Consumer units -how do they work?

OK Toby and Nick

I am a dum dum where electrical definitions are concerned , so I employ a professional electrician to solve my problems of ignorence , if I can't use domestic and workshop equipment after paying for his services what are my options to remedy what seems to be a problem???

First thing is to make sure the root cause is understood.

1) A simple overload. Solution is to seperate high current stuff onto a seperate circuit. Eg put your workshop on its own circuit, perhaps even with a serperate RCD.

2) If it is start up current problems then perhaps a change of MCB type (or even RCD type) might solve it. But it would need an electrician familiar with your house to advise. Or it could be you need to look at the drive circuits (eg startup caps etc) for your machines.

3) If it is a fault. (eg earth leakage etc) then find and fix the fault.....

fwiw, on my house (given I am qualified as an electrician) I replaced my consumer unit with the following:

Low usage circuits like washing machine, cooker, shower etc on a RCD, lights, sockets on their own RCBOs and garage on a type C MCB without RCD protection. Supply to the garage is armoured cable and it has its own consumer unit that gives separate RCBO protection to power and lights. That way, if I have a particular item (eg my lathes VFD) that will trip a standard RCBO/MCB I can swap that out to a separate circuit with protection to suit.

Basically, you need an electrician prepared to design the installation properly. Unfortunately a fair few are only prepared to fit standard circuits and are not much interested in fault finding!

btw. a type C MCB is one that will cope with a higher inrush current than normal and an RCBO is basically an MCB with RCD protection built in, so it does not need separate RCD protection.

Edited By Toby on 06/01/2017 11:29:35

The wiring regulations (BS7671) are not retrospective so, unless this a is a property subject to a periodic inspection regime (rented) then rewireable or cartridge fuses, no RCD and without earthing to ceiling pendants and light switches is permitted on an original installation dating to say the late 1950's where PVC insulation became the norm.

Sadly Part P of the Building Regulations (a separate document to the wiring regulations) and probably the £250 cost of building regs approval for standalone wiring changes has appeared to have driven the general public away from any permanent wiring installations towards multiway socket extensions that, along with multiway adaptors the fire brigade now deem a new potential source of fire in the home. (despite them generally all being constructed and fused to BS1362 & BS1363)

Also with the introduction of the 17th edition 3rd amendment to BS7671 somehow the tens of millions of plastic (and wood) enclosures that have been used for decades for consumer units are also now 'unsafe' and new installations have to use a metal enclosure or be housed in a fireprooof cupboard. The cynical would suggest they only become unsafe when wired for intensive indoor horticultural purposes when operating at 300% of their intended design rating.

Edited By Martin 100 on 06/01/2017 11:42:31

I agree with most of what Martin has said with a couple of provisos....

1) although I agree on the fireproof consumer bit being a massive over-reaction, I have seen a worrying number of "modern" plastic enclosures where the terminals are loose to the point of overheating and hence causing a fire. I think this a two fold problem. First, too many "electricians" who have not been taught the right technique for "snugging down" a connector. Just tighten it up and leave and you can guarantee it will loosen later. 2nd, the old boxes used to have double screw connections, the modern cage clamps are good if used properly but are easier to get wrong.

2) I would class a metal light fitting without an earth as a code C2, ie potentially dangerous and requiring fixing. This is in line with industry guidance. If it a plastic class II fitting then yes, no problem.

Toby

Thanks again that explanation suggests a solution, however should I not have confidence in a professional and qualified electrician to work this all out for me?

yes, you should. Unfortunately, as in all walks of life, there are "electricians" and "electricians". Meaning some are experienced enough and have the will to do it right, some either do not have the experience or just don't see there is enough money in it to bother so will just fob you off.

I understand you don't want to get too involved in this yourself but I am guessing that, as an engineer, you have a logical mind and understand how to problem solve. Given that I would advise you get an electrician in, explain the problem and ask for his advice. If he doesn't suggest an initial investigation to find out if there is a fault then I would be suspicious but otherwise I would expect a clear explanation of what he intends to do and why. If you don't get that then I would suggest trying another electrician.

Btw. often the one man bands are better than someone from a big company. They have more incentive to do it right.

Oh, and unfortunately you need to expect to pay a reasonable amount to get it sorted. Eg, to turn up at your house and spend a few hours testing circuits and appliances I charge at least £100 with a proposal to review the situation if that money did not find a solution (ie I could not find a fault/problem to fix). From there you are talking anything from no extra (ie I found a problem and fixed it within the £100) to £hundreds to install a new circuit. And I think my prices are on the cheap side (judging by how little money I made as an electrician......)

It is possible but unlikely the problem has been caused by poor work by the guy who changed your consumer unit so unfortunately this is extra cost. I learnt very early on to warn customers about this when changing a consumer unit as it potentially save a lot of "you broke it, no I didn't" type arguements later on!

Hi Martin - my comments were really aimed at John Flack's post where he said he had had his CU replaced by an electrician. In which case, the regs should be adhered to in terms of 30mA RCDs, surely. The alternative would be to bypass or respecify the RCDs that came in the replacement CU, which would be very dodgy territory. I doubt you'd recommend that?

Murray

BTW, there's a handy electrician's guide to implementation of the wiring regs, published by the IET. Doesn't cost much and it explains the practical implementation of the regs. I doubt most electricians would understand the formal document which costs £70-90.

Edited By Muzzer on 06/01/2017 13:08:54

I am assuming Martin was responding to Johns comment

"our fuse box was not up to code and required replacing, though it had been OK for the 30 years"

Strickly that is wrong, as Martin said, there is no need to update a unit just because it doesn't meet the current regs. However if new work is being done (eg adding a socket or circuit extension) it can often be more cost effective long term to update the fuse box to gain the RCD protection etc. Plus it is no bad thing as safety has come along way in 50 years!

I think the book you linked to is the building regs guide, for the wiring regs you want this one: **LINK**

I agree though, both are excellent summaries (although my copies have some minor errors).

Any registered electrician should understand (and own) the full regulations as they have to pass an exam on the subject before being accepted by a registered body.

That said, that highlights a problem with the Part P registration schemes and larger companies. It is completely within the rules for a company to have one "qualified supervisor" who has past the exams and registered supervising a number of "electricians" who have not. So there is no guarantee the guy that turns up at the door has even read the regulations, let alone understood them

EDIT: just re-read your post and I might have misunderstood your meaning. Yes, fitting a new consumer unit but bypassing the RCD etc is definitely not allowed, all new work needing to meet the current regulations.

Edited By Toby on 06/01/2017 13:26:26

Thanks again Toby,

Fortunately he has yet to invoice me for the work. He has agreed to return and have a look.

I shall, within reason, attempt to astound him with my (your? ) expert teachings.

Thanks again Toby,

Fortunately he has yet to invoice me for the work. He has agreed to return and have a look.

I shall, within reason, attempt to astound him with my (your? ) expert teachings.

Ah, in which case take care how you handle him. If he has not yet been paid for the fuse box change he might be reluctant to get too involved in discussions of the current problem, perhaps thinking he will be blamed for "breaking it, because it worked before" and being told he has to "fix it" before he gets paid. Neither of which is likely to be fair as it is probably a pre-existing problem.

Best case, he is keen to sort it out but perhaps needs your understanding it might not be for free, worse case he just wants his money and shot of the problem. In which case you might get a "can't find anything wrong guv" type response.

There again, I might be over thinking it

Bear in mind with RCDs that they also trip on Neutral to Earth faults as well as Live to Earth -something a fuse or MCB doesn't do.

I had one of these faults at work late last year, which turned out to be a neutral wire having been sheared off at a connector block by the screw & the loose end had sprung to contact the side of the earthed adaptable box containing the connection. So it is necessary to check all the neutrals, as well as all the lives, for shorts to ground with an insulation tester if an RCD is tripping.

I had the CU changed at home as part of having a kitchen extension built - it was easier (and hence cheaper) for the electrician to fit one, larger, CU rather than add a small suplementary CU for the extension. He initially had some difficulty getting the new CU to stay set, which turned out to be a Neutral to Earth fault in a dining room light fitting, wahich had not shown up on the old CU. A while after the extension was completed, the kitchen circuit RCD started tripping, which I eventually traced to the washing machine. First thoughts were that the several years old washing machine immersion heater element was getting "leaky", but it turned out to be that the plumber had not properly tightened the washer waste trap, which was dripping. Unfortunately, the waste trap was positioned above the washer 13A socket, which had filled with water ! So the RCD tripping was as a result of a dodgy situation & it did the job it was intended to do. These things trip for a reason - it is best to get to the bottom of that reason & fix it properly.

Nigel B

Yes, I've had that problem with loose Neutral wiring shorting to earth. You can isolate the power by flicking the NCB to make it safe while you install wiring etc but if the Neutral connection brushes against an earthed box etc while you are doing so, the RCD can trip.

And all this has been generated because Robin needs to replace a fan motor?

.

Yes ... that's the joy of conversation

To add futher fuel to the fire

"There is no legal requirement, and no regulation in BS 7671, requiring an existing electrical installation to be upgraded to current standards. However, there is a requirement under the Building Regulations for England and Wales to leave the installation and the building no worse in terms of the level of compliance with other applicable parts of Schedule 1 to the Building Regulations than before the work was undertaken. (Schedule 1 gives the requirements with which building work must comply,)

Electrical Safety First Best Practice Guide No. 1 (Issue 3)

Replacing a consumer unit in domestic premises.

Link replaced  (quote is taken from the best practice on consumer unit replacement where lighting wiring historically does not not have an earth connection) 

Edited By Martin 100 on 06/01/2017 15:36:57

I am not sure why but that link gives me a file not found error. If others have the problem you want guide 1, issue 3 from here.... **LINK**

To avoid confusions though, that quote refers to upgrading the existing installation. If the new work done relies on the existing installation to meet the regulations (eg bonding, earthing, RCD protection) then the existing stuff does need to be upgraded to support it. Sorry, I am labouring the point but I have talked to electricians who do not understand the distinction and think it is ok to add a new socket to a circuit without adding RCD protection or worrying about loop impedance and disconnect times.

Yes the OP has a problem and as you pointed Toby out quite early in this thread it could be existing leakage on the existing installation, I agree entirely!

My point immediately above was not all the installation nor indeed the entire consumer unit requires replacement for the sake of one new circuit, or additions or modifications to an existing circuit

Like for like requires nothing more than what was there originally. Indeed by 'improving things' such as extending RCD protection to lighting circuits it removes shock risk but increases the risk of falling down the stairs in zero lighting conditions. The former could be survivable, the latter quite possibly not.

.

Yes. But the title of the OP was "How does a consumer unit work"

Once this had been explained the thread developed and expanded. - Have you never been to the pub with a crowd of like minded guys.?

Nick

I need a C type CB in my box to stop occasional trips but I can't seem to get anyone to fit one. The last two sparkles that have done work for us conveniently forgot to do it.