Type of earthing to sub-board

[OldSpark44]

I am a retired electrician and I have build a new extension to my house (brick building) and as this will contain several electrical items (washing machine/dishwasher/tumble dryer etc) I have run a new 3 core 10mm SWA cable from the mains position at the front of the house to this new room at trhe rear of the building. In this extension room (that is attatched to the main house) I am installing a CB Board with it's own circuit breakers for ringmain/lights & boiler etc. I am proposing to use the third core of this SWA as the earth between the mains incomming supply and the new extension CB Board. My questions are:-

1. Should I fit an RCD to the existing mains board to supply this sub-board?
2. As the copper pipework runs through the house to all radiators and water system and is currently bonded should I cross bond the 2nd incomming water supply in the new extension to the sub-board ? (The new watermain in the new extension is a PVC main).
3. I assume that if I do install a RCD in the mains board the SWA sheath should be bonded at the mains board end only - and should not be bonded to the earth ternimal in the sub-board, is that correct.
4. Would it be better to fit the RCD into the sub-board rather than the mains board?

Since I retired, I am aware that the regs have changed concerning earthing - hence my questions.

 

[Steve]

JohnD will be along in a minute, practically advertising RCBOs. Until then . . .

1. It is generally taken that RCD protection should be as local as possible. So it would be best to have the RCD in the submain CU rather than the origin.

3. No, the sheath can be connected both ends - this might help your EFLI at the submain. I think you're confusing this with the scenario of having a PME source and TT submain, at a shed, for example. Then, you wouldn't connect the sheath at the shed, as the shed has a seperate earthing system. Here, you have the same earthing system - so you ought to do everything you can to make sure earth continuity is maintained. This is especially true if you have a TT earth.

4. isn't this just 1. rephrased? If you fit one at the origin, dont fit one in the submain.

 

[JohnD]

RCBOs are great, I think you should have some....

(pulling your leg)

Its true...

If this is a 1-room extension, I imagine you have a lights and a socket circuit. A split-load board may not be worthwhile, you could use a 4-way and have room to spare. If you use an RCBO for your sockets, that should be all you need. As you are having a sub-main, it will probably be convenient to you to be able to reset in the room where you have just fixed the fault.

Although RCBOs are quite expensive at list, and not often discounted much, you can often pick up new or near-new ones in the £10-£15 region on Fleabay. I happen to like MEM, one advantage is that the Industrial range will fit in the Domestic enclosures, and there always are quite a few over-ordered industrial parts around at clearance prices (especially if you have any chums in the trade). There is a way of prising the RCD part of a MEM RCBO off the MCB part, and clipping it onto a different MCB (they are made as clip-on parts, so this is OK)

 

[OldSpark44]

Hi Crafty1289, Thanks you are correct regarding the shed type building. I have an SWA cable to the shed and in this case I earthed the SWA armour at the house end only. The armouring in the shed is NOT connected to the earthing within the shed at all. I have no connection from the earth terminal at the mains board to the shed at all. I have fitted an RCD in the shed with it's own earth stake.

Regarding 4 I was questioning about the location of the RCD, from what you have confirmed I will bond all cables and SWA sheath and fit thye RCD in the sub-board. As you said this will be better being in the location. As a poiny of interest would you perhaps NOT fit an RCD at all in the situation ?

Thanks once again

 

[JohnD]

p.s. an RCD or RCBO is well worthwhile as you have watery appliances in the extension, and presumably might reasonably be expected to plug in a lawnmower or other appliance to use outdoors.

 

[OldSpark44]

Thanks JohnD & Crafty1289, John's reply must have crossed with mine. I will fit the RCD (already have) in the extension room, as you say it will be easier to reset in any fault conditions. I think you have all answered all my questions - Many thanks for such a quick responce. Regards

 

[Spark123]

So you are making it into TT and not using the 3rd Core nor the armours earth? The 3rd core and armour should be earthed at the supple end. As you intend to go for TT you need to make sure that the earth of the supply i.e. the armour is not simultaneously accessable with any of the TT supply earth. If you have a plastic supply which converts to copper inside the building you should main equipotential bond the copper pipe as close to entry as possible. (don't ask me why, the iee seem pretty persistant in the OSG that the metal pipework from a plastic supply needs bonded).
There is some further info here on earthing outbuildings:
autumn_wiring_matters_electrical_installations_outdoors.pdf

 

[OldSpark44]

Hi Spark123, I think may have confused matters a little since my first posting. I have two types of buildings - Extension and Shed

EXTENSION: I have an extension ON the existing building with a 3 core SWA cable, the third core (earth) will be connected with the armouring at both ends one to the main CB Board and the other end in the extension to the sub-board.

SHED: This also has a 3 core SWA cable from the main CB Board, The mains CB end I have connected the 3rd core and armouring to the main CB board - the other end the cable is terminated into a PVC adaptable box. neither the third core OR the armouring cable is connected to the sub-board in the shed. I have however fitted an RCD and earth stake, this is the only earth the shed has. I worked on the principal that IF anyone damaged this cable in the ground (18in deep) It would trip one of the CB in the main board.

I hope thats correct.

 

[Spark123]

I'm starting to get the picture, as the extension is part of the same equipotential zone as the building, the rising water main etc will need main eq. bonded back to the MET.
The shed sounds OK, I take it the gland is not accessable? There is some info in the iee pdf file I posted above re outbuildings.

 

[OldSpark44]

Hi Spark123, The extension is in the same zone as the house and the extension has its own water mains (two incomers).

As the hot water system and heating is all copper I have bonded the pipes throughout with one 25mm earth bond to the earth terminal at the meter position. The gas meter is also bonded in the same way to the earth block at the meter position. I have also run a earth wire from the sub-boad to the nearest copper pipes (heating and cold mains). Is that acceptable for part "P" of the regs.

 

[OldSpark44]

I omitted to say that the SWA cable to the shed is in a PVC box and the gland has a PVC sheath over the gland - no connection between the gland and the shed electrical installation. The thirs core in the SWA cable is terminated in a connector and also NOT collected to anything within the shed.

 

[Spark123]

Hi Spark123, The extension is in the same zone as the house and the extension has its own water mains (two incomers).

As the hot water system and heating is all copper I have bonded the pipes throughout with one 25mm earth bond to the earth terminal at the meter position. The gas meter is also bonded in the same way to the earth block at the meter position. I have also run a earth wire from the sub-boad to the nearest copper pipes (heating and cold mains). Is that acceptable for part "P" of the regs.

As it has a separate service the service where it converts to copper is required to be bonded directly through an uncut cable to the Main Earth Terminal of the premises. The gas meter sounds like it is done correctly, are the hot water/ch pipes extraneous? 25mm sounds nice and big btw, depending on supply type and length of run 10-16mm is the norm for main eq. bonding.
Part P is a section of the building regs which is statutory, see Ban-all-Sheds part p summary 04/06 and Guidance to approved document P. One recognised method of compliance is to comply with BS7671 the iee regs. LABC is required to be notified of certain work.

 

[OldSpark44]

I was told by a electrician who works for the local electrical supply company that as all the copper pipe work to the gas central heating/radiators and sink units/baths etc was bonded together within the house including the extension and it was all within the same building one earth wire back to the meter position was all that was needed.

It would seem a bit over the top to run a second earth wire to the 2nd incoming watermain its only running to a dishwasher/washing machine and a sink unit. The total length of pipe being around 3m in length. The hot pipes are already earthed throughout the house being copper and connected direct ro the earth terminal at the meter position.

I would have a major job at this stage to run another 10/16mm cable all the way from the front to rear of the house. So many carpets to lift.

The mains incoming supply is a lead cable and fused by the supply company at 60amp so as you say 10mm earth bond cable would be sufficient. But I like to be safe rather sorry hence the 25mm.

 

[Spark123]

I think you are confusing with supplementary equipotential bonding which applies to bathrooms and the likes, see here:Earthing plastic pipes.
I don't entirely agree with what the on site guide says either, imho it is daft to have to main bond such a small length of copper pipe. In contrast to what the OSG says, GN5 says to determine wether or not an item is extraneous you can check its resistance to the MET. If the result is more than 23000ohms then it is not extraneous hence doesn't require bonding.
Another option open to you would be to replace the copper pipe with plastic which does not need main eq. bonding.

 

[JohnD]

Why is it main bonding? This is just another room in the house. There is a tap in the room, but the water main comes into the existing house and is main bonded. The extension is not a separate building, nor is it a separate dwelling.

The extension does not appear to be a bathroom so it doesn't need supplementary bonding either.