Ring main earth voltage drop


Postby t10ake » Mon Feb 11, 2008 12:17 am

I'm having a problem and could do with some guidance if possible. I have been having issues with my boiler throwing up error codes after my bathroom was replaced over the last two weeks. I have had the engineers out twice now to replace the PCB in the boiler (in line with the error code).

On the second visit the engineer fitted the new pcb and immediately got the same error code. He then tested the electrical supply to the boiler and found that Live to Earth voltage was only 189V (L to N was 210).

The central heating is on its own circuit from the consumer unit.

Upon further testing it appears that this drop only occurs when the ground floor ring main is powered on at the consumer unit. I have unplugged all items from the ring and tested all the sockets for continuity, but this does not seem to help.

Can anyone provide any ideas as to what might be causing this? My current thoughts are that there was an existing problem with the earth that has been highlighted by the new crossbonding of pipework done when the bathroom was installed. But I have no idea why the ground floor ring main would be the affected circuit as this goes nowhere near the bathroom. I also don't quite understand why activating that circuit causes the live to earth voltage drop in all circuits?

Any thoughts and guidance would be much appreciated, until I get to the bottom of this, my boiler and downstairs sockets are mutually exclusive...

Thanks in advance

Tim
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Postby ericmark » Mon Feb 11, 2008 1:49 pm

If you are on a TT supply your neutral is not bonded to the neutral and as more current is drawn the voltage between neutral and earth will vary. The supply voltage is 230 volts - 6% at the supply so 216.2 volts min plus volt drop on internal wiring 5% so 205.39 volts min I may be wrong and minus 10% which makes it 196.65 volts either way your well within it. In general terms any earth fault must not allow more that 50 volts to appear before something trips so again you are well within the limits.
Having said that it does seem you have an earth leakage problem somewhere and I wonder what are the results if the system is tested? You speak of new work on bathroom etc. I wonder if you had any electrical work other than cross bonding done at that time and if so what were the results when it was tested on completion? The one called ELI is of interest and the type of earthing either TT or TNC-S what protection is fitted to your sockets i.e. 32 amp MCB and 30ma RCD would be normal maybe if TT also a 100ma RCD do these trip OK when the button is pressed? I would have expected the voltage live to earth to be higher not lower than the voltage live to neutral and I worry in case you have some earth leakage. But the reading given should not affect your boiler.
ericmark

Postby t10ake » Mon Feb 11, 2008 9:55 pm

So have spent the afternoon & evening with an electrician testing everything... Cut a long story short, it appears that the earth bonding to the mains water supply pipe got damaged with all the turning on and off of the stop cock etc. we ran a new length of earth back to the consumer unit from this point and the problem is solved. we are not entirely clear as to why this fixes the problem but it did. I guess ultimately the bonding will only work if the system has a proper route back to earth and in this case it didn't apparently.

Thanks for your helpful ideas Ericmark.

Tim
t10ake
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