For my sins I am currently editing a 40 page MOV application guide. As
the guide was written by a Chinese expert I have the additional task of
translating the Chinlish to English. However this forces understanding
and I found the document contains many gems I did not appreciate. On
degradation three areas are mentioned:
1. Varistor voltage at 1mA has decreased by more than 10 % of the
initial value.
2. Surge limiting voltage at a specified impulse current has increased
by more than 10 % of the initial value.
3. Leakage current or watt-loss shows a steady increase.
Then the comment is made that generally if the Varistor voltage has
decreased by 10 %, the limiting voltage will have only increased by 3 %.
Thus the Annex Q limiting voltage change limit of 10 % would be better
replaced by a Varistor voltage change.
On fuses I did some calculations and found an anti-surge fuse in the 4 A
region was required to withstand the Annex Q MOV requirement of a 3 kA
8/20 surge. It should be remembered that most fuses will not interrupt
an 8/20 surge as the fuse link plasma carries on conducing the 8/20
impulse current. Ted makes the excellent point that in the equipment
additional series impedances will exist that reduce the peak the peak
surge current from an 1.2/50-8/20 generator.
Tests are now appearing for thermally protected MOVs, which disconnect
the MOV in the event of exceeding a pre-set body temperature limit.
Thermal disconnect, rather than current disconnect, gets to the heart of
the matter. But, as far as I'm aware, none of these tests apply a surge
voltage to check for open thermal switch arc over.
Regards,
Mick Maytum
Safety and Telecom
Standards
[email protected]
https://ictsp-essays.info
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