From: Patrick Jankowiak (recycler@swbell.net)
Date: Tue Oct 05 2004 - 15:51:53 PDT
Here's a post from the Boatanchors list. GFI tripped when
operating a power supply which was grounded to a jeep. The jeep's
tires seemed conductive when measured. A nearby Ford Taurus'
tired did not condust. Keep in mind these guys are pretty smart
about measurements.
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--------- Forwarded message ----------
From: "Chris Bowne" <radiobwn@riconnect.com>
To: Old Tube Radios <boatanchors@theporch.com>
Date: Tue, 5 Oct 2004 07:17:03 -0400
Subject: Conductive Military Vehicle Tires???
Message-ID: <200410050717.AA156106896@riconnect.com>
We had a real head scratcher going at the NH Hosstraders hamfest
this past weekend.
While trying to run Dale, KW1I's jeep mounted GRC-19 from an
external 50A 28V DC supply from the AC power available from the
adjacent show barns on site, we kept tripping the GFCI supply
breakers.
The problem initially was solved by placing the external supply
on a plastic storage box to keep its frame (literally) off the
ground. Later in the day, after the dew started to set, the
trippng started again.
Now it would only trip coincident with connecting the DC power
cable at the T-195 DC front panel power input, even with the
radio off. So there was a ground fault at the jeep, but how?
Its sitting on 4 big rubber tires!!! Diconnected the external
80M dipole, no effect.
Well, out came the VOM, and lo and behold, readings at a tire
sidewall near the tread to the frame of the radio were in the 10K
ohm range!
We successfully solved our problem by rolling the jeep on to
small plastic sheets under each tire, which eliminated the GFCI
tripping and proved the leakage path was through the tires.
Out of curiosity, we checked the Goodyear Eagles on my 97
Taurus, which appeared to be essentially perfect insulators in
comparison.
Most likely the inital path the fault was taking was through AC
line bypass caps in the DC power supply to the frame of the unit,
which is why we had to "float" the system in the first place. Of
course, the frame was at ground potential for the negative side
of the output DC.
Is there something special (by accident or design) about military
tires that causes them to have a relatively low electrical
resistance? Possibly by design to prevent static buildup when
fueling or carrying ammo? The tires in queston were new, and had
not been driven on salt treated roads.
73, Chris, AJ1G
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Patrick
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