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Re: what is the joule rating on surge protectors?

Subject: Re: what is the joule rating on surge protectors?
From: John Popelish
Date: Sat, 12 Aug 2006 23:21:01 -0400
Newsgroups: sci.electronics.basics
Sjouke Burry wrote:
John Popelish wrote:
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.
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)
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.
I agree with all of this, to some degree.

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
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.

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