I see lots of confusion on this subject. First we should define the
difference between power line surge trash and lightning. They are different
in the way that they are handled.
Power line trash is typically between hot and neutral.
But since the neutral and ground are tied together at the service panel then
the power line trash is also between hot and ground as well as neutral.
An MOV across hot and neutral and hot to ground will keep all at the same
reference during the trash spikes. (almost)
There is nothing wrong with a 3-mode MOV device.
It will keep hot, neutral and ground at the same potential (zero
difference)(almost) with respect to the line trash.
A single MOV at this point, between hot and neutral, would allow a
difference in potential to ground at the equipment during trash spikes. This
of course depends on the distance the equipment is from the neutral/ground
bond in the main panel. The further away the greater the voltage difference
to the ground wire at the equipment. With the addition of an MOV from hot to
ground then the ground will have trash transmitted to it if it is any
substantial distance from the main power panel bond due to the impedance of
the long ground lead
Power line trash/spikes usually are not the problem of killed equipment
although they can be the source of noisy equipment. Lightning is the biggest
concern to do damage.
With a lightning strike whether it is a direct or indirect strike can cause
Remember that lightning is always trying to find EARTH ground to equalize
Lightning unlike power line spikes can come in on the hot, neutral or ground
line. Some type of equalization is needed so that the strike energy does not
come into the equipment on the hot line and leave the equipment on the
ground line. That will cause damage to the equipment. Clamping all three
lines to the same level will keep the energy out of the equipment even
though the chassis may rise well above zero volts during the event. 3-mode
MOV's can accomplish this. A single MOV can not!
Where the problem arises is when there are multiple equipment ground points
with multiple pieces of equipment!
An example would be a computer in one room with a data cable running to
another computer in another room. Data cable shield bonded to each
computer's chassis and each computer plugged into their own outlets. Now we
have a loop between the two pieces of equipment with power being fed
individually to each. If the power is fed to each from the same circuit
there is much less chance of voltage differences between chassis.
Or a TV being fed power from an outlet and also connected to the cable TV
line. A place for the energy to enter and another for it to leave.
This gets back to our "single point ground system" that has been discussed
many times here. All power and other cables connecting to a piece of
equipment need to go first through a common ground/bonding point (single
A whole house protector is a good idea but be sure to also bond all other
cables such as phone lines, cable TV, and your antenna cables etc to the
same ground point or very near where the whole house protector device is
installed. If you don't do that the whole house protector could make things
worse than not installing it at all because the whole house protector is
going to force the ground leads to your TV, radios etc. up to the same
potential as the hot lead carrying the lightning strike! If your antenna, TV
or phone cables are not referenced to the same place as your whole house
protector then there will be a difference of potential at your equipment.
Here is something to always factor into your lightning protection:
"No matter how good a ground system you install the voltage on that ground
system will always rise above true earth ground during a lightning strike".
Sometimes by many thousands of volts! This means that ALL equipment and
lines connected to that ground system must rise together to avoid damage.
By the way, I haven't looked at whole house protectors in awhile but lots of
then used to be just a couple of leads to form spark gaps and were filled
with sand to quench the arc. Nothing fancy but effective.
> I would definitely install a whole-house SPD solution at the service
> entrance. Then, only consider single-mode SPD devices (e.g., SurgeX) on
> branch circuits.
> Right now, I'm only using a whole-house protector. All 3-mode MOV
> previously used on my branch circuits were hauled away with the trash.
> add single-mode SPD devices as time (and my budget) permits.
> Paul, W9AC
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