Surge protector MOV replacements. Part replacement recommendations please! - Page 1
Forum section: electronics / beginners

author: msat
author_rank: Regular Contributor
timestamp: March 13, 2021, 11:34:23 pm
content: So I got a little paranoid (probably unnecessarily so) about the age of the handful of surge suppressors I use. I bought a few replacements, but I have an old Tripp Lite isobar that I like, and considering the price of a handful of MOVs are a lot less than the price of a whole new unit, nevermind the waste, I want to replace the MOVs in it. The unit consists of 6 Maida brand Z131 - 70UL MOVs, which according to Maida is rated for 130V continuous AC (175V DC), max clamping voltage of 340 @ 100A, 1571pF capacitance, and max transient energy of 130J. Since Digi-Key doesn't carry these, and I don't see why I have to retain these specs, I was hoping I could get someone to help suggest an alternative.I like the idea of having a fairly low clamping voltage, even if that means they'll wear out quicker. I'll just buy a bunch of replacement MOVs. But what clamping voltage is too low for practical purposes? Also, what other values should I look at for improvement? Suggestions wanted! Thanks!

author: Per Hansson
author_rank: Unknown
timestamp: March 14, 2021, 09:46:09 am
content: I like the Littlefuse TMOV series because they have a built in fuse to prevent your house from burning down unlike contemporary MOV's.Here is one example: TMOV20RP130E if you exchange the E at the end for an M you get the iTMOV series that has an output pin to signal that it has failed.https://www.mouser.se/Search/Refine?Keyword=TMOV20RP130

author: msat
author_rank: Regular Contributor
timestamp: March 14, 2021, 08:56:57 pm
content: Interesting. Looking at the example circuits, it shows a line fuse, but if the clamping MOVs go open, overvoltage will go to the rest of the circuit and that input fuse stands less of a chance of blowing, rather than blowing due to the short across the MOV, no?

author: TimNJ
author_rank: Super Contributor
timestamp: March 14, 2021, 09:02:09 pm
content: The point of the thermal fuse is to prevent fire, more than anything else. Sure, once the MOV is out-of-circuit (due to the thermal fuse opening), then there are other problems that can arise. But, the point of the TMOV is to prevent fire in the case that the MOV is overstressed.

author: TimNJ
author_rank: Super Contributor
timestamp: March 14, 2021, 09:04:56 pm
content: Often, better quality surge protectors have one or more thermal fuses bonded to the MOV bank (externally). This accomplishes the same goal, but takes some more space. I'd also guess that Littelfuse TMOV is still more expensive than a MOV + external thermal fuse.

author: msat
author_rank: Regular Contributor
timestamp: March 15, 2021, 01:24:04 am
content: That sounds like it's passing the buck on to some other components, and perhaps in the case of the surge suppressor, potentially any MOVs inside the PSUs that are plugged in to it. It's hard to see how that is the more desirable option.

author: wizard69
author_rank: Super Contributor
timestamp: March 15, 2021, 03:26:58 am
content: I'm pretty sure you can duplicate the specs with another part but did you try a different supplier to see if somebody stocks the part you need? It is often, most times, easier to spend a few minutes searching for the right part. Also; is this current production at Maida, if not they may have a recommended upgrade path.Quote from: msat on March 13, 2021, 11:34:23 pmIf it is a UL listed part you will want to replace with parts also UL rated. Ideally with the same or better specs.QuoteI'm not sure I agree with you here. You don't want the varistor tripping due to a modest power line fluctuation. Further early wear out means you could end up unprotected and you need to understand the manufactures spec sheets. The varistor voltage and the allowable AC voltage are two different things. Frankly I'd want a little head room above the max allowable AC voltage.As for your concerns with respect to worn out MOV's, if that is all your surge suppression devices include it would be a good idea to get rid of them. In an ideal world you would have gas discharge tubes and other filtering. To an extent it depends upon what your fear will happen but relying upon one device doesn't inspire me.Oh please do read the manufactures data sheets safety warnings and such. At least one strongly suggests inline fuses or breakers to assure trips for across the line implementations and ground leakage detection for line to ground. Personally I don't like the idea of in device fusing as that can leave you unprotected. You are better off with a fuse that requires a technician to get to it and when he does hopefully the parts get replaced.I really don't think much of a lot of the so called surge suppression strips on the market.

author: TimNJ
author_rank: Super Contributor
timestamp: March 15, 2021, 03:50:09 am
content: When the thermal fuse opens, it removes the failed MOV from the circuit. Sure, now the circuit is without surge protection, but that's a moot point because when a MOV fails, it won't be an effective surge supressor anyway, and more likely, it will fail in a way that causes the main over-current fuse to open.TMOV is superior in that once the failed MOV has (safely) been removed from the circuit by the integrated thermal fuse, the main circuit (which it was protecting) can still function normally. Of course, this is with the caveat that it is now more susceptible to surge related damage. But, hey at least it still works! And, if you chose iTMOV (indicating-type), you can even signal (via an LED, etc.) that the MOV has failed.When a MOV fails, it develops some random shorts between the discs, large current flows, which causes internal heating and potentially fire. Even if the MOV does not actually catch on fire, most likely it conducts enough current (as a shunt element) to cause the main fuse to blow. You may argue that un-protected equipment should not be allowed to operate when the MOV has failed. There is some validity to this, but in general, most people would prefer that their equipment does not become completely inoperable under these scenarios. And in some scenarios, the risk of fire may be even more important than anything else.

author: Monkeh
author_rank: Super Contributor
timestamp: March 15, 2021, 04:00:54 am
content: Quote from: msat on March 15, 2021, 01:24:04 amWhen MOVs fail they can catch fire under normal line conditions. It's hard to see how that's a desirable option.

author: TimNJ
author_rank: Super Contributor
timestamp: March 15, 2021, 04:06:18 am
content: Quote from: wizard69 on March 15, 2021, 03:26:58 amTrue. Some applications may be tolerant to down-time. For other applications, medical for instance, down-time may be unacceptable, may risk loss of life, etc. But, it's still kind of a catch-22, because, the unprotected equipment can just as easily get slammed some time in the future, while trying to save some other guy's life. In this case, these is still the net benefit that *at least the MOV doesn't catch on fire*.To really get the benefit of a thermally protected MOV, some sort of indication should be provided. This way, the equipment still continues to work under critical conditions, does not cause fire, and indicates to the operator to decommission the equipment or have it serviced before it's next deployment/usage.But, I guess this isn't exactly the application the OP is referring to.

author: james_s
author_rank: Super Contributor
timestamp: March 15, 2021, 04:10:40 am
content: I typically remove the MOVs from power strips and UPS's and throw them away or when I can find them, I buy power strips that lack MOVs in the first place. I've seen them burn up more than once, and I'm skeptical of their value. I've never had anything damaged by a power surge although maybe my experience would be different if I lived somewhere with frequent lightning storms, we very rarely have those here.

author: perieanuo
author_rank: Frequent Contributor
timestamp: March 15, 2021, 10:13:03 am
content: Quote from: james_s on March 15, 2021, 04:10:40 amthey certain have valueone of my subordinated wired a 220V automation to 380V supplyresult, MOV exploded, fuse deadreplaced them, saved some 500 euros pcb and 3 weeks wait timeelectric company can do you the same joke, i saw it more than 2 times, also reversed phases (no relation to OP...)so think again, this was only one random example

author: wizard69
author_rank: Super Contributor
timestamp: March 16, 2021, 11:57:22 am
content: The medical industry could be a long discussion all on its own. There is the flip side where a surge that blows a fuse or trips a breaker might be a good thing if there is a robotic surgeon on the other end of that line. Imagine a robot getting a jolt that cause the robot to send a knife into somebodies heart. Robot surgeons scare me really, I've been working industrial controls for years and have seen what happens when a servo does something it shouldn't.Its been a long 12.5 hour day and I've lost my train of thought. Best wishes.Quote from: TimNJ on March 15, 2021, 04:06:18 am

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2021, 05:46:20 pm
content: Quote from: perieanuo on March 15, 2021, 10:13:03 amWell I don't have a 380V supply available in my home so this is a non-issue. The electric company could theoretically do something like this but in my entire life I've never had it happen. I *have* had MOVs burn up though, at a former job there was a big hole burned in the carpet in one room where a power strip caught fire after the MOVs in it burned up, and I had a UPS at home melt down for the same reason. I'd rather have some equipment damaged by a surge than my house burned down by a plastic power strip that caught fire due to MOV failure. In addition to this dangerous failure mode, they also can fail silently, wearing out and becoming ineffective while the "protected" indicator is still lit. I don't like MOVs and don't trust them.