Electrical Earthing Advice


Postby jackychan » Tue Apr 29, 2008 9:42 pm

Hi Friends

I had an extension with a shower/toilet built. Ocassionally, I noticed there is some tingling feeling when I touched the tap or in the water but it's not always there. This convinces me that the earthing is somehow not correct. I have a similar problem with the upstair bath taps too. Both rooms have an electric shower installed. As said, the problem is not there all the time.

Firstly how can I check if there is an electrical earthing problem. I strapped the hot/cold water pipe to provide equi-potential seem to help for awhile but it came back ocassionally again.
Secondly, is the earthing in the shower unit not adequate, hence the problem. How can I check that?
Thirdly, what can I do to ensure the earthing is adequate?

Your advice is always be appreciated.

Jacky
jackychan
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Postby ericmark » Wed Apr 30, 2008 12:23 am

Unfortunately the equipment required to test is expensive. As a result not really a DIY job. I can detail how to check it but insulation testers and earth loop impedance testers at approx £500 for cheap set mean in most cases you will need to call an electrician.
ericmark


Postby ericmark » Wed Apr 30, 2008 6:34 pm

For you to get this problem you need at least two faults. The earthing system as you say should make everything equipotential this does not only include the pipes. To earth damp floor boards and include them in the equipotential zone is near impossible so we need to ensure the pipes also stay the same potential as the one thing we can rely on as being the same potential as the house and that’s the ground below it. Two methods are used. One uses a metal rod in the ground at your house and the other uses many items in the ground one the supply to your house. Either way it becomes the main earth into your house and should be bonded also to all incoming pipes. If this fails then the metal pipes may become a different voltage to the ground below.
Our standard test is the earth loop impedance and this measures on a fault how high the voltage on bonded metalwork can rise before it will open a protective device. It is measured in ohms but we look at the relationship between volts ohms and amps to ensure it can never exceed 50 volts for 0.4 seconds. Because of the time involved this means for an extended period with a B type breaker we are looking at 1/46th of this value less than two volts. A D type would give 1/11.5th and even hear less than 10 volt. RCD’s would allow higher volts but unlikely not to keep tripping anyway. The meter does connect live to earth through a limiting resistor for a very short time in the order of 0.01 seconds the electronics then store this information and calculate the external resistance that the supply must have to allow this to flow and displays this as ohms on the screen and holds the display so the user can read it. It is all this electronics that results in the meter costing around the £300 mark. But even if there is something wrong with the system of earthing for the bonded earthed metalwork to have a different voltage to the ground there must be a second fault. Something connected to the metal work must be allowing line voltage to be connecting to earth and here is also a problem two main methods are used. The high voltage installation tester which uses 500 volt to test for leakage at a voltage exceeding the normal supply voltage and is called in the trade a Mega in the same way as vacuum cleaners are called Hoovers. And the other is an all in one machine known as a PAT tester which also uses 500 volt and both are expensive. But unlike the previous test sometimes you can find the faults with a multi-meter set to highest ohm range but the multi-meter can’t show no fault exists it only shows up some faults and some need the extra voltage and measurement while running that only the PAT tester can do. Even then items like washing machines can take hours to complete there cycle and the PAT tester is normally only used for a few minuets so can still miss faults. Here we come to a bit that is affordable the RCD. Where we suspect something may have an intermittent fault using an extension lead with earth leakage built in can locate which of the items in the house has this fault. Mine uses a 10ma RCD so it trips first before general house one. Fridges and Freezers especially with auto de-frost, Washing machines, Dishwashers and central heating systems all can have hidden faults which only appear at one point of their cycle. The combination of expensive meters and a lot of experience to locate to double fault required to give you these small electric shocks mean it really does need an electrician on site sooner rather than latter. Some faults can be very hard to find like when some plastic piping is used. Enough to mean the main earth bonding is no longer connected to the main mass of pipework but not enough to isolate items like immersion heaters showers etc from taps. In theory the earths connected to immersion heaters showers etc with there supply cable should be enough in practice faults like the one you have are normally a combination of a number of faults so it can still happen. It only takes the builder to brake the cable going to a earth rod when knocking down a wall to start a fault like this. That’s why on completion of any new electrical installation inspection and testing is required and the results documented and since 2004 also copies sent to building control under Part P regulations. Before Part P many builders thought they could make a little extra money doing their own electrics and often the testing and inspecting was never completed. They thought is was a lot of money to get a few wire connected and never realised what testing was done. Even Electricians would have one meter between 10 and when the other guy had it would fudge up some results again the prospect of being caught out with more and more spot inspections by governing bodies has in the main stopped that practice. May be yours was the one that slipped the net. Following links to Part P in projects on page 23 I think is shows you the forms that should be filled in I would expect you never got a copy? It wouldn’t be first time and I expect not the last that builders or electricians fail to hand them over. Claiming loss is safer for them than getting caught out with wrong readings.
ericmark


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