MOV is an acronym for Metal Oxide Varistor. These devices behave like
bi-directional zener diodes but, for their size, can absord much higher
amounts of energy (Joules) for brief periods of time. They're available in a
wide variety of sizes and electrical ratings and are used to limit peak
voltage excursions. They limit voltage by becoming low impedance above a
threshold voltage and shunting high current from the surge source, thus
limiting the voltage.
A typical setup would be in something like a "surge-protected" outlet strip,
where 3 MOVs are used: one from L to N, one from L to G (safety ground), and
another from N to G. I do not like to see MOVs that connect to safety ground
because, during a surge event, very high peak currents then are forced to
flow in the safety ground system (as the current finds a pathway back to the
N-G bond at the main panelboard). This high peak current flow, often in the
thousands of amperes, causes very high (often 2,000 volts or more) voltage
differences to briefly exist between pieces of equipment in a system that
may be plugged into the power grid elsewhere. Therefore, the signal
interfaces are exposed to very high transient voltages ... which often
releases factory-installed smoke!
Series-mode suppressors (such as Surge-X units) don't *shunt* high currents
but rather *block* them with a high-impedance (to surges) series element.
These are much preferred on branch circuits since they don't dump current
into the safety ground.
However, MOV suppressors don't present such dangers if they're installed at
the main panel and connected directly to the N-G bond. No system-wide
voltage differentials can be created by incoming surges from the utility
lines.
The remaining issue with MOVs is lifetime. The life of any MOV is determined
by its cumulative exposure to surge events (which briefly heat the MOV
internally). Every surge causes a chemical change in the MOV ... which also
tends to increase the amount of "idling" current it draws. Eventually, with
enough cumulative exposure, this causes the self-heating to enter "thermal
runaway" which causes meltdown, explosion, or fire. Since MOVs in power
strips have been shown to start a substantial number of fires, modern MOV
devices are "thermally protected" by a series cutoff switch activated when
the MOV reaches a predetermined temperature. These units are sometimes
called TMOVs and will also drive indicator lights showing that the MOV has
disconnected.
In practice, the MOVs used in a "whole house" protector (installed at the
main panel) are very large (about the size of a hockey puck) and are often
rated to withstand repeated surges up to 50,000 amperes or more. The MOVs
used in most "outlet" protectors are about the size of a nickel and rated at
much lower repetitive surge currents (these are the kind SurgeX "blows up"
in their demonstrations).
My experience with whole-house MOVs goes back to 1962 when I installed a
then new GE three-MOV module at our family home in St. Petersburg, Florida.
Because there was little in the way of "solid-state" equipment around in
those days, our primary failure due to lightning storms was light bulbs.
After the MOV module was installed, we never had another light bulb failure
during storms. As far as I could tell, the module was still working well
when the house was sold 14 years later.
Cordially,
Bill Whitlock, president & chief engineer
Jensen Transformers, Inc.
WELCOME TO JENSEN TRANSFORMERS, INC.
AES Life Fellow - IEEE Life Senior
(from my home office in Oxnard, CA)
"Marketing is, all too often, the art of deception by omission."
