electrics


Postby dicko24 » Thu Dec 27, 2007 3:32 pm

how do you earth an electric box i live in bulgaria i had an english electrician to renew the wiring but when the bulgarian electric company came out they ripped the earthing wires out i have the british standard electric box with one main switch going to the rest of the box with the trip switches in. there is three wires from outside to top of main switch then two wires going from bottom to main box now the bulgarian company say it is not earthed but they ripped it out and said dont need so how do i earth the whole system
cheers rob
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Postby 333rocky333 » Fri Dec 28, 2007 5:32 pm

Do you mean totally ripped out or just disconnected.
What was the reason they did so

In this country you would normally see green and yellow cables.

maybe16mm linking all the boxes together, sometimes connected to an earth bar inside ,or lugged and bolted to the casing, usually connected to some form of earth bar near the supply fuse.
Also other earth wires slightly smaller connected as well bonding other things like water and gas etc
Is the third wire from outside an earth wire maybe this needs linking to your trip box.
maybe the board not responsible past the main supply earth presumably inside the main switch.

Not sure about bulgaria though, may be wise to get in local elect with equipment to test the earth.It may just need new link from mainswitch to tripbox with suitable cable
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Postby ericmark » Fri Dec 28, 2007 5:34 pm

Hard to reply to these questions when we don't know the system used. But I will have a guess and you can then see if I am right.
We have a few different ways of supplying electricity. TN-C (not used on British houses), TN-S, TNC-S (Standard system in UK when supplied under ground), TT (Standard system with overhead cables), and IT. Only with the TN-S and TNC-S (also known as PME) does the supply authority supply the earth and with TNC-S (PME) the same wire as is used for a neutral is also used as an earth as it comes to the house. For this to be safe there are multiple earths along the cable run. Otherwise a earth rod is used. With earth rods they are seldom good enough to ensure if there is a fault that the system will automatically disconnect in time using fuses or MCB's so we fit two earth leakage trips. One at 100ma and the other at 30ma.

I guess the supply authority has given you a TT supply but your electrician though it was a TNC-S supply? Even in the UK we have problems here as the rules say that houses in an estate must all have the same type of supply so where it was overhead then is latter put underground often the houses still have a TT supply and have to provide their own earth rods. And nothing in UK law says the supply authority has to supply the earth it says the supply authority has to inform the user as to what type of supply it is providing and what the ELI is where it is a TNC-S or a TNS supply.
In UK it is further complicated since the Billing Agent may not be the same as supply authority.

Putting earth rods in seems to be way forward. There are two ways to test earth rods. One uses a very special meter that has two spikes that you put in the ground and can be used without there being a supply the other needs the supply to be there first and is called an earth loop impedance meter but can only be used when the supply authority earth their step down transformers. I expect you can use an earth loop impedance meter but can't be sure. Where supplies are from local generators the other system has to be used. In sandy soil where I was fitting earth rods to get 8 ohm I needed 4 to 8 x 1.2 meter earth rods all screwed together to get reading. With a 100ma Earth leakage trip you don't need anything like that low of a reading but you do need to check best when your in a dry spell as will always pass if wet. My boss always sniffed the rod to make sure I had not pissed on it to get reading down.
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