Test a cheap Surge Protector Power Strip (US 110V)?

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Summary:

A cheap surge protector doesn't have the LED monitoring for the MOVs/circuitry. Can I test it myself? I have an idea.

Main Question or Discussion Point

I figure I could connect a high voltage source with a high impedance, and verify that it clamps that voltage. The easy way would require very careful consideration to keep it safe - that would be to use a 220V outlet connected through a 220K Ohm Resistor on each leg.

Apply the current limited 220V to Hot-N; then Hot-GND; then N-GND. I think my meter will read peak V, but I should also see some reduction of RMS with the power strip connected.

Is there an easier/better way? Obviously, one must be aware and careful when dealing with line (mains) voltage, but I am familiar with the precautions, and this would be safe if normal precautions are taken. I could build little switcher and run it from a battery, but I really don't want to invest the time right now. Hmmm, maybe charge a couple caps to the 110V peak ( ~ 150V), put them in series, and monitor with a scope (assuming my meter can't catch that fast peak), or charge from a 220V sorce (small transformer, current limited, isolated).

I tried a search, either special equipment was used (not practical for a consumer), or the non-helpful advice of "don't take a chance, throw it out!".

TIA -
 

Answers and Replies

  • #2
anorlunda
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Of course safety is first priority, but utility is second.

As I understand it, MOVs failure mode is short circuit. A cheap surge protector with MOV, I would expect to be a one-shot device. Once it conducts, that is end of life.

You might also consider the difference between a surge and a temporary overvoltage. It is important because MOV failure is temperature dependent.
Here's an article on that topic.


EDIT: here is the belated link.

https://interferencetechnology.com/...oxide-varistors-in-end-use-equipment-designs/
 
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  • #3
berkeman
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Here's an article on that topic.
I don't think the article made it (maybe Greg's experimenting right now...)
 
  • #4
berkeman
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A cheap surge protector doesn't have the LED monitoring for the MOVs/circuitry. Can I test it myself? I have an idea.
What are the specs on the MOVs? I would expect the fire voltages to be (maybe) high enough to not be tripped by just 220V...
 
  • #6
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Thanks for the replies - that link looks great, I will look in more detail later, but yes, it appears these are for protecting against much higher voltage spikes, not a 2x over-voltage condition.

Hah, I just looked closely at my power strip, it says "Type 3 SPD 800V(L-N)". So I assume it has an 800V MOV L-N only, nothing from either to ground.

The switch has a built in circuit breaker, so if the MOV did fail short, that would keep tripping. I'm more concerned with a fail open, as then you have no real protection. I should look at how the indicators work.

Will read more later....
 
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The problem with cheap 'surge protectors' (which should just be called 'switchbars,' IMHO) is the lack of upstream series impedance. Tying a couple of tight knots (or clamp-on ferrites, if you have them laying around) in the surge protector power cord can improve performance by a surprising amount.
 
  • #8
berkeman
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The switch has a built in circuit breaker, so if the MOV did fail short, that would keep tripping. I'm more concerned with a fail open, as then you have no real protection. I should look at how the indicators work.
Yeah, the MOV should clamp a few times to protect against transients, and eventually (sooner for cheaper MOVs) it will fail short, which will open the breaker. Usually MOVs fail short (in my testing experience), and then fail open if they are subjected to more hits (often with a very loud bang).

I learned the "loud bang" part the hard way in our test lab a few years ago. Another engineer was running life tests on various brands of MOVs so that we could recommend the best part to our customers in our powerline communication reference design. Unfortunately he would often leave the Surge tester unattended while running these tests, but at least he would put a thick Lexan cover over the test fixture. Too many times I would be working in the lab on something, and I'd hear the "tick" sound of the Surge tester firing every couple of minutes. Then sometimes you would hear a little louder "tick", which it turned out was the MOV failing short on that hit. Then a couple minutes later, "BANG!" it would destroy itself on the next hit. Sacred the Bejeezus out of me too many times... o0)
 
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  • #9
Tom.G
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The problem with cheap 'surge protectors' (which should just be called 'switchbars,' IMHO) is the lack of upstream series impedance.
I can't find it at the moment, but there was a thread several months ago involving Surge Suppressors. Here in the States, the authoritative organizations specified that (at least consumer grade) surge protected outlet strips are have at least 10 meters of wiring between them and the overcurrent device (circuit breaker).

Ahh, found the old thread, but it's pretty long winded. https://www.physicsforums.com/threads/surge-protector-specs.955697/post-6062621

The 'authoritative organization' is UL (Underwriter Laboratories). The reason for the 10 meters of wiring is it adds 10uH of inductance, thereby supplying an impedance to drop the surge voltage when the Surge Suppressor conducts.

Cheers,
Tom
 
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