earthing


Postby ener » Thu Jun 30, 2011 3:18 pm

my electricity is by way of overhead cables.Having a new boiler fitted the electrician said he could not wire the new electrical fittings & fitments as the house is not earthed.In a cupboard with electrics in there are two earthing wires visible(do not know where they lead) but he was adamant the wall sockets would not be earthed even though he did not check any.Before he could do any new wiring we would have to get our electricity company to spike the property. Advice please on his comments.
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Postby ericmark » Thu Jun 30, 2011 8:41 pm

Earthing is not up to the supply company although they with under ground supplies do often give you an earth with what is referred to as a TN supply.

If your supply is TT then it is up to you to provide an earth. In the main this means sticking an earth electrode (steel rod covered in thick copper) into the ground.

The rod can be tested with two types of meter.

The standalone meter is quite complex and requires test earth spikes to be put in the ground at 15 and 30 meters from the earth rod and often there is not the room or access to ground to use these meters.

The other is to use the suppliers earth as a reference and since the supplier is allowed 21 ohms then this method is up to 21 ohms out in respect to true reading.

However with this type of earthing we always have an earth leakage trip called an RCD. As a result of using these RCD's we are allowed up to 200 ohms (A value exceeding 200 ohms may not be stable). Often we get a reading of around 60 ohms with a single rod so this is not a problem.

The electrician therefore needs two meters.
1) Earth loop impedance meter to test rod with.
2) An RCD tester to check the RCD trips not only with the rated current but also within rated time.

Now what becomes confusing is the difference between an "earth electrode" and an "extraneous-conductive-part" and in fact the same bit of metal in the ground can change names when the supply is changed from TT to TN.

Your water pipes and gas pipes are always extraneous-conductive-parts and have to be disconnected when testing the earth together with all other metal parts. But other than for testing are earthed to the earth electrode.

In times gone by the gas and water were often used as the earth but as the mains in the street were converted into plastic this practice was banned. This does not mean they are not connected to the house earth in fact they have to be connected to house earth but there must be another bit of metal in the ground which has the label "earth electrode".

In real terms of course the pipes do act as an earth electrode but they are not given that name they are called extraneous-conductive-parts.

So likely the wires you can see go to the water and gas pipes. Plus any other metal in the ground like cast soil pipes.

The electrician likely had either a proper earth loop impedance tester or a plug in tester which will also measure earth loop impedance to a limited extent. When the readings failed he aborted the work as I would guess fitting an earth rod was not in his remit.

However to tell you that you need to get your electricity company to spike the property makes me think he is a little lacking in his education as with a TT system it is nothing to do with the supply authority. However I would ring them and ask what type of earth your property should have. It is up to them to tell you. If they say TT then you need to get a proper electrician to fit a rod and if you don't already have one a RCD.
They (the electric supply company [Not billing agent]) may not know what supply you have. In which case they may send out some one to inspect. I have often told lies and said next door has a TN supply and if the readings are within limits they may provide you with a TN-C-S supply in which case you don't need an earth. Cups of tea etc may help get guy to give you a TN supply! You may still need to get an electrician to connect this to your consumer unit.

I would have normally at least given the customer the phone number of local supply company, and normally a script of what to say, plus in case they would give a TN-C-S supply, an earth wire ready for the supply company to connect to their earth.

Likely the supply company with an over head supply will not provide you with an earth. So you will need an earth rod and a RCD.

The earth rod can get a dangerous voltage to it when there is a fault although only for a short time as the RCD should disconnect the supply. So normally the rod is put under a plastic inspection earth pit see ([url]http://www.awin1.com/awclick.php?mid=1228&id=61071&p=http://www.screwfix.com/p/plastic-inspection-earth-pit/16092[/url]) hope the admin allows the link as it will show you what to look for.
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