From: ke6myk@sbcglobal.net
Date: Tue Mar 09 2004 - 21:56:08 PST
"ygmir" <ygmir@onemain.com> wrote:
> Any way, I tested the plug (round, 8 connector half male, half female) ( now
> I know you're laughing....).
Hmm, interesting. My S250 has two 4-pin connectors, one for 120V AC
and one for 28V DC. If I'm not mistaken, the AC connector uses three
of the pins, one of them being grounded. There's also a big stud with
a wing nut on the cable entry panel for grounding. My shelter is
configured to hold an AN/GRC-122B radio teletype set. Maybe yours was
set up to hold something else, and thus has different electrical
connections.
I guess I'd need to know more about your shelter, which sounds like
it's different than mine, before I could offer much specific advice.
In general, I'd check for the following things:
1) Determine which pins are (or at least should be!) connected to the
AC hot, neutral and ground lines. You may have to trace wiring from
convenience outlets in the shelter (the round or U-shaped hole is
ground, the smaller hole is hot, and the larger hole is neutral),
study wiring diagrams for your specific shelter (maybe in manuals for
the system that was built into your shelter, maybe on diagrams on the
inside walls, etc.), get information from somebody familiar with your
version of the shelter, etc. You may need to do some or all of the
above, looking for any discrepancies.
2) Make sure that your power cable is wired properly (i.e., ground,
hot and neutral from your power source are connected to the proper
busses in the shelter).
3) Make sure that your power source is wired properly (yes, even if
it's an outlet in your house). You can use a meter, and/or a plug-in
tester from the Home Depot (or similar). You could have an open ground
or neutral in the house, swapped hot/neutral, etc. Ground and neutral
should be bonded together in exactly one place in the house, in the
main service entrance panel. Thus, if wires were perfect, there would
always be a zero-volt potential between ground and neutral in the
house. In real life, wires have resistance, so there will be a voltage
drop in the neutral wiring in the house in circuits that are under
load. Thus, you might see a small voltage between neutral and ground
(under a volt), because if everything is wired up properly, current
should flow through the neutral line but NOT the ground line.
4) Step one verifies that the shelter is wired properly, step two
verifies that the shelter is connected to the house or other power
source properly, and step three verifies that the problem isn't in the
house or power source. If the problem hasn't been fixed by this point,
then you need to look for a ground fault or open ground in the shelter
somewhere. Ground and neutral must NOT be bonded together in the
shelter. The incoming ground line MUST be connected to the shelter
body at one point. Although there should be little or no voltage
measured between neutral and ground in the shelter (with the small
voltage varying depending on the loads on the circuit), there should
NOT be continuity between neutral and ground in the shelter. There
should especially not be any continuity between hot and ground in the
shelter! Unplug or disconnect any loads in the shelter, and trace down
the ground fault by a process of elimination, through physical
inspection of the wiring, resistance measurements (with the power
disconnected!!), and voltage measurements.
It used to be customary in older electronic equipment for both AC
lines to be capacitively coupled the the metal case. In modern US
120VAC electric power systems where the two AC power lines are not
balanced with respect to ground, this places the case (if not
otherwise grounded) at a potential of around 60VAC, although the
connection to the AC line has high impedance, so touching the case
will generally cause a tingle without killing you. When I get such
equipment, I disconnect those caps and replace the power cord with a
3-wire cord, connecting the case to the ground lead. Maybe you have
some piece of equipment like that (or even a noise filter with that
configuration) in the shelter, and it's energizing the shelter body.
These are my best suggestions based on my own knowledge and
experience. I'm an electrical engineer, a ham radio operator, and I've
studied the National Electrical Code a bit for home projects. Still,
I'm not a professional electrician or a licensed Professional
Engineer, so you may or may not be wise to accept my advice. I would
have replied to you off-list to avoid starting a thread which wanders
away from the topic of military vehicles, but I decided to post
on-list so that if I've made any blunders somebody might catch it.
Please contact me off-list if you have any more questions about what
I've written. On the other hand, please replay on-list if you think
I've given any bad advice!
-- Mark J. Blair, NF6X <ke6myk@sbcglobal.net> PGP 2.6.2 public key available from http://www.keyserver.net/ Web page: http://www.qsl.net/ke6myk/
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