Earthing isolator switch

[Steve Hart]

Hi - Can the panel advise on the following please:
My REC have told me they need to replace my meter. I'm planning on using the oppportunity to add an isolator between the meter and my CU.
I'm not sure about earthing arrangements. I'm out in the sticks on an overhead supply and my earth is provided by a rod in the garden.
Do I earth the isolator back to my CU? Sorry if its a daft question but my reasoning was that if a fault occurs in the isolator switch I could have a lot of power from the meter earthing via a 4mm earth wire. I've searched the forum and can't find an answer so your help would be much appreciated.
Cheers
Steve

 

[ban-all-sheds]

The tails come out of the meter, go into the switch, come out of the switch and go into the CU.

What 4mm earth wire?

 

[Sparky Jim]

There should be a point in the Installation where the main equipotential bond from the Earth stake is terminated, it is from this point that a bond should be attached to the Isolator Earth Terminal. If this main termination point happens to be in the Consumer Unit, then the Bond for the Isolator should come from the earth bar in the Consumer Unit.

These main bonds should be a minimum of 10mm and that means from the Stake to the Main Earth point should be 10mm, Bond to Gas, water and the Isolator should all be 10mm. If there are any other Main equipotential bonds originating from this point, they too must be 10mm.

The only time that smaller bonding cables are used is for supplementary bonding, and these can be 4mm if mechanically protected ie: installed in conduit or trunking, or 6mm if they rely on their sheath for protection. These cables tend to be used for cross bonding of pipework and other extraneous metalwork in the installation that could concievably become live under fault conditions.

 

[HDRW]

Do I earth the isolator back to my CU?

What sort of isolator are you planning to use? If it's something like this: http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=70622&id=53010 in an insulated housing, there's nothing to earth!

Cheers,

Howard

 

[Sparky Jim]

Howard, that will still be in an enclosure that has an earthing terminal and so will need an Earth link.

 

[mapj1]

if you have an earth rod only, and no earth wire to company service head (where the company fuse is) then you have TT.
Therefore the only type of switch allowed before the first RCD, is a plastic one in a plastic box. Then there is nothing to earth until after the RCD.
Otherwise make that first switch be a 100mA RCD - then you have no problem.

 

[Sparky Jim]

Therefore the only type of switch allowed before the first RCD, is a plastic one in a plastic box. Then there is nothing to earth until after the RCD.
Otherwise make that first switch be a 100mA RCD - then you have no problem.

Incorrect Information.

The enclosure of the switch will still have an Earth Terminal, and this must be connected to the common earth terminal for the installation to comply with BS7671:2004

 

[Lectrician]

It would be dangerous to place an isolator before the RCD on a TT install unless the enclosure is insulated. (or those special grommets used).

Your best bet is to install an insulated enclosure.

Sparky jim - your info is incorrect

 

[mapj1]

Perhaps its unclear what I was describing.
The basic principle is that we don't want to be able to get a fault between the rod earth, and the supply live, that will not be isolated by the firing of the RCD. The reason is the TT earth will be too high impedance to clear such a fault by blowing the company fuse (probably too high to blow ANY fuse anywhere in the house come to that), and the upshot would be a live earth rod, live equipment chassis in the house, and a larger electricity bill than expected for all the ground current, but no automatic disconnection of supply would take place.

Therefore to avoid this risk the TT earth comes nowhere near the incomer until after its been though the RCD. If the RCD is in its own box like this http://www.tlc-direct.co.uk/Images/Products/size_3/BHCT4.JPG, a design that has a plastic 'dinrail' for the RCD to clip to, there is no need to even bring the TT earth into the box, just double insulated meter tails in and tails out to the CU on the other end.
Actually, even if it were a metal DIN rail, I'm fairly sure that because it requires a second fault to opose a risk to life (cable to Din rail, first fault) broken case exposing DIN rail, second fault) there would still be no need to worry.
If it were and isolator i a box feeding an RCD, it would have to be similarly constructed, and for the same reasons.
Apologies if my intention was mis-understood.

 

[Sparky Jim]

Sparky jim - your info is incorrect

Is it really, then I suggest you take this up with the IEE and the NICEIC. We just had an inspection on some Domestic new builds we have completed, the NHBC got independent inspectors in to verify a percentage of our test results for the Guarantees. The spec for the job called for a 300mA Time Delayed RCD to be installed between the Meter and the Consumer Unit, which is a split unit (63A Isolator for lighting etc , 80A 30mA RCD for protected circuits).

As the RCD enclosure is fully insulated plastic, we did not see the need to install an Earth to this unit, we took the Earth from the Main Earth terminal to the Consumer unit only (Obviously feeding the Main Equipotential Bonding too). The NICEIC Inspector said that this was not acceptable and that the enclosure needed to have an Earth connected to the Earth terminal in the Enclosure. We argued the point but he would not budge on this point, so our Contracts Manager telephoned the IEE Technical Line for Clarification, and they agreed, it must be connected to earth. This was also clarified via the ECA (To which we are also members). The question was asked if this was due to the unit being an RCD rather than an Isolator, the answer was apparently an unequivical NO, the type of device is irrelevent, a Bonding connection must be present to comply with BS7671:2004.

I'll be honest and say that I have not had the inclination to look this up in the book yet, but as I have to do some things over the weekend I will try to find where it says, or more likely implies, this.

 

[ban-all-sheds]

What function would such an earth serve?

 

[fireman22]

sparky Jim, i thought NICEIC state 100 mA time delay up front 30 mA socket side of split board for a TT supply?????

 

[andy]

What function would such an earth serve?

bugger all. its there for show

in most of those plastic cases, there isnt an earth terminal there. i used a terminal block to join the earths in 1 before (the earths were needed)

 

[ban-all-sheds]

b*****r all. its there for show

Or to start a fire if perchance the live incomer should fall out of its terminal and land on it....

 

[andy]

b*****r all. its there for show

Or to start a fire if perchance the live incomer should fall out of its terminal and land on it....

and a small firework show