Water mains and earthing

[Ian Cundell]

Sorry if this is mildly OT, but I'm not sure where else to ask.

Three Valleys Waters has just very kindly relaid the mains outside my house with spiffing new plastic ones. Among the notices they dished out were warnings that if my electrical system was earthed to the old metal mains, this would no longer work.

Neither Powergen not...er, whatever 24-7 is called now...seem terribly interested or knowledgeable. Any idea how to establish whether it is worth calling an electrician in?

• The house (a 1950s ex-council house) was last rewired c1989 by the council.

• Obviously the Water Co only replaced up to my street side stop c*ck. From there to my house is still the old iron mains feed.

• My shower has an earth wire connected to the cold water tap of the sink basin. I ran it slightly absent-mindedly and didn't get electrocuted.

• In my meter cupboard there are a couple of whacking great earth wires near the meter, running from the circuit breaker box. One is attached to a metal poll apparently drilled into the wall, another to a copper pipe coming from (or to) the gas meter (!).

Am I just being paranoid?

Off to boil some water....

[Andy Wade]

They do that to cover themselves.

...EDF Networks.

If it was competently re-wired at that time you should have no worries, and nothing should need changing.

That's bathroom supplementary bonding, only part of the overall earthing and bonding scheme.

See the FAQ at

you're still unsure, post a photograph of your electrical intake, meter and consumer unit location on the Web somewhere and further comment will doubtless be offered...

[Stefek Zaba]

Andy - aren't the b*ggers - however sliced-n-diced-n-microprivatised - still under a legal obligation to tell you what your earthing arrangements are, and what the maximum supplier's earth loop impedance is (even if the latter will almost always be the Conventional answer of '0.35/0.8/yourproblem' respectively for TN-C-S, TN-S, and TT ;-) ?

[Andy Wade]

Yes, there's a list of data they are obliged to provide in the BS 7671 Amendment 2 which I've just mentioned in another thread. Here's a quick cut'n'paste. I've added the usual answers (for LV supplies) and some comments in square brackets:

Regulation 28 of the Electricity Safety, Quality and Continuity Regulations 2002 requires distributors to provide the following information to relevant persons free of charge:

- The maximum prospective short-circuit current at the supply terminals [1-ph: 16 kA; 3-ph 25 kA; can be higher in the former LEB area]

- The maximum earth loop impedance of the earth fault path outside the installation (Ze) [TN-S: 0.8 ohm; TN-C-S: 0.35 ohm, TT: 21 ohm]

- The type and rating of the distributor?s protective device or devices nearest to the supply terminals [usually BS 1361 Type 2, 100A, or whatever records show for older stuff]

- The type of earthing system applicable to the connection [TN-S, TN-C-S or TT, but their records may be inaccurate - consumers can to some extent change the means of earthing without reference to the distributor]

- The number of phases of the supply [this tends to be either 1 or 3 ;-)]

- The frequency of the supply and the extent of the permitted variations [50 Hz ±1%]

- The voltage of the supply and the extent of the permitted variations. [230/400 V -6%/+10%; -10% after 2008, perhaps]

[Ian Cundell]

Thanks for the feedback chaps.

On a related question, presumably as a consequence of detritus getting in the mains feed (?), my electric shower keeps overheating and cutting out. Is there an easy way to clean one of these things out?