Pete C. wrote:
HeyBub wrote:
wrote:
On Apr 17, 3:50 pm, "HeyBub" wrote:
Intermatic surge protectors do not use MOVs - they are electronic.
What is this claim based on? If they don't use MOV's what exactly
do they use? Also, this would seem to imply that MOV's are not
considered electronic components, but I believe by any reasonable
definition, they are electronic components.
You're right, of course. MOV are classified as "electronic" components. But
metallic-oxide-varistors work like reverse fuses: they short their terminals
together. And, like fuses, they (usually) only work once with no indication
(other than sometimes smoke) that they won't work again.
MOVs will continue to work indefinitely if their rated clamping current
isn't exceeded, so they will readily clamp on the modest surges seen
every day. The mega surges from a really close lightning strike or a
tree branch dropping the primaries into the secondaries is what will
cause the MOV to self destruct and usually trip the circuit breaker in
the process thereby sacrificing itself to save the stuff downstream of
it.
While MOVs are good with very short duration surges, dropping a primary
wire onto secondaries will rapidly destroy a MOV as you describe. Then
protection is gone.
UL requires thermal disconnects for failing MOVs. The specs from trader4
include “200k AIC Surge Rated Fuses” which indicate that suppressor can
be used on in a panel with an available fault current (not surge
related) of 200,000A. The suppressor may also trip a circuit breaker,
but protection should be internal.
I believe the maximum likely surge current on one of the hot service
wires is 10,000A based on a very strong 100,000A surge hitting the high
voltage wire on the pole behind your house. The spec from trader4 is
40,000A per wire for that suppressor, which is well beyond what is likely.
MOVs also have an energy (Joule) rating, which is cumulative. The
40,000A rating goes with a very high Joule rating, which means the
suppressor can take many hits. If a MOV had a 1000J rating, it is for a
single surge. If the individual hits were much smaller, like 100J, the
cumulative rating is much higher than 1000J. When the energy rating is
exceeded, the MOV starts to conduct at lower voltages, eventually
conducting at ‘normal’ voltages and failing in thermal runaway.
Without a service panel suppressor, there will be arc over to panel
ground at about 6,000V. After it is established, the arc voltage will be
hundreds of volts. That dumps most of the surge energy to earth. For
plug-in suppressors, the impedance of the branch circuit wiring greatly
limits the current, and thus the energy, that can reach the suppressor.
Combined with arc over, the energy that can reach a plug-in suppressor
is surprisingly small (unless the branch circuit is very short).
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bud--