Date : Thu, 21 Feb 1985102:46:00-MST
From : d3unix!jhs@mitre-bedford.ARPA
Subject: MOV transient protectors
" What is the proper configuration for hooking up an MOV to a power
line? (i.e. in series or in parallel?).
If it hooks up in parallel to the power line, why wouldn't one MOV
work for your entire house? It seems that it would, because all the
lines in the house are connected to together in parallel.
Joe "
The MOV is a shunt device: it has a high impedance until a transient
causes a large voltage to appear across it, at which point its
resistance becomes small, or more precisely, it prevents the voltage
from getting much bigger by shunting a large current through the MOV.
To protect equipment with MOVs connected to the AC line, you would
want to choose an MOV rated for a voltage which would not NORMALLY
appear across the line. Be sure to find out (does anybody know?)
whether the MOV is rated in terms of instantaneous peak voltage or RMS
line voltage. The peak voltage is sqrt(2) times the RMS value of 115
or 120, thus about 170 volts. To allow for manufacturing tolerances,
an MOV which limits peaks to about 190 or 200 volts would probably be
about right.
Another parameter you need to choose is the maximum shunt current you
need to handle in the MOV. The right choice depends on the
application. For EMP protection, FEMA recommends a rule of thumb of
25 to 120 Amps per FOOT of exposed conductor if the conductor is
vertical and 5 to 10 Amps per foot if it is horizontal. In either
case, a practical limit would occur at about 2000 Amps because more
than that will arc over around even your MOV. So probably a 2000 Amp
peak current rating would handle most transients in the real world.
I'm not sure just how LIGHTNING transients would differ from EMP, but
it is generally claimed that the risetime is longer (slower) for
lightning. I suspect that the above rules of thumb for EMP would
result in a conservative design that would handle lightning nicely
also.
The MOV would be connected across the line (in parallel, in your
terms). A single MOV at the power entrance point would be very useful
if all wiring beyond that point is in metal conduit. However, note
that MOVs may become ineffective after being zapped a couple of times,
so it might be well to install several of them in parallel at the
entrance point. It is also a good idea to inspect or test them
periodically.
Bear in mind that both EMP and lightning are fairly wideband
phenomena, EMP particularly extending well into the VHF region.
Because they are so wideband, they act like (they in fact ARE) radio
waves and can get picked up by even a few FEET of conductor betwee the
"central" MOV and the device in question. MOVs at various other
points in the system would provide better protection to specific
(expensive) equipment especially if the entrance-point MOV(s) failed
or if the distribution wiring in the building is not shielded.
The LEAD LENGTH and inductance in series with the MOV will greatly
reduce its effectiveness in limiting voltage peaks. It is best to use
techniques similar to those used in building VHF/UHF amplifiers: very
short leads, wide "straps" rather than ordinary wire, or else large
diameter conductor, avoid bending the conductor (because bends
introduce inductance) etc.
Additional protection can be provided more locally by using Zener
diodes on low-voltage lines and gas gap devices on antenna feedlines.
NOTE THAT AIR GAPS ARE NOT EFFECTIVE against EMP transients because
the risetime is much faster than the time required to ionize air. Buy
a gas discharge device specifically designed for EMP or lightning
protection, and then check it periodically (after every major
electrical storm, for example).
I hope the above is helpful. Please note that I am only an "instant"
expert on this subject rather than a "real" expert, so I may not be
able to give you much more detail than the above. However, I would be
glad to try to answer any further questions or point you at someone
who can do so.
73, John Sangster,
W3IKG jhs at
mitre-bedford.arpa