Connecting MOV's in parallell

MOVs are the basic protection elements in both service surge protectors and plug-in surge protectors. MOVs do provide effective protection from most surges including those caused by lightning. They may or may not protect from a direct hit to overhead wiring very near your house. Or the surge suppressor may not survive but protected equipment will.

I would not recommend building your own surge protector.

I don't think it has been mentioned - the voltages are peak voltage. If a MOV with a MCOV of 130 volts is connected to a 120 volt RMS line (168 volt peak) it will rapidly destruct.

------------ Martzloff, who was the NIST surge guru, has said the major cause of surge protector failure is temporary overvoltage, not unusually large surges.

bud--

Transients from the refrigerator, et al are not surges. Put numbers into perspective. It is only noise. Noise that is well below what every electronic appliance internal protection makes irrelevant. How many days this week have you been to hardware stores to replace dimmer switches, smoke detectors, or GFCIs? If internal appliances created surges as myth purveyors so often proclaim, then we are trooping to the hardware store daily.

Primary purpose of a surge protector is lightning protection. Damage from direct lightning strikes to your appliances (ie when lightning strikes utility wires above street) means a human has failed to learn and install effective protection. Your post implies lightning is massive energy. Lightning at the strike location is massive power and not much energy. As Alan Taylor from the US Forestry Service demonstrated long ago, well over 95% of all trees directly struck by lightning leave no appreciable indication. Effective and properly earthed protection makes lightning irrelevant. Damage from lightning is directly traceable to human failure because such damage is so easily averted - once we learn.

MOVs are superb protector devices. Once the telco used gas discharge tubes (GDTs). That was replaced maybe 50+ years ago by an MOV like device more commonly called 'the carbons'. You may have seen it:

Now we use MOVs to earth direct lightning strikes which is why joules for that protector must be sufficient. Notice that as joules increase, the transient voltage decreases. MOV life expectancy also increases with joules. Therefore protectors use many MOVs in parallel.

Appreciate that all appliances already have internal protection. Your MOVs must earth lightning distant from that appliance so that lightning will not overwhelm protection already inside that appliance. This is what telcos, connected to overhead wires everywhere in town, have done for more generations than you have existed. Telcos don't install protectors adjacent to electronics; that would be ineffective protection. Telcos prefer protectors up to 50 meters distant from transistors - and near zero feet to earth ground. Then protection inside that $multi-million switching computer is not overwhelmed.

MOVs are some of the best protector devices we have, > thanks.

It was a hypothetical, which should be obvious. I was trying to force fit something as gently as I could to point out that your statement could use a little work. It is clear that my hypothetical only confused things.

I'll be blunt so the point won't be missed this time. You said: "Varistors also have manufacturing tolerance. If you put a two varistors in parallel, and one is at the top of its tolerance range and the other is at the bottom, virtually all the current will go in to one of them, and hardly any onto the other - hence the short reply of no."

My stupid example was intended to illustrate that even if one varistor operated as you say, with virtually all of the current going through it, the overall rating is in fact increased. The rating would be essentially doubled, give or take innaccuracy in the rating. Additionally, unless there is a large manufacturing variation, both MOV's will conduct appreciable current in a surge event that causes one of them to conduct heavily.

Right. I did not specify a 130 v varistor. I provided a bad hypothetical to try to illustrate a point. You are right to challenge it - it stunk.

Not at all. Nothing you posted here addresses it.

No. Are you? Some surges events are destructive to MOV's, some are not.

Well the aren't, unless abused. In general, you should select a

Well, I don't know what your example has to do with my way of looking at things. My way of looking at MOV's in the real world is that some surge events are destructive and some are not. Some surge events may damage an MOV, without destroying it. Such events degrade and therefore subtract from the capacity of the MOV. Some surge events don't damage the MOV.

Testing (as opposed to real world) is assumed to degrade the MOV, while a real world surge may not.

That just dodges the issue. Putting MOV's in series is useless as a means to double the joule rating, as you pointed out. Putting them in parallel is effective at doubling the rating. (Increase would be a better term than double, because there may be some variation from rating in the actual devices.) It is also increases the current that can be drawn during a surge event, because it will split between the two MOV's, but in the case of current, the word increase does not indicate essentially double the amount. The differences between the MOV's would determine the proportion of total current each MOV would draw.

I *think* we agree that joule rating may be misleading to the public.

Can you spell it out? Seems to me that if we apply the same limitation - "safely contained" - to both cases, they would be equally benign. So I'm not sure what you have in mind.

Ed

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