Robert wrote:

Electric trip problem.

Hi, any ideas on the following.

I have a mains wiring problem on one circuit in the house. Note fuses
and separate trip not MCB's, house and wiring late 1980's.

I have a 30ma earth leakage trip that is tripping every time a
significant load (about 3KW) is applied to the "downstairs" ring main.
The immersion heater will always trip it, the dishwasher in heat mode
will usually trip it and I have seen daft things like the fridge trip
it. Note that if the dishwasher is plugged into a different circuit
but with all other appliances remaining 'as-is' the device won't trip.
This doesn't make any sense to me as the trip is seeing the same load
(and fault) regardless of which circuit is carrying the load.

Rusty nail through a cable bridging neutral & earth, or bare & tarnished
earth wire contacting neutral in the back or a socket, or some other
high-ish resistance neutral-earth connection on the affected circuit? Or
maybe the other circuit you plugged the dishwasher into isn't properly
earthed and the fault is with the dishwasher?

I bought a Maplin socket and RCD tester. The trip trips at a setting
of 25ma with no significant appliances running but this reduced to
10-15 ma as the circuit is loaded with say 2kw.

I have tested the resistance of the circuit with everything unplugged
or isolated and get around 1Mohm. this seems to consistent with the
other circuits.

Although 1 MOhm is within the Regs, generally anything less than 2 MOhms
could suggest a fault developing if the test was done using a proper
insulation resistance meter, a normal multimeter doesn't test with
sufficiently high voltage. Having said that, I wonder what the
resistance of a rusty nail would be? Also, did you disconnect neutral &
earth from the CU before taking the reading? I'm a bit puzzled that you
got that consistently low a reading for all circuits, yet you're not
seeing problems on other circuits.

As I load the circuit I can measure 20-100 mv between neutral and
earth.
The earth tag is bonded both to the Gas main and to the electric
company's main fuse 'block'
There doesn't seem to be any bonding between neutral and earth. This
is strange, as I would have expected it?

Depends on your earthing arrangements, for a TT (local earth rod only)
supply there isn't any bonding between earth & neutral, although if the
earth is connected to the company's earth terminal then this suggests
TN-S or TN-C-S, in which case I certainly wouldn't expect anything like
1 MOhm between earth & neutral. Sounds as if the earth-neutral bond has
come adrift, or it's actually a TT supply but the elec. supplier has
used the wrong kind of incomer for installation. But having said that,
your RCD trips when tested with an independent device, so it would seem
you're getting an earth from somewhere. Wierd...

Does anyone have any experience of this type of problem and how to
sort it?

I reckon you really need to do or have done some proper tests on your
installation - insulation resistance & earthing for starters...

HTH