John Larkin wrote:
> If "earth" is zero volts and you dump, say, 10,000 amps into it,
> what's its power dissipation?
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.
> Any lightning protection device must be composed of two
> "subsystems," a switch which is essentially some type of
> switching circuitry and a good ground connection-to allow
> dissipation of the surge energy. The switch, of course,
> dominates the design and the cost. Yet, the need for a good
> ground connection can not be emphasized enough.
> Computer equipment has been damaged by lightning, not
> because of the absence of a protection device, but because
> inadequate attention was paid to grounding the device properly.