Consumer units -how do they work?

[David Standing 1Participant @davidstanding1]

John

The mcb's in the CU you linked are screwed to the busbar. You just can't see it, under the busbar cover at the bottom .

[Clive IndiaParticipant @cliveindia]

Posted by Nick_G on 06/01/2017 16:49:17:…… – Have you never been to the pub with a crowd of like minded guys.? Nick

Oh yes, forgot – will try it out with my mates down the pub tonight, although some of the group are power engineers who have little respect for the average electrician.
Just for the record – I didn't say anyone here is an average electrician.

[TobyParticipant @toby]

Posted by David Standing 1 on 06/01/2017 19:35:27:

Posted by Toby on 06/01/2017 19:33:04:

Posted by David Standing 1 on 06/01/2017 19:19:50:

I put a C curve mcb in a spare way in the garage consumer unit myself, and took a direct feed via a switched outlet for the compressor off this.

Voila, no more tripping.

You forgot to mention that you also did a loop impedance test to check the C type will trip correct….

I have been sitting here expecting a comment/question from you .

I didn't want to disappoint

[TobyParticipant @toby]

Posted by Clive India on 06/01/2017 19:54:29:

Posted by Nick_G on 06/01/2017 16:49:17:…… – Have you never been to the pub with a crowd of like minded guys.? Nick

Oh yes, forgot – will try it out with my mates down the pub tonight, although some of the group are power engineers who have little respect for the average electrician.
Just for the record – I didn't say anyone here is an average electrician.

Edited By Toby on 06/01/2017 20:23:05

[Nick HulmeParticipant @nickhulme30114]

Posted by Clive India on 06/01/2017 19:54:29:Just for the record – I didn't say anyone here is an average electrician.

Average electricians?

Like guys who sell up a small maintenance job into one which isn't actually required and involves much greater expense and then fail to conduct an adequate site survey and fit equipment suitable for the job?

[Robin GrahamParticipant @robingraham42208]

Thanks for the (myriad) replies guys – didn't expect this one to run to three pages and counting! Not objecting at all though – an interesting read, and I've learned some stuff I didn't even ask about.

My original question was answered in the first few replies – thanks especially to Toby for a very clear and comprehensive answer at #3. I now know that it is an earth leak I'm looking for – it was the switch with the test button that tripped, so the RCD – I just wasn't sure about the difference between that and the MCB's.

The machine is a Record extractor manufactured in 1999. It's probably been sitting unused in a garage for years. Multimeter shows >40Mohms between live and earth with the power switch on and measured at the plug pins, but of course that's 1.5V DC so says nothing about a capacitative leak or higher voltage insulation breakdown. It's a brushed motor, so no start capacitor but a 1.0 microfarad supressor across the live/neutral terminals where the power cord enters. Then it goes to a mystery component (pic below), the power switch and finally the motor. If anyone can recognise the 'mystery component' from the crappy photo, I'd be interested to know.

Regards, Rob

[AjohnwParticipant @ajohnw51620]

Too small really but shows slight signs of being a filter for noise suppression. Some times just done with 2 capacitors.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Agree with Ajohnw …

Manufacturer's website is here: **LINK**

http://www.schaffner.com/index.php?id=44&tx_kesearch_pi1%5Bpage%5D=1&L=0&tx_kesearch_pi1%5Bsword%5D=mains+suppression+filter

You should find a direct replacement, or suitable equivalent at RS Components, or similar.

Note: This could well be the cause of your problem.

MichaelG.

[Mike PooleParticipant @mikepoole82104]

As John says it looks like a filter Schaffner make filters and it looks like the diagram of a filter circuit in you picture although it is not too clear. Filters are often bad news when they meet RCDs. Try just running the fan without the filter, this may cause interference with other equipment so ok for a test but should not be used if it does interfere with other equipment.

Mike

[Les Jones 1Participant @lesjones1]

I have zoomed in on the picture of the filter and I THINK I can see three capacitors. The one that is easily visible is between live and neutral this should not cause a problem. Of the other two which I think I can one will see will be between earth and neutral and the other between earth and live. On older filters these were normaly the same value. (I think about 0.1 uf) 0.1 uf has a reactance of about 32K ohms at 50 Hz so the reactive current through the one between live and earth will be 240/32000 = 7.5 mA This is not enough to trip a 30 mA RCD if there were 4 or more devices wil this type of filter the reactive current to earth would be enough to trip the RCD. We need to know the value of these capacitors to see if there value will cause a reactive current of 30 mA In modern filters this capacitor (If fitted) is a much lower value. Even if these capacitors don't cause enough reactive current to trip the RCD they can break down at a higher voltage so then need to be tested with an insulation tester which uses a higher voltage. DO NOT test them with more than 500 volts as it can destoy a good capacitor.

Les.

[Michael GilliganParticipant @michaelgilligan61133]

Presumably, Robin will have sight of the actual item, and can [hopefully] identify the part number and schematic.

MichaelG.

[John FlackParticipant @johnflack59079]

As someone responsible extending this discussion, may I say that my gas lit childhood and reading the Beano by candlelight things were a little simpler. No this is not a leg pull !!!!!!!!! It really was like that in my parents home in the 1940/1950s

[TobyParticipant @toby]

I've not much to add except I think that RCDs were around in the 1990s so the fan (and hence the filter) was hopefully designed for them.

But yes, I agree with Mike, the easiest approach is to bypass the filter and see if it still causes a problem. If it is ok then a new filter should sort it.

[AjohnwParticipant @ajohnw51620]

I might be inclined to remove it and also if possible check the rcd. If that is faulty a replacement might be ok. It might not be.

Brush dust is a well known cause of the problem. It can be difficult to get rid of it entirely as well.

RCD's were about well before the 90's. Not sure when they started being regularly put in houses though. I married in 85 so it was well before that. My electronics workbench at work was initially fitted with a stupidly low one. Later increased and later still modified 2 way adapters were provided with the earth disconnected from the supply. Painted red. Earth loops were interfering with measurements. There is still some scope for having gear that isn't protected this way but I think anyone trying to get round it that way in a private workshop or house could be breaking the law now.

I've never lived in a property with RCD's but do recollect that there were a lot of screams and shouts when they started appearing in houses. All sorts tripped them out. I don't know what the current levels were initially. I do know that there were thoughts about preventing electrocution but having once been involved with worries in that direction I don't think that is actually possible. Depends what is holding what. I'm fairly certain various trip levels were available.

The current regs were more or less established by i would say 1980 or there abouts. Some aspects seem to have been dropped. The voltage drop aspects etc were all there. When I rewired my fathers property he flatly refused to have a plastic consumer unit fitted. He got some one to find him and old metal one that had been removed from some where else so I'm not surprised that the plastic ones are forbidden now. Good job too. Stupid idea. Apart from bonding and at the time the lack of conduit the wiring would meet current regs. Bonding was specified but not needed then. The need for conduit seems to have disappeared in houses anyway.

Whoops Toby might pick me up on that. Conduit of some sort – seems to depend on where hidden cables run.

John

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Edited By Ajohnw on 07/01/2017 10:57:24

[Russell EberhardtParticipant @russelleberhardt48058]

I thought it strange that plastic consumer units are regarded as unsafe in the UK but are the norm in most of the EU then I noticed that "Electrical Safety First Best Practice Guide No. 1 (Issue 3) " is published by Electrical Safety First whose address is The Metal Box Factory etc.

Am I just being suspicious?

Russell

[TobyParticipant @toby]

No, I am not going to pick you up on that John

Yes, conduit is not generally used in houses nowadays unless additional mechanical protection is needed. Even then I think the recommendation is 3mm steel which rules out normal conduit! Although I think that 3mm is a figure picked out of the air by some guy at NICEIC so can be argued about. RCDs and running cables in prescribed zones is the usual way of handling things nowadays.

You are also right that RCDs were around well before the 90's, I wasn't clear but I meant common in houses (ie mandated on sockets etc) which I thought started around 2000.

That said, I have just looked at wikipedia and it says the wiring regs were started in 1882 but I think it was the 16th edition that mandated RCDs on sockets and that was in 1991, so earlier than I thought.

As to whether RCDs stop electrocution. They don't, but they certain reduce the risk dramatically in the most common situations. As you say though, it depends on what you grab! I like RCDs and think that (generally) they are a good idea. However can cause problems, particularly with motors and inverters so I can understand why some people are not so keen.

Actually, on that subject it is worth mentioning that it is still well within the regs to supply equipment without an RCD. It just has to be done the right way (and with a risk assessment) and you certainly cannot use a 13A socket! I am of course not advocating that as a solution when the problem is actually faulty equipment

[TobyParticipant @toby]

Posted by Russell Eberhardt on 07/01/2017 11:55:12:

I thought it strange that plastic consumer units are regarded as unsafe in the UK but are the norm in most of the EU then I noticed that "Electrical Safety First Best Practice Guide No. 1 (Issue 3) " is published by Electrical Safety First whose address is The Metal Box Factory etc.

Am I just being suspicious?

Russell

To be fair, I don't think they have made metal boxes there for quite a few years…..

[AjohnwParticipant @ajohnw51620]

The reason for the metal boxes is simple – anything goes wrong in there – rats chewing cables etc can happen and it will most definitely blow a fuse if it causes a problem. The other aspect is fire risk and if so fumes. Fire risk is also now the main reason for rcd's but I feel that they are still a bit bound up on other aspects. In other words how much current to cause a fire? Bit of an imponderable. I'm pretty sure higher current trip level RCD's are still available. Not sure that anyone can use them but believe that the official feeling is that any rating is better than none.

I didn't want to mention don't have to be used on here. It's a bit like PAT testing there are variables. I do wonder why an industrial plug though. Suggests there is something wrong with what we usually use. I would also wonder if fixed wiring regs were different to plugs.

I used to talk to some one who was directly involved in establishing the regs. Conduit would make it easy to replace cables. It's fortunate for many people that rubber was bigger than pvc so the pvc could generally be pulled through. The other thought was drilling holes in walls. The metal type they used in houses probably didn't help at all with that.

The latest addition is bugging me. 10mm^2 bonding on gas pipes just past the meter. If you have one of the people in who can work on the supply side of things they have to leave a card saying this should be done. There are also some cowboys about leaving card saying must do it now. The person who left one here was apologetic about it and shook his head when he told me about the 10mm^2. He said he has to leave them just about everywhere he goes. I said no way. He said we will just run it around your door – our front door in this case on the outside. So I said no you wont you'll fit an earth spike to which he said that the problem with those is ensuring they are really in contact. Testing I suppose. Well there are metres and metres of metal pipe work running off the things so I am pretty sure it would pass just as it is. Same with the water pipe. It seems that both can come in on plastic now but why 10mm. In a lot of more recent house the run would be very short. It's long on ours but the water pipe bonding cable on most house will be long. Then I remembered a complaint from the suppliers a long time ago. So much current was being dumped to earth that the local potential compared with the substation earth was often too high so wonder if this is a cheap way of them improving it as something doesn't make sense.

Anyway leaves me wondering why regs don't state test it and fix if needed as far as bonding is concerned.

John

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[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Ajohnw on 07/01/2017 13:49:14:

The reason for the metal boxes is simple – anything goes wrong in there – rats chewing cables etc can happen and it will most definitely blow a fuse if it causes a problem. The other aspect is fire risk and if so fumes. Fire risk is also now the main reason for rcd's but I feel that they are still a bit bound up on other aspects. In other words how much current to cause a fire?

Our French rats are far too fat to get into a consumer unit. (Most of them are in Parliament ). I doubt if the fire/fume risk is much different when you consider the amount of plastic in all the circuit breakers. I would hope that the enclosures would be made from a fire retardent grade of plastic for these prices I would hope so! **LINK**

Russell.

[Frances IoMParticipant @francesiom58905]

RCDs were around but not mandated or generally in use in early 1980s (+ also expensive + rather restricted in choice IRC) – I fitted them when I had to rewire a house but advice at time was to leave some non RCD circuits specially for fridge + freezer spurs.

My flat in IoM has a plastic box (fitted pre current metal box regs) retrofitted with RCBOs but current fire regs for flats on Island required a heat detector in sight of box!

Edited By Frances IoM on 07/01/2017 14:50:09

[MuzzerParticipant @muzzer]

RCDs have been mandatory on TT systems for several decades, possibly the 60s? They were fitted by the supplier and it would have been a serious offence to tamper with them.

[AjohnwParticipant @ajohnw51620]

There are loads of plastic consumer units on ebay. One mentioned must be fitted with into a fire proof box. I doubt if they really allow that. A number of them are water proof. The regs imply that these can't be used for any purposes in a domestic environment – or some seem to think they do.

One thing I wondered is if the switch and rcd was taken out and just the current trips left would it still be a consumer unit. Maybe even if just the rcd was taken out. To make up a sort of distribution board / super junction box for spurs etc.

I think I read somewhere about the height of consumer units above the floor being regulated. If we ever updated our and it was moved indoors the most sensible places are all over 6ft off the floor. Can't be under the stairs so some child at some point might play with it. Our consumer units are good old Bakelite that has stood the test of many many years.

Personally when comprehensive regulations are introduced I think a lot of people should be retired off when they are reasonably ok. A problem with people. If they have a job they are almost bound to carry on thinking up other things to do.

One new idea which seems to be purely aimed at circumventing one aspect of the regulations is being rolled out when smart meters are installed. They fit a totally separate isolator in the feed to the consumer unit so that ordinary electricians can work on them. Pulling the fuses isn't ok any more – I mean they are sealed. That's what the man told me anyway – so it doesn't need some one like him to work on it.

John

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Edited By Ajohnw on 07/01/2017 15:35:14

[Gordon WParticipant @gordonw]

Way back in 1970 we rebuilt an old house, got grants etc. so had to comply with the regs. .Consumer unit had an RCD.

[TobyParticipant @toby]

Posted by Ajohnw on 07/01/2017 13:49:14:

I didn't want to mention don't have to be used on here. It's a bit like PAT testing there are variables. I do wonder why an industrial plug though. Suggests there is something wrong with what we usually use. I would also wonder if fixed wiring regs were different to plugs.

…….

The latest addition is bugging me. 10mm^2 bonding on gas pipes just past the meter. If you have one of the people in who can work on the supply side of things they have to leave a card saying this should be done. There are also some cowboys about leaving card saying must do it now. The person who left one here was apologetic about it and shook his head when he told me about the 10mm^2. He said he has to leave them just about everywhere he goes.

……

Anyway leaves me wondering why regs don't state test it and fix if needed as far as bonding is concerned.

John

I don't think omitting an RCD is quite like pat testing. Pat testing is there to ensure that something isn't damaged or faulty and unsafe. ie works as designed. It is hard to argue that is not a good idea.

Whether you have an RCD or not is a design issue. There are some circumstances where they are not a good idea as they will cause problems. eg machine tools with out of phase currents, medical equipment where an interrupted supply could cause harm, etc.

Hence the regs say RCDs should be used on socket outlets not exceeding 20A and mobile equipment used outdoors with a current rating of less than 32A. The point being that both of these are higher risk, either because you cannot control their use (eg sockets) or because of the environment (outdoors). For lower risk stuff the need for RCDs is not so important.

In fact there is also a exception – for a specific socket that is for use for a particular item of equipment and labelled as such. So I have one socket in my garage/workshop that is not RCD protected. It is specifically for my door entry system because I would really like to be able to get in, even if a bit of water gets in the electrics

So if someone had a machine that was not faulty but still caused RCD problems I would have no problem fitting a non RCD outlet for it. I would of course want to make sure the machine was wired in such a way there was no chance of cable damage or other accidental access to a live part and that the machine was properly earthed.

Bonding is a real can of worms! This is one area where it should be sorted even it was ok against the regs when installed. Hence the gas man giving everyone leaflets saying it must be done. In fact the 10mm2 min is actually for PME/TNC, for TNCS or TT it can sometimes be 6mm2.

Like I say, it is a real can or worms and can actually cause a hazard – I have seen a video of a gas pipe smoking due to a PME system fault being earthed through the bonding! However in general it can save lives if there is a fault somewhere outside the property and can be essential in ensuring an RCD works properly for an internal fault.

EDIT: forgot to say, the guidlines DO say test and fix for bonding. Strictly no work should be done unless bonding is present and measures less than 0.05 ohms, but it doesn't necessarily need to upgraded to 10mm2 if it appears ok.

Toby

Edited By Toby on 07/01/2017 17:41:17

[TobyParticipant @toby]

Posted by Gordon W on 07/01/2017 17:19:29:

Way back in 1970 we rebuilt an old house, got grants etc. so had to comply with the regs. .Consumer unit had an RCD.

I am guessing that house had an TT earth system (earth rod, rather than earthed through the suppy). I think that as the only reason for needing one back then.

EDIT, just noticed Muzzer already mentioned the need for an RCD on TT systems.

Edited By Toby on 07/01/2017 17:35:34

Edited By Toby on 07/01/2017 17:35:49

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