what is the joule rating on surge protectors?

That's OK, John. My Dad taught me many years ago to just consider the source.

Thanks! Rich

As they say in Texas, call me anything you like, just don't call me late for dinner.

John

Ah! Another pseudonym for goof-ball posts: "Late For Dinner". ;-P

Cheers! Rich

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It doesn\'t.

If it did, it would vaporize.

As JF says, a 39 j MOV is rated to absorb 39 J in a single-shot event.

"Earth", as in a good ground, doesn't absorb the transient energy. The transient is partially reflected and partially dissipated *in the generating source*, and the MOV absorbs a bit of it.

Earth absorbs transient energy only in proportion to how bad a ground connection you make.

If "earth" is zero volts and you dump, say, 10,000 amps into it, what's its power dissipation?

John

Earth is zero volts when a 10,000 amp current travels four miles to earthborne charges? Not true. When earth is conducting lightning, a four legged animal in that path may become a victim. A tree is struck. Shortest path to earthborne charges is down tree, through earth, up that cow's hind legs, down its fore legs, then on through miles of earth to earthborne charges. Why is the cow killed? If earth is zero volts, then electricity would never travel through and kill a less conductive cow. Earth is not zero volts because earth is not zero ohms. Earth is dissipating lightning energy. A cow's body makes a more conductive path; therefore cow is killed. Earth is so non-conductive that energy in those 10,000 amps is dissipated in many miles of earth and takes an electrically conductive path through cow. Earth is where energy is absorbed because earth is not a zero ohm conductor. Earth is not zero volts. Miles of earth is where energy from 10,000 amps is absorbed.

Effective protection means a low impedance (as possible) shunt to earth so that little energy is absorbed by the shunt (MOV, GDT, avalanche diode, or lightning rod) and so that significant energy is absorbed by earth. A low impedance shunt does not absorb the energy just like wire does not absorb energy. Better protectors are even lower impedance when to provide even better protection. Shunt energy to earth where energy is absorbed. Franklin demonstrated same in 1752. Did his ligthning rods, instead, absorb the energy of a lightning strike? Of course not. Earth absorbs the energy. Just like MOVs, the lightning rod make a low impedance connection to earth.

MOV joules is how life expectancy is measured. Just like wire, a 39 joule MOV absorbs little energy while shunting massive energy into earth. Unlike wire, the MOV eventually degrades with use. We measure this MOV life expectancy in joules. A more conductive and superior MOV absorbs less joules. Therefore a more conductive MOV will shunt more energy to earth, absorb less energy, and has a longer life expectancy.

They are shunt mode protectors. To protect by absorbing energy, a protector would be a series mode device - in line with electronics. MOVs are not series mode devices. MOVs are shunt mode devices. MOVs are effective when energy is diverted to and absorbed by earth. If MOVs absorbed energy, then voltage on protected appliances must increase - completely undesireable. Effective MOVs make a low impedance connection to earth just like a switch. That means less voltage confronting protected appliances, less energy absorbed by the MOV, and more energy into earth. That means even better protection. Notice: better protection means even less energy absorbed by the MOV; more energy absorbed in earth.

Shortest path? A standing cow is a shorter path than no cow?

The energy of a lightning bolt is mostly converted into light and heat within the thousands of feet of length of the bolt itself. Where else would it go? The fraction of a bolt's energy that winds up heating the earth is minute; just do the math. If the earth at the point of contact spikes up to 10KV (not a grossly unreasonable number) and the cloud potential is a few hundred megavolts (up to a gigavolt for positive lightning) then the earth dissipates well under 0.01% of the total energy of the bolt.

In a series circuit, the lowest resistance dissipates the least power. The function of the MOV is to keep this piece of equipment from absorbing enough energy to fry it, but it actually diverts more energy

*upstream* than it does into ground.

Nobody has yet answered my question: what are typical one-shot versus lifetime joule ratings for MOVs?

Collecting folklore can add up to a lot of contradictions. Understanding the physics has no such problems.

John

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He means that the resistance through the cow is less than the
resistance of the earth between the cow\'s front and rear hooves.

Does he? Why didn't he say so? "Electricity follows the shortest path" is qualitative folklore. I wonder what the inductance of a cow is.

So any MOV can absorb a megajoule?

John

Well, convert a mega joule of electric energy to heat, if given enough time to do it. 4 weeks should be long enough for most.

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_Cubic_ miles, which means there are zillions of _parallel_
resistive paths through which the charge travels until it\'s
dissipated.

Yeah, gotta protect against those slow lightning bolts.

John

100 of the largest of these in parallel might. (10 kj each)

Yeah, but the big one clamps at about 7 kilovolts!

Some of the curves sort of imply a lifetime of 1 to 1e6 shots, depending on the energy per shot, with a fuzzy disclaimer.

John

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Yes.

Oh, you have a clamping voltage requirement, also? ;-)

I would expect any MOV that absorbed its rated joules in a single pulse to have fulfilled its purpose and be ready for replacement. Derate that peak energy to half and it is an entirely different situation. Derate it to 1/10th, and it can be expected to last a long time, as long as it cools off between pulses. I don't think integrated energy is a very good way to calculate life. It wears out by hitting peak temperature.

You want to replace the MOV anytime your air conditioner switches? Its no 1shot device, it acts like a zener diode, one voltage for any current you apply. It reduces any line transient to its rated voltage, whatever the current. Lightning by the way causes most damage by secondary induction, up to 1000+ volts/meter (or yard), and this happens much more than direct hits, those are almost always fatal. Those induction spikes can be found in a wide range of voltage and current, for which the MOV can be a nice protection, dont forget that a Megawatt pulse of one microsecond duration is only 1 joule, so that a 40 joule MOV can dissipate 40 MW for 1 usec, and thats much more than your electronics can stand. The rated joules is just the amount of energy/heat, that will cause a tolerable temperature jump,and some damage, because the MOV is overheated in some spots,with local failure. When to much local damage is done, the MOv voltage will go below the mains voltage,and it will burn out/blow the fuse.

Your question was answered last month even with numbers. MOVs are not 'one-shot' devices. A one-shot device is grossly undersized; ineffective protection. Some plug-in protectors are grossly undersized so as to be damaged by a first surge. Being grossly undersized sells more ineffective protectors. Answer to John Larkin's question reposted:

One shot operation means an MOV was grossly undersized for that design. Again, as MOV joules increase, joules that it may shunt to earth increases exponentially. Answer starts with manufacturer datasheets such as John Popelish provided:

. John Larkin would not be making his claims and not be asking this question had he first read manufacturer datasheets and application notes. He would then understand why an MOV shunts energy to earth; why it does not absorb the surge.

A 20,000 amp lightning surge will only contain 200 joules? Of course not. First MOV in that datasheet will shunt (divert, earth) a 20,000 amp transient - and in the process that entire 20,000 amp transient is only 200 joules? Of course not. That 20,000 amp transient may dissipate hundred of thousands of joules into earth while only dissipating 200 joules in the MOV.

My air conditioner switching off does not dump rated peak joules into any MOV I have installed across my power lines. (and I have many)

I agree with all of this, to some degree.

Yep.

Yep. Maybe you should read what I said, again. I did not mean to imply (and certainly didn't say) that most line spikes come any where near to dumping rated joules into any MOV. For instance, I have has a very small one across my furnace burner motor starting winding, to keep the starting switch contacts from flashing as they open. It absorbs essentially all the stored inductive energy from that winding each start, and has been doing so for twenty something years. But each pulse is a small fraction of its peak energy rating.