MOV's In Surge Arrestors Quest. ?

Hello:
Two questions regarding the MOV's one finds in the typical surge arrestors:
When the MOV clamps due to a spike, is it all over for the MOV, or are they
typically good for many clampings ?
When they do fail, do they usuall fail as an open, or as a short circuit ?
Thanks,
Bob
Reply to
Robert11
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MOV's are tested by UL once. They work the design passes. There is no more testing for another hit. Manufactures test the random one off the manufacturing line and as long as they work as rated the line continues to produce.
My experience is if the MOV sees a max rating they usually fry, open. I have installed a lot of Class 1 and 2 surge arrestors. ( distribution and service ). They have fused protection and usually if a fuse blows we just replace the circuit board with the MOV's. Testing has shown the odds are the board is bad.
I have a whole house arrestor installed and then point of use strips. Ya need 2 of the 3 zones for protection. The house arrestor clamps down to a level that the point of use ones can handle. I check to see if the lights are green after each lightning storm at the service. If the lights change color I go buy another one. I have lost 2 in the last 3 years. I replace the point of use ones every 3 years or earlier if I think it is needed. My opinion is that they are pretty cheap insurance. Compared with the big screen HD TV and the DVR.
Reply to
SQLit
Hi,
Thanks for info.
Can you recommend a point of use type of brand and model ?
Want to protect a new furnace circuit board that runs off a dedicated 110 V line. Has already been fried once, recently, in a lightning storm.
It has a fuse, but have real doubts if the board has any MOV's.
What do yo think of the Intermatic AG2401 ?
Thanks, Bob ----------------
Reply to
Robert11
UL tests protectors only for human safety. Plug-in protectors can fail on the first transient (which means ineffective protection) and still obtain a UL 1449 approval. UL does not care whether a protector provides transistor protection. Purpose of UL is human protection. UL only cares that the protector does not burn down the house - harm humans. Indeed, many protectors in some 1987 tests by PC Magazine did spit flames. One technique to meet UL 1449 is to disconnect the protector faster - leave more transient to find other paths such as through the adjacent computer.
How dangerous are plug-in protectors? Others demonstrate the problem. Or why plug-in protectors that are also undersized should not be on a desk covered in paper, on the rug, or behind furniture in a ball of dust:
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Do not locate a surge protector or power strip in any area where
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An employee arrived at the Greensport Yard in Houston this
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or
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From February 1994 to February 1995, two families on Brainbridge
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On August 28, 1999, a Kensington Power Tree 20, model # 62162
MOVs are designed to shunt transients and remain operation. MOVs (properly sized) fail due to degradation - not vaporization. One Taiwan MOV manufacturer even defines a number for degradation. A 10% change in the Vb voltage. They provide examples of how an MOV can degrade by 10%: for the 18 series MOVs, a 200 amp (classic 8/20 usec) transient is applied 10,000 times. No open circuit (vaporizing) condition in these tests. 18 series MOV degrades after about 10,000 pulses. Degradation - not vaporization - not open circuit failure - is how MOVs fail when properly sized. An MOV that vaporized - well the manufacturer does not even provide specs for that because the protector should not be that grossly undersized.
To sell ineffective and excessively profitable plug-in protectors, some manufacturers undersize their product. Then the naive will promote that grossly undersized protector with myths. A vaporized protector provided no effective protection - but did promote myths that increase sales.
UL does not test protectors for protection. That is not what a UL test does. The protector can completely fail - provide no transistor protection - and still get UL1449 approval because it did not threaten human life. That is all that UL 1449 2nd edition tests for - human safety and not transistor safety.
SQLit wrote:
Reply to
w_tom
First off absolutely nothing is lightning proof. The closer the lightning gets the worse the results. Second just protecting the furnance board is silly to my way of thinking. ( lets not go there right now )
IEEE says if your going to protect from Transients, and that may or may not include lightning for some. It does for me. You need two of the three levels of protected. Levels are distribution, service, and point of use. Distribution would have the highest let through service lower and point of use even lower. Coordinating the zones is the key. Your situation installing the intermatic at the service gives you one level of protection. You still need another level that can withstand the 490v and clamp down further.
What is the voltage tolerance of the equipment being damaged? Furnance boards are usually powered by a transformer not line power. I have not worked on any of the new stuff but I think that the manufactures still keep below 120v level for controls. Following this line the voltage was induced into all of the wiring in your home. Only the furnance was damaged. I lost a garage door opener last monsoon season, it was the only electronics, THEN, that was not protected by a point of use TVSS.
Someone mentioned that the 490v clamping voltage seemed high. I tend to agree. But this unit is being sold as a lightning protector. Just looking at the voltage is not the whole story, you also need to look at the joules that the unit will withstand/protect.
As a matter of prinicpal I do not care for "wired" solutions. I once worked for an OEM, CH. They came out with a bus connection TVSS series called Clipper. Yep they make wired solutions as well. Just like all of the other manufactures. The techy's that I worked around explained to me that TVSS connected to the bus are better cause the distance is shorter. Wired connections are longer. Programs with the APQA, Arizona Power Quality Association also agree with this. My recommendation is to look for a bus connected surge protector/tvss by your panel manufacture. As far as I know they all make them. Then look for a point of use device
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this one is a pretty blue color. (humor)
The bus connected unit should have a failure light, not that they are a guarntee.
Nothing is lightning proof. Mov's are not tested for a second surge. I replace my equipment everyother year.
good luck
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Reply to
SQLit
You cannot test or detect how well your installed MOV will work. Throwing MOV's at your problem is not the right fix. Bond the duct work. That is a human safety requirement, and one possible source of your controller failure if it is not properly bonded. No exposed metal in your heating system should be permitted to be above ground potential.
But, since you asked: MOV's fail shorted - but that is an over simplification. The short usually leads to an open, unless power is removed. There are an enormously huge number of possible paths for current through an MOV at the atomic level. Each path can be thought of as two conductors separated by an insulator. When the voltage across an MOV exceeds some specific level, one or more of these insulators has a hole torn in it, resulting in a short circuit (actually, a very low resistance) between the conductors. The energy that tore the hole is converted to heat, and does not reach your equipment. That's a good thing - but the hole is permanent. As further current flows after the hole is made, it is converted to heat in the MOV, and does not reach your equipment. How much current flows through the MOV (and therefore not through your equipment) is a function of path impedance, voltage, and how much energy the MOV can absorb. If electrical energy exceeding threshold is applied to the MOV for a long enough time, it will burn out and become open. Each time the threshold voltage is exceeded, at least some of the paths are destroyed, and the MOV's capability in trems of how much energy it can absorb is reduced by the amount already absorbed.
Ed
A shorted MOV in your point of use protector will quickly become an open MOV, unless a fuse blows or a breaker trips. An MOV *works* (not fails) by degradation.
Reply to
ehsjr
Each one of those transient events also causes a slight reduction in the conducting threshold voltage. Eventually, it can reach the normal peak voltage, at which point the MOV begins conducting every half cycle and self destructs.
Ben Miller
Reply to
Ben Miller

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