Unexplained Earth Fault

Postby hiwj » Sun Dec 14, 2008 7:59 pm

My 13A sockets are on a radial circuit with a 32A MCB and my cooker is on a seperate MCB (both protected with an RCD).

Last week I got a nasty shock and discovered that there was 115v between the earth on my toaster (on the radial circuit) and my cooker. There was also 115v between the neutral and earth in the socket.

I tested my sockets on the radial circuit with one of those plug-in socket testers starting from the socket closest to the CU and working out. Most of the sockets passed OK, but then there was a fault with every socket from a certain point onwards (showed up as LN REVERSE).

I have now replaced this faulty cable and all my sockets pass OK. There is now no voltage between neutral and earth.

[b]Why didn't my MCB/RCD detect the fault and stop me getting a shock?
Would I be better converting my radial circuit into a ring circuit?[/b]
Posts: 3
Joined: Sun Dec 14, 2008 7:48 pm


Simply Build It

Postby ericmark » Mon Dec 15, 2008 1:17 am

1) A RCD does not prevent you getting a shock it only reduces the time you can get a shock for. i.e. 40ms
2) A MCB will need massive current to flow to trip much more than can travel through the human body.
3) Are you aware with radial circuits on a 32A MCB we use 4mm² not 2.5mm²
4) I am not sure what would come up with reversed line and neutral with one of the testers you used at a 115 volt it looks more like a bad connection rather than reversal and considering the ohms to volt on most modern meters the current available could be well under the 15 to 30ma required to trip a RCD.
5) In the main radial circuits are safer than rings as if any cable becomes disconnected with a radial it is easy to detect but with a ring the ring status can be lost and as a result overloading of the cables and as a result risk of fire.

To test a RCD needs an expensive meter and 6 tests. 3 on positive half cycle and 3 on negative half cycle. Test 1 is it will not trip at 15ma Test 2 it will trip at 30ma Test 3 it will trip within 40ms at 150ma you can hire testers but there are other tests which an electrician would also make for example the earth loop impedance but this test unlike the RCD test can cause danger and as a result I would not recommend hire.
A periodic inspection report should be done on change of ownership or every 10 years which ever is shorter and I would consider if this would not be best option?

All faults found are given a code. Code 1 faults need doing quickly where as Code 2 you still need to correct but not as urgent. Code 3 means unable to test this is normal for incoming fuse size as normally sealed but asked for on inspection and can be ignored in most cases. Code 4 means it would have passed with an old regulation but will not pass on the new regulations no action is needed on this but it warns you that if you were to modify or add then you may also need to upgrade to new standards at the same time. Hardly any house will get clean bill of health but at least you know where any faults are.
The button on a RCD is only to test the mechanics of the RCD as when not used for some time they can stick.

Postby hiwj » Mon Dec 15, 2008 10:00 am

Thanks for the explanation.

The only reason I thought a ring circuit would be better was so there was an alternative path to earth should a section of cable be faulty.

I find it hard to believe that a CU will not treat the fact that there is 115v between earth and neutral as a fault (and switch off the current).

Anyway, I am going to get an inspection report done, as this hasn't been done since I moved into the property.
Posts: 3
Joined: Sun Dec 14, 2008 7:48 pm

Postby ericmark » Mon Dec 15, 2008 1:03 pm

I remember complaining about a TV which had 90 odd volts at aerial socket and being told it was OK as under the ma allowed!
My comment was it's not the shock that kills, it's not even the falling off the ladder. It's only when you hit the ground the problem arises!
But someone had decided that it was within allowed tolerances.
Although we can have a good guess we can also get it wrong. Out of interest how did you find this site compared with DIYnot? I noted you posted on both.

Postby hiwj » Tue Dec 16, 2008 9:01 am


I did post on both sites because I wanted a reply quite urgently to be honest (and I thought why not try two different sites).
Posts: 3
Joined: Sun Dec 14, 2008 7:48 pm

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