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.

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.

--
Peter Parry.
http://www.wpp.ltd.uk/