to the letter.

Having said that you may wish to look at 537.4.1.3 Means for emergency switching shall act as directly as possible on the appropriate supply conductors. The arrangement shall be such that one single action only will interrupt the appropriate supply.

Now that regulation could be very well read as meaning a single point of isolation is required. Again it's down to interruption.

Just last week this was being discussed as to the requirement for a cooker isolator in a kitchen where the supply was from a sub-main consumer unit installed in the kitchen right next to the exit to outside.

My point was the cooker switch was directly behind the cooker so if there was a fault one could place ones self in danger so it did not comply with 537.4.1.4. However in real terms with the RCBO and whole kitchen isolator bar for lights being within 3 meters of the hob there was no real need for an isolator anyway. However since it was an induction hob which could if it malfunctioned affect the pace maker of the user to place an isolator in a position where the user could be tempted to lean over the hob to isolate was not really a good idea. Since then there has dated the 3rd Nov 2010 been a directive not to fit induction hobs where the users have a pace maker so it's being ripped out again.

Seems although electrical manufactures have to follow EMC rules medical manufactures have not been complying with regulations and some pace makers can be affected and the users heart rate falls in line with the hob pluses!

Hope you are better. Some day may be I'll set up HF and we can have a chat.

73 Eric
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