490 V Clamping Volt For MOV: Seems High ?

Hello,
Thinking of installing an Intermatic AG series surge protector across a single 110 V line to help protect a circuiit board in a furnace.
Clamping voltage is given as 490 V.
Seems awfully high.
Wouldn't most solid state devices blow (well) before this level ?
Thanks, B.
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It does. Are you _sure_ it's not intended for a 240V circuit?
IIRC, a MOV for 120V should clamp somewhere around 180-200V.
It might be worth calling Intermatic, it may be a communication goof between engineering and tech writers.
--
Chris Lewis, Una confibula non set est
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That's a decent figure, actually, considering that there is already protection there on the ckt board, even though you're getting storm damage or whatever. It should help substantially, but that's not the primary protection figure.
In my experience they were 375V, but it varies by manufacturer and purpose, level of protection, application, etc.. "clamping voltage" is the steady state voltage that will occur across the protective device during a fault condition, and the fault conditions are specced to be able to exist for a period of time, usually in microseconds, sometimes milliseconds. instead of thousands of volts, this component will only allow 490V to the secondary protection ckts already present in a standard ckt board design. At that voltage, the part is about to become an open ckt after x amount of time. If the load goes away in time, you still have a good part. If not, the part is no longer functional and can no longer protect anything. Other things fried, too, but they lasted a lot longer than they would have. This is a typical scenario for trying to protect against lightning surges, etc.. In a system, the rating of the intermatic has to be higher than the protective devices already present in the ckt board. Otherwise they cannot share the load during a surge, and successfully direct current away from the protected equipment.
The more important figure is the "knee" voltage, the point at which the device begins to conduct current away from the equipment being protected. Then there is a time/current and energy curve, detailing what happens as the part becomes overloaded and nears its own destruction. The two most important figures are the joules of energy it can absorb (basically watts, but not exactly) and for how long it can handle that level of energy, which is measured in anything from micro to milli seconds. 1,000 milliseconds - 1.0 second. If the clamping voltage is reached across the part, that probably would indicate, based on design, that the actuall applied voltage is in the order or many thousands of volts. So, say a 15,000 volt spike will clamp at 490 volts due to the component being there, and only 490 V is left for the remianing protective cktry to handle. It's sort of a staging cycle in most good designs. The switch you have isn't as simple as an MOV, but it operates almost like one. Try a google for MOV or Metal Oxide Varistor and you'll get a feeling for what the part you have can do.
Hope I haven't just made things more confusing!
Pop
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Hi,
Thanks for such good info.
BTW: When an MOV fails due to being unable to dissipatge too many Joules, or overcurrent, how does it fail ? As an open or as a short between the lines it was trying to protect ?
Thanks, Bob -------------------

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These short the line so that the fuse blows out, thus effectively disconnecting the load from the power source.
--
SVL







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MOVs generally fail _open_, and the fuse don't blow. That's why most protective devices have LEDs - to tell you whether the MOV is fried or not.
Consider the situations under which they trip - very high transient voltages and effectively an arc. The guts of the MOV are destroyed.
--
Chris Lewis, Una confibula non set est
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wrote:

PMFJI, but...
MOV's fail open, and AFAIK, that's universally true. A bit sad since, if you have no indicator light showing they're still alive, they can take their last hit and depart the circuit leaving you none the wiser and unprotected to boot.
A more hard-core approach is something like a TransZorb(R), which is designed to fail short. In this case of course there must be a fuse or fast-acting circuit-breaker to open or you have a fire hazard. With a TransZorb and suitable current limiting in place you have far superior protection than a MOV, and you can get a clamping voltage much closer to what you want to protect.
http://www.transzorb.com/diodes/protection-tvs-esd/trans-zorb /
When I was using these they were a General Semiconductor product. It's Vishay now I guess.
-=s

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An MOV fails open-circuit. No surge protection ckt I ever knew of would fail open circuit as that presents a further hazard in the wiring to the device during the overcurrent until it pops the breaker. The poster who mentioned tranzorbs, etc. is also correct in his post. A "safety" device will not create an additional hazard (as in popping a breaker) when it operates.
I hope you'll come back and let us know how your solution works; I've seen your past posts about this and found them interesting.
Just a final note: You do know that the ground to the whole system is good, right? Surge protection such as you need requires a good ground to be reliable. You probably already answered that, but ... can't hurt to check.
HTH, Pop

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