>When I mentioned earlier that a hi-pot tester should be used I had
>that the ICE units have a coil to ground. A hi-pot tester would do no good
>With Polyphaser units there is no coil to ground, only a gas tube. A hi-pot
I have a whole bank of Polyphasers and they are all solid state. Not a gas
tube in the bunch. My Alpha Delta switches are listed as having gas tubes.
>tester is what Polyphaser recommends for testing their units. In fact they
>even sell one for that purpose. It is called FIST.
>To test MOVs Polyphaser also recommends the hi-pot tester. It is true that
After working with MOVs in industry for many years I find this suprising and
it raises a bit of doubt.
>the normal failure mode is shorted but they can fail open too.
>The low current ( a few microamps) from a hi-pot tester will not harm them.
According to industry literature; _any_time_ an MOV is taken above it's
normal peak operating voltage there is a degradation of the device. It's
admittely very small (depending on how high you go), but for that reason my
GE manual recommended not to test them with a hipot device, or do it no
more often than absolutely necessary. Typically MOVs are rated by working
voltage and peak rated voltage and not a break down voltage as they don't
breakdown even when clamping. (Unless they have changed the wording since I
retired) They are a Metal Oxide Varistor which is a voltage variable
resistor made of metal oxide. This oxide is basically many thousands of tiny
diodes. Below a given voltage they conduct very little. Above that
threshold the resistance of the device varies inversely with the voltage
increase, but the curve is not linear and at some point will have a rather
sharp knee which has sometimes been referred to as the break down voltage
although it's not truly a break down voltage. How many of us would want to
test a bunch of diodes with a hipot tester?
I don't know if they are still available but GE used to give away a
manual/catalog that covered the theory, construction, and handeling of the
devices as well as a listing of what they sold.
When using an MOV on AC lines we had to calculate the peak operating voltage
from the RMS. IOW, at 110 VAC it was 110 * 1.414, or 156 volts plus a safety
factor to account for line voltage variations. If you know the highest RMS
in your area (I've seen as high as 127 which is out of spec) then use that
for a base. 127 VAC works out to about 180 Volts peak. So you need to have
the voltage rating high enough to allow for slightly more than normal line
excursions, but low enough to protect the equipment from higher voltage
spikes that could cause damage.
>The capacitors (if they are ok) will withstand higher voltages than the
>breakdown voltage of the MOVs.
When I mentioned the 50,000 Joule MOVs in an earlier post I really wasn't
BTW (Over 20 years ago) we used to run those 50,000 Joule MOVs across a pair
of water cooled, 1200 Amp SCRs running at 480 VAC RMS. The MOV was about 2
X 3" and 1" thick mounted on a 1/8" thick aluminum plate. If the down
stream taps were changed under load the results were a thing to behold.
Parallel 500 MCM cables jumping as if they were spastic snakes, a fireball
accompanied by *lots* of noise when the MOV blew, and several ton
transformers actrually trying to bounce on the concrete floor. It was a
truly spectacular event <:-)). Expensive too as well as a tad hard on the
nerves if you were working close to one.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com (Use return address from home page)
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of K8RI on TowerTalk
> Sent: Wednesday, August 09, 2006 12:46 AM
> To: TowerTalk
> Subject: Re: [TowerTalk] Testing ICE arrestors
> Test Metal Oxide Varistors (MOVs) at their working potential. Every over
> voltage, even testing, degrades them.
> I would caution uses of Polyphasers not to use this method of testing.
> Normal MOV failure mode is shorted. If they have failed open it is
> quite evident.
> MOVs failing open are usually nothing but a pair of leads pointing to
> the MOV had been located.
> You should try standing next to a 50,000 Joule MOV when it lets go. <:-))
> Roger Halstead (K8RI and ARRL 40 year Life Member)
> N833R - World's oldest Debonair CD-2
> www.rogerhalstead.com (Use return address from home page)
> The surge suppressor for ICE rotator controls uses MOVs in parallel with
> capacitors for each lead. Typical failure modes of each is a short
> circuit or low resistance. I wouldn't hi-pot them, either. Energy from
> the hi-pot may cause an MOV to fail if it is applied for too long a time.
> 73 de WOØW
> Jim McDonald wrote:
> >How can one tell if used ICE coax arrestors or 8-conductor control cable
> >arrestors are still good? DC continuity on the center conductor of the
> >arrestors? DC continuity of the control cable arrestors? SWR connected
> >a dummy load for the coax arrestors?
> >Jim N7US
> >TowerTalk mailing list
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