surge protectors - I am told that they only protect once


Mar 21, 2004
The IT in my local college told my friend that surge protectors are not all that safe. He claimed that after the first time they "only work once" they become mere power strips, unable to protect against further surges.
I have looked online and I cannot see any basis to such a claim, however, this little bit in wikipedia bout surge protectors:
Clamping voltage — better known as the let-through voltage. This specifies what voltage will cause the metal oxide varistors (MOVs) inside a protector to conduct electricity to the ground line.[1] A lower clamping voltage indicates better protection, but a shorter life expectancy. The lowest three levels of protection defined in the UL rating are 330 V, 400 V and 500 V. The standard let-through voltage for 120 V AC devices is 330 volts.[2]
has me wondering whether the MOVs might degrade with use, eventually failing to clamp.

anyone knows more about this and can clarify things for me?

Mark R

Diamond Member
Oct 9, 1999
When the mains voltage rises above a preset limit, they a MOV activates and diverts the surge to neutral or ground. However, the MOV actually diverts the surge current through itself. As surge currents can be large (thousands, or even 10s of thousands of amps) this heats and stresses the MOV and can damage its structure.

How many surges the MOV is able to divert depends on how big the surges are, and also how frequent.

The normal MOV rating that you might see on a surge protector (e.g. 4,000 A) is the 'ultimate' surge current rating. A surge of that severity will be successfully diverted, but is expected to cause destruction of the MOV. Larger surges may not be succesfully diverted.

Lower current surges will cause less damage - e.g. the same MOV as in the previous example would be expected to survive 1,000 surges of 400 A each (unless the surges came in such close succession that the MOV would overheat and burn out)

Even smaller surges (e.g. 40 A) would cause no significant damage - and the MOV should survive many millions of such surges (subject to cooling conditions).

MOVs' maximum current ratings are given for very short duration surges (< 20 microseconds), such as surges produced by switching electric motors and industrial equipment. Events like lightning can last 500 microsecs and this long-lasting surge is much more destructive than the short surges produced by machinery (e.g. a nearby lightning strike resulting in a 40 A surge current would likely destroy the MOV given in the earlier example).

Finally, don't think you can get around this problem of wear by connecting loads of surge protectors together. MOVs naturally vary in their trigger voltage, and they are 'binned' into narrow voltage ranges. However, due to the shape of their resistance curves, very small changes in voltage sensitivity result in massive changes in resistance. If you connected 2 MOVs, from the same voltage bin, in parallel, they would be very unlikely to share the current equally - and could easily split the surge 999:1. Rather than sharing the load, the slightly more sensitive MOV would sacrifice itself before allowing the slightly less sensitive MOV to take any of the surge itself.


Senior member
Apr 25, 2009
The IT in my local college told my friend that surge protectors are not all that safe. He claimed that after the first time they "only work once" they become mere power strips,
Surge protectors that are undersized have higher profits and get the electrically naive to recommend them. For over 100 years, effective protectors earth even direct lightning strikes - and remain functional. A properly sized 'whole house' protector also makes lesser transients irrelevant.

Effective protection has always meant that nobody even knew the surge existed. But that would not get the naive to recommend them.

Another problem: scary pictures seen by most fire departments when a protector is grossly undersized to fail on a first surge:

Even a fire marshal describes why plug-in protectors cause the fire threat.

Even plug-in protectors need protection provided by one properly earthed 'whole house' protector. Protection has always been about where energy dissipates. Either energy is inside a building hunting for earth destructive via appliances - with or without a plug-in protector. Or that energy is harmlessly absorbed outside the building in earth. Your choice. A 'profit center' plug-in protector that fails so that the naive will promote it. Or one 'whole house' protector so that even direct lightning strikes cause no damage. No damage even to the protector.

Essential is a short ('less than 10 feet') connection to single point earth ground. Why? What provides the protection? Whole house protectors (for about $1 per protected appliance) make that always required short connection.

Protection for 100 years was always about where energy dissipates. A protector is only as effective as its earth ground.


Diamond Member
Aug 31, 2003
you know what you dont see in all those links. Monster cables, the proof is in the pudding folks. Run to circuit city and get protected.