Peter Parry wrote:
On Fri, 22 Apr 2005 21:28:09 +0100, "Christopher Key"
wrote:


You then place the multimeter from the appliance earth to supply
neutral, and it read 2.9A. To me, this clearly doesn't sound
healthy.

In most appliances I would agree but induction hobs have a heap of
electronics in there to produce the high frequency high current
needed to operate. I'm not altogether convinced the test is valid
hence the suggestion that an earth-earth test be carried out on a
non-RCD circuit.


I can certainly imagine the the high frequencies being leaked could confuse
a DMM, but can't see why the results would be any different if the leakage
was measured to earth and not neutral. Certainly no harm in trying if it's
possible though.

There are a number of inconsistencies:-

If the leakage current really was consistently a few amps the RCD
would always trip immediately. It isn't. It is tripping after some
time and somewhat unpredictably - this points to a situation where
the total asymmetry in the house is hovering about the minimum trip
current.

The voltage is about half mains - this is exactly what you would
expect from a noise filtered circuit with a floating earth - any
leakage path would alter this reading.

In a healthy
appliance, the earth should isolated from both live and neutral
(with the exception of a high impedance link via the noise
suppression caps), and should hence not be able to supply any
current to neutral.

I'd expect a few mA leakage through the filter.

If my understanding of the tests that have been performed is
correct, then this does sound clearly like a faulty device.

Except that the results of the tests and the reported fault symptoms
are mutually exclusive. With a permanent leakage of this level the
RCD would always trip immediately but apparently it isn't. The
voltage reading is also inconsistent with a single fault capable of
supplying this current.


I take your points onboard, but still maintain my original argument that the
multimeter was correctly connected to measure any leakage to earth. Whether
it was actually doing that, or reporting spurious values as a result of
induced noise, or an odd waveform is another matter. Would be interesting
to see what happens with a moving coil meter.

As you say, there is no way it could be leaking 3A to earth consistently,
and only occasionally trip the RCD. It is conceivable that it is an
intermittent fault however, as reading the OP, I got the impression that
typically the RCD tripped instantly, but occasionally was ok for a while.
This could be consistent with an intermittent short internally, that was
open when the 120V was read, and whilst the RCD wasn't tripping. That does
seem a little far fetched though.

There are combinations of whole house faults that could cause the behaviour
described, but they are so contrived that I doubt they could be created even
if one tried!

It seems like the hob is being replaced however, so I shall be interested to
see the behaviour of the replacement.

Chris Key