On 5/31/2015 7:53 AM, Chris Jones wrote:

however they show a gas discharge tube between the Earth
wire and the MOVs. It would be interesting to know if this is common and
if so, why and what are its ratings.

As a guess, the GDT prevents leakage Phil is worried about, and the MOV
prevents "follow-on" discharge through the GDT right after the surge is
over and normal power is present.


Whilst I would agree with Phil that a used MOV is
likely to be so leaky as to result in unacceptable current flowing from
live to earth,

As I responded to Phil, this is actually quite unlikely (at least in the
US).

I'm not sure that many of the appliance standards would
catch this, unless the sequence of tests (during product approval not
PAT) would first damage and later test the leakage of the MOV.

US-UL tests include a series of test surges, after which the device has
to be functional (in particular, it is not 'leaking').

Later tests may cause the device to fail, but it must fail safely.