nuckolls.bob(at)aeroelect
Guest
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 Posted: Mon Mar 23, 2015 9:06 am Post subject: Busy contacts are happy contacts |
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| Quote: | Turned out the main bus CB (original factory unit in my '65 185) had
opened up without popping the indicator button. I was dumb-founded
by that as it was about the last thing I checked.
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MAIN BUS BREAKER? What's that?
I've experienced two instances of old switches 'going open'
with age. Contact corrosion is always 'thicker' than the
surface from which the corrosion is developed. The two
instances I personally encountered were on low current,
press-to-test switches that were not routinely operated . . .
and when they were operated, their 'high current' (silver
cad) contacts did not benefit from the naturally occurring
arc that has a cleansing effect on thin films of naturally
occurring corrosion.
Do you still have the old breaker? I'd love to have it
for an autopsy.
The first time I saw the effect, a tech had already pulled the
switch and replaced it. I cabbaged onto the carcass and
discovered that it was a perfectly good but seldom exercised
switch loaded with too little current. A constant current power
supply set for 10A was 'shorted' with the suspect switch
and wig-waged a few dozen times. Contact resistance fell to
catalog limits and the switch was probably good-to-go for
another 10-15 years.
The second time I saw it, I was able to advise the tech
and suggest an experiment before we pulled the switch
from the panel (and gigged the customer $300). I hooked
a CC power supply across the open contacts and wiggled
the dead-short switch a few dozen times. Again, contact
resistance fell to milliohms and the switch was 'refurbished
in situ'.
Years ago I recall advising techs who attended my
seminars that one of the best things they could do
for an aging breaker panel was to exercise the all breakers,
under load if practical, every time the airplane showed
up for service. An exercised contact is a happy contact.
Unfortunately, breakers in that era of Cessnas were
push-to-reset only. To 'clean' those switches, the techs
were advised to clip-lead a 30A load resistor to the downstream
side of the breaker . . . force it open a couple of times.
Bob . . .
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