In my experience (and opinion), only slightly more than 15 watts is necessary 
to raise plastic material to ignition temperature.  

If a MOV is used as a SPD, then it should only operate (be conductive) in the 
event of a transient over-voltage.  If the transient event is the 1.2x50 
waveform, the MOV should conduct for about 100-500 usecs.  How long before the 
MOV returns to quiescent resistance?  I ask this question because the recovery 
time will determine the heating power dissipation (from the mains voltage).

Rich

-----Original Message-----
From: Ralph McDiarmid [mailto:ralph.mcdiar...@schneider-electric.com] 
Sent: Friday, November 17, 2017 10:05 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on 
AC mains ports?

I wonder if a 1A fuse would protect against MOV flameout just as well as a 
100mA, if these MOV fail as an avalanche.  (get hotter = more leakage) I've 
seen some standards use 240VA (assume 240W) as a power limit for protection 
against fire.  However, I wonder if a MOV could burn nicely at say 0.5A without 
further increase in current.

Ralph McDiarmid
Product Compliance
Engineering
Solar Business
Schneider Electric

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