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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Equipotential Bonding https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1317584037 Message started by GarethH on Oct 2^nd, 2011, 8:33pm

Title: Equipotential Bonding
Post by GarethH on Oct 2^nd, 2011, 8:33pm

Hi All,

I need to get a spark in to replace my old wylex fuse box with a new 17th edition CU. I understand this would require main equipotential bonding to the gas and water services (which are both in copper).

My 1970s house doesn't have main equipotential bonding but installing it is non-notifiable so I want to do it myself to save some cash. Sod's law has dictated that the gas and water meters are nowhere near the electricity meter and CU so running the 10mm2 earth cable will take time.

I tried to find the MET so I looked in the meter cupboard. There's an earth terminal in the box that houses the 100A fuse. Connected to that is a piece of thin (probably 2.5 mm2) earth cable going to the consumer unit. There's no additional earth wiring in there. 3 cables (two meter tails and the earth go through the wall to the fuse box).

The earth bar in the fuse box just connects that earth wire and all of the earths from the other circuits together; there's no earth going off to an earth rod.

My questions are:

1. When the new CU is installed, will it contain the MET (i.e. do I run the 2x 10mm2 earth bonding cables to the location of the new CU

2. Is the current 2.5mm earth cable typical of a 1970s house (I presume this needs upgrading when the CU is replaced)?

3. Is the system definitely TN-S or TN-C-S (not TT) - there isn't an earth rod / MET that I might have missed.

4. Is it acceptable to run the 10mm2 earth bonding cable on the outside of the property?

Cheers

Gareth

Title: Re: Equipotential Bonding
Post by Lectrician on Oct 2^nd, 2011, 8:49pm

The 10mm² bonding conductors can be terminated into the consumer unit - the earth bar of a consumer unit can be used as your MET.

You would need to replace the 2.5mm earth to the service head.

Certainly sounds like a TN-C-S or TN-S - a photo would confirm.

The bonding cable may be run externally, although not cable tied to a gas pipe etc. Extra mechanical protection is at the installers discretion (likely hood of damage etc)

If the gas and water are close by and it is easier, you can use a single run of 10mm². You would pass it through one clamp and onto the next WITHOUT cutting the cable.

Title: Re: Equipotential Bonding
Post by GarethH on Oct 3^rd, 2011, 6:41pm

Thanks for your reply. Here's a photo of the service head and meter.

[gallery]GarethH/1317662967.jpg[/gallery]

I've just had a thought - my water main is a plastic pipe which is converted to metal just before the meter. As the metal does not extend outside the equipotental zone of the house, is equipotential bonding required for that service?

What I've read suggests that only extraneous conductive parts of the location require bonding. I interpret this as parts of the location which are both extraneous and conductive. The water main is extraneous but isn't conductive.

Title: Re: Equipotential Bonding
Post by Lectrician on Oct 5^th, 2011, 5:27pm

You have either a TN-C-S or TN-S system. Not a TT. Difficult to tell for sure if it is TN-C-S or TN-S.

If the supply pipe is plastic, the NICEIC still strongly recommend that main protective equipotential bonding is installed if the pipework within the installation is copper.

Title: Re: Equipotential Bonding
Post by GarethH on Oct 8^th, 2011, 5:34pm

I'll bond it then!

Thanks for the help

Title: Re: Equipotential Bonding
Post by L.Spark on Oct 12^th, 2011, 8:24pm

Lectrician wrote on Oct 5^th, 2011, 5:27pm:

If the supply pipe is plastic, the NICEIC still strongly recommend that main protective equipotential bonding is installed if the pipework within the installation is copper.

I'd forgotten about how the NIC like to make up their own rules. We all know earthing can cause dangers as well as prevent them!