Jim Yanik wrote:
"David" wrote in -
september.org:


A MOV is somewhat like two back-to-back Zener diodes.
It
is
a voltage clamp.
no,it's not. it does not "clamp" the voltage.

You do not pass energy to ground, you pass
current to ground just like you do with any load. The
energy
is totally dissipated in the MOV.
Uh,"passing current to ground" IS passing energy to
ground.

David



totally wrong.
Wiki has a nice article on metal-oxide varistor,I
suggest
you read it.

--
Jim Yanik
jyanik
at
localnet
dot com
Jim, I am not going to get into a flame war over this
topic.
Maybe you should check this out:

http://www.cliftonlaboratories.com/metal_oxide_varistor_(mov).htm

David


http://en.wikipedia.org/wiki/Metal_oxide_varistor

Varistors can absorb part of a surge. How much effect this
has on risk to
connected equipment depends on the equipment and details
of the selected
varistor. Varistors do not absorb a significant percentage
of a lightning
strike, as energy that must be conducted elsewhere is many
orders of
magnitude greater than what is absorbed by the small
device.

--
Jim Yanik
This is my final say on this topic. In the quote above, you
assume the section saying that "... energy that must be
conducted elsewhere ..." goes to ground through the MOV.
This is where your error resides. The energy is going
elsewhere but being dissipated somewhere else completely
such as blowing up a transformer. The article should also
use the term dissipated elsewhere to make things clearer.

feel free to edit it.
You also assume that passing current is equivalent to
dissipating energy.


No,that's what YOU assume I said. Wrongly.

Current can *move* energy somewhere, but
electrical energy is only dissipated when the current causes
a voltage drop. A perfect ground will not have a voltage
drop so that is not where the the energy is being
dissipated.

HA,now you're talking about "perfect grounds".Sheesh.
you don't know what you're talking about.

I agree. The vast majority of energy in a lightning strike is passed on
to the earth.

Assume a surge of 10,000A on a service wire (maximum that has a
reasonable probability), a very good resistance to earth of 10 ohms and
a duration of 100 microseconds. If I am multiplying right that is
100,000 joules dissipated in the earth.

If you had a service panel suppressor with UL let through voltage of
330V (measured at a specified current much lower than 10kA) the actual
voltage across the MOV might be 500V and the energy dissipated for the
same surge would be 500 joules.

Most of the energy that was available at the cloud is dissipated on the
trip down - in heat, light, sound....

In about any lightning strike there are multiple paths to earth -
multiple utility earthing points, multiple houses, ....


BTW,when a lightning strike hits a ground,it dissipates it's energy -in the
ground-. literally.

Nice example.

It even makes a fulgurite.(fused earth)