Richard wrote:
>
>â?¬ With luck and good engineering, the MOV does not destruct when the
>wolf knocks at the door.
>
That's certainly true in this case, where the switching surge and the
energy to be absorbed are completely predictable. To choose the right
MOV for this kind of protection you only need the good engineering.
Save all your luck allowance for the other situation, which is where the
MOV is protecting against power-line surges coming in from the big bad
world outside. These come in all sizes and are not predictable, except
in a broad statistical sense. Thankfully the bigger the surges are, the
rarer they are too.
The scenario that Steve described is of this kind. The MOV handles the
frequent small surges with no damage at all, but the larger, less
frequent surges can cause some cumulative damage. Eventually the MOV
accumulates enough damage to cause it to fail. Or a huge surge can
destroy the MOV in a single hit.
Equipment operators in lightning-prone areas often routinely change MOVs
after a hit or a near miss. It's simply more cost-effective to change
them than to test them, and that sometimes gives MOVs a worse reputation
for damage than they deserve.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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