Chris Jones wrote:



At least some testers measure the insulation resistance (between
live/neutral and earth) at either 500VDC or 250VDC. The 250V setting is
labelled "MOV".
This document explains the testing process:

http://uk.megger.com/getmedia/a5f14a...AN_en_V01.pdf/


** That note is dated *2014* and admits that PAT testers without a 250VDC setting cannot distinguish devices with MOVs to ground from faulty devices.
This is not an acceptable situation.

IIRC, the "Megger 4DV" that I saw had no such setting.


The note from Megger on testing these surge protectors above also seems
to imply that connecting MOVs between live and earth is a common
arrangement,

** So common that they noticed it only last year !!!

Yawnnnnnnnnnnnnn.....


Other pages seem to suggest that at least in some cases/countries, MOVs
are connected directly from live to earth.

** Sure - in the USA, but only in some surge protectors.


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, I'm not sure that many of the appliance standards would
catch this,


** No need to "catch" it, the fitting of MOVs to ground inside a plug-in appliance is simply prohibited.

The standard 500V DC leakage test would indicate the existence of a MOV wired to ground - if eyeballs were not able to.



.... Phil