PME earthing system and RCDs

[webde]

I am considering having the existing fuse box (which has the old wire type fuses) replaced by a consumer unit with an RCD. However, I note that the existing system has PME type earthing and it's therefore not clear to me the installing a RCD consumer unit will add much by way of safety, particularly as with a split load unit many of the circuits will be no better protected than at present. Your comments please.

However as I am having an existing mains fed extractor fan in the bathroom replaced by a SELV fan, I assume that I must have a RCD installed in the lighting circuit which feeds the fan. Is this really necessary as the transformer will be outside the bathroom?

Lastly, if I re-thought the whole wiring arrangements and instead fed the SELV transformer and a new low voltage light from an existing socket totally outside the bathroom would this work still come under Part P of the Building Regulations?

 

[ban-all-sheds]

I am considering having the existing fuse box (which has the old wire type fuses) replaced by a consumer unit with an RCD.

Good idea.

However, I note that the existing system has PME type earthing and it's therefore not clear to me the installing a RCD consumer unit will add much by way of safety,

Why wouldn't it?

particularly as with a split load unit many of the circuits will be no better protected than at present.

Why wouldn't they?

However as I am having an existing mains fed extractor fan in the bathroom replaced by a SELV fan, I assume that I must have a RCD installed in the lighting circuit which feeds the fan. Is this really necessary as the transformer will be outside the bathroom?

Regulation 701.411.3.3 requires RCD protection for all circuits of the location. Opinions differ on whether that means it has to be added if you do any work to bathroom circuits, but the majority opinion is that it does.

Lastly, if I re-thought the whole wiring arrangements and instead fed the SELV transformer and a new low voltage light from an existing socket totally outside the bathroom would this work still come under Part P of the Building Regulations?

Yes it would - Part P (in fact all of the Building Regulations) applies to any work whatsoever on fixed electrical cables or fixed electrical equipment located on the consumer’s side of the electricity supply meter which operate at low or extra-low voltage and are—
(a) in or attached to a dwelling;
(b) in the common parts of a building serving one or more dwellings, but excluding power supplies to lifts;
(c) in a building that receives its electricity from a source located within or shared with a dwelling; or
(d) in a garden or in or on land associated with a building where the electricity is from a source located within or shared with a dwelling.

 

[ericmark]

Regulation 701.411.3.3 requires RCD protection for all circuits of the location. Opinions differ on whether that means it has to be added if you do any work to bathroom circuits, but the majority opinion is that it does.

Can you explain how one fits a RCD that will work to a SELV piece of equipment?

First most RCD's will not work with 12 volt. And second even if one leg of a SELV system did go to earth no current will flow so no imbalance so will not trip.

OK I can also read BS7671:2008 but same applies to cables buried less than 50mm it can't really be applied to door bell wires. Seems the writers have made a few errors and some common sense must be used.

As to fitting RCD's my father-in-law said to me back in 1990 would I ever forgive myself if my children ever injured themselves with an electric shock which a RCD would have protected them from and I could see his point and fitted a pair of RCD's and since then they have protected my children especially my son who got into amateur radio and then also became an electrician.

I will agree there are times especially as the number of computers in the house has grown to a point where the RCD's were tripping a bit too regular and if I was doing it now I would used RCBO's but now they have flown the nest no more problems with computers tripping out the RCD.

As to Part P it all depends where you read and how you interpret it. However since most DIY people don't have the test gear to be able to be sure the job is OK then the only way to be safe is to get the work done for one and not to DIY.

To my mind the permitting of DIY on electrics in the home is to allow the industrial electrician to do work in his own house and is not really so Joe Public can DIY. Mainly because the test gear costs around £750 so most people can't afford to DIY safely.

 

[ban-all-sheds]

Can you explain how one fits a RCD that will work to a SELV piece of equipment?

I can't, of course, but that wasn't what I was suggesting.

However as I am having an existing mains fed extractor fan in the bathroom replaced by a SELV fan, I assume that I must have a RCD installed in the lighting circuit which feeds the fan. Is this really necessary as the transformer will be outside the bathroom?

So the lighting circuit will be worked on, cables will be moved from one place to another and/or added etc.

There is a widely held opinion that that would require the lighting circuit to become RCD protected, which is what I said.

Regulation 701.411.3.3 requires RCD protection for all circuits of the location. Opinions differ on whether that means it has to be added if you do any work to bathroom circuits, but the majority opinion is that it does.

 

[webde]

However, I note that the existing system has PME type earthing and it's therefore not clear to me the installing a RCD consumer unit will add much by way of safety,

Why wouldn't it?

I accept that a RCD in the consumer unit is potentially safer for external sockets, and for bathrooms, but, as I understand things, on a split load consumer unit many of the circuits are not protected by the RCD, so that in effect one is merely replacing a fuse by a MCB. How is that safer?

 

[ban-all-sheds]

I assumed that if you were replacing the CU that you'd add RCD protection to circuits as per the current edition of the Wiring Regulations, not install a CU with RCDs in it and then perversely not use them....

 

[webde]

I assumed that if you were replacing the CU that you'd add RCD protection to circuits as per the current edition of the Wiring Regulations, not install a CU with RCDs in it and then perversely not use them....

Many thanks for your comments. As you can perhaps gather I'm generally au fait with the principles of domestic wiring (and very good at searching the internet) which means that I want to have a meaningful discussion with any electrician I commission to do work for me. I'm not prepared to just accept what the electricians I approach for quotes recommend - I want to know why they are proposing a specific approach, particularly if different electricians come up with different solutions. This is why I find your comments so useful.

Which brings me back to your latest comment, as in many respects I am actually employing the electrician because of his knowledge of the regulations. Are you therefore saying that under the new regs almost all the circuits should be RCD protected, not just outside sockets, bathrooms, kitchens, etc. If that is the case why do manufacturers still sell split load CUs? Is it just for things like freezers?

 

[GarethW]

webde;

I think that you need to understand exactly what an RCD does.

An RCD 'senses' an imbalance between the live and neutral wires.

If 10 amps goes down the live , then 10 amps comes back through the neutral. If there is a difference of 0.03 amps (30 ma), ie you touching the live or neutral wires then and the current flowing to earth, then the RCD will trip.

It doesn't matter about the lights, sockets, cooker or shower circuit, any more than 50 - 80 ma will kill you !!

 

[flameport]

Splt load CUs are made and sold for the same reasons as the other types - they are useful for some installations. RCBOs can be used on the non-RCD side, e.g. for lighting. Or perhaps some circuits do not require RCD protection at that point, such as submain to a garage or shed.

The majority of circuits in a house will require RCD protection, as it is highly likely the cables will be concealed in the walls. Unless you have massively thick walls and can cut chases over 50mm deep, then an RCD will be required.
Other options to avoid RCDs include certain types of cables incorporating a foil covering, mineral insulated cables or steel conduit however these will all cost more than using an RCD.

While some people seem obsessed with putting their freezer on a separate circuit with no RCD, this achieves little or nothing. Freezers do not instantly defrost when power is removed, and if on a circuit with other items else and the RCD trips, this will more than likely be due to another faulty appliance such as a kettle or iron. People will notice the lack of power and therefore power will be restored long before the freezer defrosts.

If people go on holiday leaving a freezer filled with thousands of pounds worth of food, they have themselves to blame when things go wrong. In fact, why do these people have so much food in their (vast) freezer anyway - shops are plentiful, frozen food deteriorates over time and huge freezer = huge electricity bill.

 

[ban-all-sheds]

Are you therefore saying that under the new regs almost all the circuits should be RCD protected, not just outside sockets, bathrooms, kitchens, etc.

Yes.

This discusses the requirements regarding concealed cables: http://www.theiet.org/publishing/wiring-regulations/mag/2008/27-cablesinwall.cfm?type=pdf

and also now all sockets, except ones dedicated for a specific purpose, require RCD protection.

If that is the case why do manufacturers still sell split load CUs? Is it just for things like freezers?

For anything which doesn't need RCD protection and/or so that RCBOs can be used.

 

[ban-all-sheds]

In fact, why do these people have so much food in their (vast) freezer anyway

Maybe they have homegrown produce and/or things that they cook. If you like home made soup for instance, it's a real pain to just make 1 or 2 portions. Many dishes (e.g. casseroles, curry) freeze well, and take virtually no more effort to make several portions than a few. If you prefer proper stock rather than little cubes of salt, MSG, onion powder and hydrolysed vegetable protein you can store bones until you have a collection, then make a batch of stock. Which you'll want to freeze in small amounts.....

 

[securespark]

I assumed that if you were replacing the CU that you'd add RCD protection to circuits as per the current edition of the Wiring Regulations, not install a CU with RCDs in it and then perversely not use them....

The way I read it, I thought Web maybe was referring to a 16th split, where one side was unprotected by RCD??

 

[ban-all-sheds]

I read it that he was thinking of specifying a "16th" CU, not using RCBOs before the RCD, and at the same time complaining that that wouldn't be much of an improvement.

I think it's pretty reasonable to call it perverse when someone deliberately chooses to do something they don't think is good and then moans that what they've chosen isn't good.

Let's hope his electrician takes charge sooner rather than later, and that he disabuses webde of his notion that he's generally au fait with the principles of domestic wiring...

 

[webde]

Most of the information posted on the internet relates to electrical systems which conformed to the 16th or earlier editions of the IEE regulations, and given the fact that electrical theory has not changed since Maxwell's equations, and little has changed in the way of available technology between the 16th edition and the 17th edition, as an outsider - I have a science degree so do know something about the principles of electricity - I find it odd that something which was considered perfectly safe ten years ago is now considered unsafe. Indeed it would be interesting to know whether in reality any lives have been saved by the advent of the 17th edition regulations. However, this is not the forum to debate the pros and cons of today's health and safety regulations. I think I now have a fuller understanding of the legal requirements - irrespective of whether they are logical of not - and can have a proper discussion with any electrician I employ. Many thanks to all those who have contributed to this string.

 

[ericmark]

There are about 4 pages on where and how RCD protection is required and there are may ways in which one can avoid using a RCD. However in the main these will cost more than using an RCD. For example one could install Ali-tube cable to feed the fridge and use the Warsall Socket

or T type socket

and mark it Fridge only but that's a lot of work to just avoid fitting a RCD.

As an electrician in theory I don't need to fit RCD's but I would be a fool not to and it would make selling the house a problem.

Use SWA cable and 32A sockets and you don't need to have any RCD protection. Yes and I really want these

all round the house.

What you really need to consider is using a pair of RCD's v Series of RBCO's and in most cases the RCBO route is the best. OK exceptions to every rule and if my father died then I would consider fitting a auto resetting RCD

Untitled

• ericmark
• 6 Dec 2008
• 34

so my mother who being disabled so can't get under stairs to reset anything is protected without the problem of being left without light and heat if it trips but at £350 each one would not normally fit them in a house.

If you go down the pair of RCD's route do you need emergency lighting? How will you split so as to comply with (iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.