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I know MOVs inside surge protectors degrade over time, but I also see manufacturer put note in box that protection warranty void if I do daisy chaining of surge protector.

So questions:

  1. Is there an estimate of what life of MOV is if:

    i. it never face surge before

    ii. it face some surge before

  2. I buying surge protectors with indicator showing effectiveness of surge protection: how they work?

    How that indicator know if surge protection still effective?

  3. Why manufacturer discourage daisy chaining of surge protector?

I assuming most off-the shelf/commercial "computer" grade surge-protectors using MOVs

I was interest in find out if MOVs are connect in parallel to AC output of surge-protectors?

If so, how does MOVs connect in parallel effect them (something like connect R or C in parallel or series change their value/property)

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Is this question about MOVs specifically? –  tyblu Jun 18 '12 at 4:30
@tyblu: I not sure. I assuming most off-the shelf/commercial "computer" grade surge-protectors using MOVs? –  sekharan Jun 27 '12 at 4:46
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2 Answers

up vote 3 down vote accepted

You should not daisy chain protective devices (fuses, MOVs, breakers, etc.) without first doing the appropriate research because generally speaking, they are rated to interrupt in x seconds given a particular fault current. When you have two protection devices with similar ratings in the circuit, they end up both trying to interrupt and may very well end up interfering with each other's interruption capability, possibly to the point where neither will properly clamp or interrupt the fault, causing excessive current flow and possibly fires.

e.g. A fuse chosen more or less at random will clear in 1s with a ~20A fault current. If you have a second fuse with similar ratings in series, they will actually start limiting the fault current as they open up, and the fault current is no longer 20A, it may be 15A, or 10A, or ... you get the idea. That same fuse will clear a 10A fault in ~10s, which could be enough time to heat up wire or traces or cause a semiconductor to fail because it wasn't designed to handle that kind of current for that kind of time.

e.g. A MOV series chosen more or less at random will clamp a surge at 130V. Two in parallel will have (slightly or significantly) different clamping voltages, usually with the lower one "winning". The breaker/fuse and MOV are usually selected so that the MOV will clamp and the breaker will open with the surge current, but when you mix and match you end up with a MOV clamping earlier, which the fuse/breaker wasn't designed to trip at, which now alters its fault ratings, leading to unpredictable protection.

In the industrial power world this kind of interaction is actually a significant part of the overall electrical design, since you have substation transformers protected with fuses, and the downstream equipment protected with their own fuses or breakers, and then the load controllers protecting their semiconductors or motors again with their own protective devices, usually a combination of MOVs or fused MOVs and either breakers or fusing. There's a lot to look at, including the I2T ratings of the protective devices, interrupting capabilities, pulse withstand capabilities, temperature derating, clearing times, current limiting effects as the devices become active, Joule ratings and so on. Here are a few good references if you wish to look into it further. The term "fuseology" has come about to describe this particular aspect of electronics design.

... and I bet you thought fuses, breakers and TVS type devices were pretty simple, didn't you. :-)

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I believe daisy chaining is discouraged to prevent overloading of the circuit(s) and the warnings are more for minimizing insurance liability.

A consumer-grade power strip is good for 15 to 20A. Put another one on and the first unit is now expected to endure 30 to 40A. (and so on). Another reason people will daisy chain is to extend the reach of the electricity. As the power strip is intended for its own 6 feet of reach, the conductors are not suitable for 10~15A over the longer distance of 3 or 4 power strips of length. Instead of daisy chaining for length, buy a thicker gauge extension cord to fit a power strip to.

Obviously, everyone does this anyway. Me personally, I put computers on their own power strip then daisy chain all the odd power adapters as needed to get all of them fed.

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I was interest in find out if MOVs are connect in parallel to AC output of surge-protectors? If so, how does MOVs connect in parallel effect them (something like connect R or C in parallel or series change their value/property)? –  sekharan Jun 27 '12 at 4:49
I'm pretty sure that anyone who plugs a 15A power strip into another 15A power strip, which is then fed off of a 15A residential circuit isn't expecting them to source 30A, or at least not for very long. –  akohlsmith Jul 2 '12 at 3:52
@AndrewKohlsmith, yes however, we have had situations where so much was plugged in that depending on the brand, the initial power strip would "trip". –  Chris K Jul 2 '12 at 20:45
@ChrisK that's because you were overloading the breaker of the first. –  akohlsmith Jul 3 '12 at 21:32
And thus illustrating when people daisychain, circuits can be overloaded. –  Chris K Jul 3 '12 at 22:58
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