Metal Oxide Varistor (MOV) Claims

Good Question, Never really thought about it, but since you brought it up, I wonder how many joules the wiring in the device can handle. Though if I understand it correctly, the extremely high current is only expected to be controlled for a few milliseconds. Never over any considerable period of time.

I hope someone who knows will read this and reply, cause now I'm curious too.

microseconds, actually, 8/20 to be more precise.

8 microseconds to rise to 90% of the maximum, then 20 microseconds to drop to 50% of the maximum.

this is one of the standard testing wave forms for surge protection.

this thread has had some very good answers.

I know of no area in EE where manufacturers are as creative in their reporting as in the field of surge protection, truly the oldest surviving pedlars of snake oil!

<Ahem> How about the home audio market.

"2000W PMPO Home Stereo System" WOW that's a lot, so what's the rms rating of the amplifier, oh its 20W rms. Well that explains why the stereo doesn't have a power supply the size and weight of a couple of house bricks.

Just a couple of points. Metal-oxide varistors do not "wear out" with repeated duty provided that duty doesn't exceed their MCOV (maximum continuous operating voltage, TOV (temporary overvoltage), or duty cycle ratings.

The individual blocks are also graded and matched to achieve near-uniform voltage distribution within the arrester asembly.

That's correct, although by their very nature they "wear" out a little bit every time they conduct and shunt a spike (to phase or ground depending on the connection) regardless of the spike duty cycle.

Thanks to you and everyone else for the excellent responses.

And I think I see what your first link is getting at...sort of. Still in this particular case even if I added up all the MOVs' Joules, it would only end up being around 1200 J. That's a far cry from the claimed 3240 J.

I'm just unclear how they're getting from A to B and if however they're doing it is how other manufacturers do it too.

Some more information to ponder

http://www.control-concepts.com/drSurge/whitePapers.html

The main point is that a MOV doesn't absorb a set amount of Joules before failing, it depends mainly on number of pulses and the energy content of those pulses. For example, a certain MOV may be able to safely absorb 100 Joules in a single spike or 100 spikes of 2 Joules (giving a total rating of 100x2 = 200 Joules).

The Joule rating is rather misleading and is used as a marketing tool a little like PMPO is used as a measurement for stereo Watts. This is rather interesting thou when you consider that MOVs are fairly standardised in that a similarly physically sized MOV of one manufacturer has very similar characteristics and Joule rating compared to another manufacturer's MOV (where both MOV breakdown voltages are the same).

A lot of information there. Double thanks!

Just out of curiosity, I dissected another "brand name" strip I happened to have. In this case I found the datahseet for the MOVs in it and found that they have a clamping voltage of 340V. Yet the power strip manufacturer claims "< 300V let-through" voltage on their webpage.

Another bit of marketing? Or something else I don't understand?

The clamping voltage varies with peak current. The clamping voltage is commonly given for a 1mA DC pulse and an 8/50uS AC pulse, the AC pulse clamping voltage is always higher. It sounds like they are using the lower DC pulse value on their website(being a smaller let-thru voltage it looks better after all).

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"In the event of fire, you should have consulted your data sheet!"

Here in the US, you can buy TVSS components that are not rated unfortunately. But UL has a 1449 testing and listing rating that they put on manufacturers that wish to be listed. As the one gentleman suggests, the rating is for the maximum value that can be seen for one time and effectively clamp the surge down. The device maybe burned to a crisp, but it must operate for that amount of energy with a specific response time and a that is all that it has to do. I can run 500 amps down a #12 wire, but it will be useless after a few seconds. Then it has to be replaced. One thing that all the manufacturer's hang their hats on is that the ground has to be short and of low impedance.