My electricity board recently fitted a double pole isolator after the meter. The tails go to a Henley block and then to 2 CUs: one for the house and one for the workshop and outbuildings. With the external isolator, do the CUs still need their own isolators or can I save the space and use it for a couple of MCBs?
Given you'll need terminals on teh busbar, you won't save any space as terminals will probably take up a module each.
But in a more general sense, it would be good practise even if not strictly required - people expect to find a main switch there and the electricty co could remove theirs when they felt like it, leaving you without a single point of isolation.
Thanks. I couldn't find anything that stipulated that there had to be an isolator, but on reflection I agree that it's the "right" thing to do.
I thought that if the isolator was fitted on the consumer side of the meter then it became the consumer's property, though I could well be wrong as I can't find authority for it.
I thought only company equipment was allowed on the meter board (didn't stop me fixing my own isolator to the board, which they "adopted" by sealing it when meter was last swapped).
Silly question - or maybe not. When the electricity company needs to do work on your wiring upstream of the consumer unit (and more specifically, upstream of their company fuse) how do they isolate the power so they can safely cut/join/move the wires - eg if the customer is moving their CU and company fuse?
And similarly, when a new house is built and the builder has installed all the wiring up to and including the CU, and the electricity board is ready to connect it to the street wiring, how do they isolate their wiring to make the final connection safely, especially if the feed comes as a tap from a set of overhead wires? Is there a designated disconnection point between the communal wiring and the company fuse? Obviously if the electricity company disconnects your supply because you haven't paid the bill, they pull the company fuse, but my question is about work on the wiring upstream of that.
Can they safely cut/join live wires provided they are suitably insulated? I suppose if they make sure the CU is turned off, there will be no current drawn so no spark.
Yes.
When I first started work repairing radios and TVs, the shop and workshops were in the process of being extended to the rear. The incomer ran beind the original building line to/from the shops on either side.
When the building work was finished there was a hole left in the shop floor where the cable pased through and to which the feed to our new consumer unit was.
A chap turned up from the electricity board with his tools and little rubber mat which he placed over the edge of the hole. He then sat down and merrily spliced our new feed into the supply while we all looked on muttering things like: "Sod that for a game of soldiers!"
When he left we went back to work on all those live chassis TVs. As many of them had non-poarised two pin connectors on the inputs, there was always a 50% chance that the entire chassis was connected to the live side of the mains.
Did we ever check? Never!
After our recent move I got a panic phone call in the pub from my wife because all the power had gone off! When I suggested she look outyside the front door she found the entire street in darkness, so not much I could do about it!
The pub was on the other side of the main road and not affected so it was business as usual. When I left, there was a lorry with a cherry picker beside a power pole on the other side of the road which, I found, was blocking the footpath so pedestrians were being shepherded along the edge of the road.
I enquired as to what was wrong and was told a transformer had failed and all the power was being switched to another one.
Sure enough, half the street lights came back on as I was half way down the road and, unbeknown to me, the power in our road had already been restored but it didn't go off again so the chap up the pole was obviously splicing all the feeds live.
Fair point - though I wonder how they keep track of which company owns what. Our DNO (UK Power Networks) say they don't own supply side isolators unless they agree to do so and they are labelled as theirs. We paid BG to fit it when they were both supplier and meter operator and they're now neither. I suppose it could be the new meter operator. Perhaps I ought to ask in case it get's stolen one day.
I think consumer units contain main switches by definition and as part of their type approval, otherwise they'd be fuseboards or distribution boards, going back to olden days when fuseboards didn't have main switches.
Owain
There does have to be a single point of isolation (IIRC) but I would need to rummage in the regs to prove that.
They grumble a bit, but seeing as it's my board (as in I paid for it and have to maintain it), they don't usually push that point. The main thing is they might refuse to say upgrade your meter if it won't fit due to your stuff being in the way.
But in reality, meter fitters are not unreasonable.
Not silly and as my old man was an LEB engineer, I can answer that:
They don't for Low Voltage - it's all done live (LV = 230/400V 1-3 phase).
High Voltage (eg 11kV and up) is isolated at the switching equipment and bonded to earth at both ends before anyone touches anything.
In my dad's time, they hot soldered the cables using two ladles and a bucket of solder on a gas burner, whilst sitting on a rubber mat. The joint was potted in hot bitumenous compound.
Think it's crimped or clamped now and the bitumen has been replaced by 2 part resin.
Sure there are some YT vids: search for "street cable jointing" or similar...
I've decided not to do what I was considering, but surely the DP isolator (after the meter) perform that function, especially when there are 2 CUs.
I shall have to check the regs and see if I can get a definitive answer...
Hasn't stopped me fitting a bell transformer and a socket to supply the answerphone onto the board and there has been no comment during the 24 years that they've been there.
SteveW
There are plenty of DIN rail mounting bell transformers, too.
Yes, mine's in the CU and since converting the bell-push to LEDs it buzzes much less loudly.
I cut my board down to half size when I moved the consumer units off it.
Owain
When they uprated the company fuse here, they also replaced the cast-iron fusebox with a hard plastic one. They simply donned thick rubber gloves, smashed the cast-iron box with a lumphammer, dropped the new box on, made the connections and resin-filled the cable entries.
SteveW
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