Much more can be said about the voltage rating of these power resistors. The
failure mode resulting from over voltage operation is an open turn (and there
are a bunch of turns, increasing the probability that one will open).
Why this happens is unknown to me but I have seen it happen in my own
equipment frequently enough that I will not use them any longer. On the other
hand,
many lower powered Metal Oxide type resistors in series have proven to be
highly reliable; just a bit more work to install.
A related issue is voltage rating of the voltmeter multiplier resistor(s).
They can have a similar failure mode and make you assume the voltage is
discharged when it in fact is not. One high voltage power supply failure mode
seen
here is a stuck mains power contacor that left the high voltage on for several
days (or weeks?) and the voltmeter multiplier opened up during this time. While
troubleshooting the no high voltage indication, thankfully the shorting stick
made enough of a KAPOW! that it was obvious that something was wrong before
disaster struck.
So the lesson was: 1> unplug the power supply from the AC power source, 2>
short the filter cap, 3> short the filter cap again, 4>remain paranoid.
Solid state relays can be even more dangerous. They can (and usually do) fail
shorted and you can't tell by looking at the contacts or listening for a
relay click on/off.
73,
Gerald K5GW
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