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national grid safety check

Postby craigm » Wed Sep 10, 2008 8:06 pm

National grid came out to exchange my gas meter for a scheduled new model but instead of replacing the meter, handed in a card in which stated:
that after a safety check was undertaken, it indicated that there may be a fault in the electrical system causing a voltage on the gas installation and therefore the engineer is unable to continue to work.
On enquiring about the problem, the engineer said there was a voltage present on the meter probably due to a faulty earth bond and could not continue, but the gas meter is safe enough that the gas does not need to be disconnected.
National grid recommend that the installation needs to be checked by a comptent electrical contractor.
Quickly visually inspecting the meter myself, the output gas pipe has an earth bond with a 6 omm cable connecting it to the incoming mains electrical supply to the house. On the same connection on the incoming mains electrical supply a cable goes directly to the earth bar in the consumer unit.

Can anyone explain my fault? or suggest a solution. On phoning a few electricans I recieved "not sure if I can do that" and never recieved a call back.

Any suggestions would be greatly appreciated


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Simply Build It

Postby ericmark » Thu Sep 11, 2008 12:14 am

First one needs to know what earth system you have. Basic two types an earth rod called TT or supplied by electric company called TN.
For a TT system you will need a earth leakage trip you may also have one in a TN system although same device not for same reason.
The earth bar either in consumer unit or near consumer unit should connect to boards earth or earth rod and to gas and water pipes.
To test an earth rod there are two ways one needs access to ground for 30 feet from earth bar and the other uses dangerous voltages and is really only to be used by an electrician.
The second meter will also check a general earth loop impedance which in layman's terms checks how good the earth is.
It is common for earth wires to be missing as years ago very thin uninsulated cables were used and often the whole house got it's earth from the gas or water rather than earthing the gas and water.
But also the fault may not be at your house but could be down to the supplier where a TN-C-S system is used the supplier should sink multi earth rods and connect them to his neutral and sometimes he does not use enough.
Step one is phone supplier and ask his what earth system you have they are duty bound to tell you. If it is not on their records they will send someone out to check who may correct the problem or may advise you as to what it is.
but it is not a DIY job. You may also have a periodic inspection report PIR should be done every ten years or on change of occupant this will tell you what type of earth you have. But sorry to say very few owners have these inspections done only rented properties seem to get checked.
With a little look Sparx will answer this also he's more informed than me.
PS just looked in Part P document and on page 36 it explains it quite well. If you go to projects and follow Part P links you can download PDF free I think that may help you. Also read post on "Cross Bonding".

Postby TOPSPARK » Fri Sep 12, 2008 6:49 pm

First off a 6.0mm earth cable does not comply with current regulations it has to be a 10.0mm earth cable to both gas and water supplies for main earth bonding.If he detected a voltage best thing to do is call in an electrician and have a full periodic inspection and test done on all electrical circuits and then if all clear then you will have a certificate to show your gads supply company
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Postby ericmark » Sun Sep 14, 2008 2:13 am

Until you state supply type I could not comment on earth cable sizes.
Earth bonding conductors need to be at least 6mm² except with PME see regulation 544.1.1
With PME then Table 54.8 if neutral under 35mm² then 10mm²
So with TT then min = 6mm² and with TN-C-S min = 10mm² I do think it is confusing how the name changed from TN-C-S to PME which mean the same.
For other earth wires then without mechanical protection supplementary bonding conductor 4mm² see reg 544.2.1
I know the regulation numbers mean nothing to craigm but they will to TOPSPARK who can then check as it is so easy to miss read the regulations.
So 6mm² may comply with the regulations but only if you use a earth rod. If the supplier provides the earth then as TOPSPARK says it needs to be 10mm²
We look forward to any further information.

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