[TowerTalk] Grounding, portable generators, field day
jimlux at earthlink.net
Thu Jan 20 17:44:45 EST 2005
At 12:18 PM 1/20/2005, Keith Dutson wrote:
>To me, the answer to your question(s) is simple. FD grounds should be for
>RF. It would be nice to have a safety grounding system with SPG,
I don't think you need SPG in this application..
And, I can't see why you should operate unsafely, with respect to AC power
distribution. Just because you're at field day doesn't mean you don't have
to meet various and sundry safety rules (such as RF exposure).
The reason I brought the issue up is that I think one can come up with a
general approach that works. It might be as simple as "use three wire
extension cords and GFCI". The wrench in the works might be that what
works OK from a personnel safety (i.e. three wire cords and GFCI) might
have some interactions with the RF that could be dealt with in advance.
What you don't want is somebody at field day saying "That darned ground
wire in the power cord was causing RFI on our 40m station, so I just cut
the pin off and it solved the problem."
I spent entirely too much time on film location shoots with generators and
old style pin connectors for the 120/208Y power in poor conditions to be
very confident that people will take the time in a field situation to
carefully analyze the situation. This is a classic "temporary installation
so much of the usual NEC rules are not applicable" situation. For instance,
it's perfectly according to code to massively overload the conductors,
because the underlying assumption is that the system is under continous
supervision by some trained person who will know what to do when the
insulation starts melting. There are also useful, but potentially
dangerous, things like snakebites and suicide jumpers, basically consiting
of cables with male connectors on both ends.
It is because of this sort of thing, on construction sites as well, that
the rules were changed: to require GFCI protection, and to ban single pin
connectors (these are essentially giant brass banana plugs about 1/2" in
diameter) in favor of Camloc types with interlocked receptacles.
So.. the real question is, what's a safe power distribution approach that
works in a field day environment: portable generators, power distributed
over hundreds of feed, coax running this way and that, that doesn't also
cause problems with "ground loops" or aggravate RF isolation and noise
I recognize the difficulty with lightning protection, but presumably, it
might be possible to construct the system so that if you DO get a hit
nearby, the damage is minimized. Heck, if the coax is laying on the ground,
it might provide a nice path (sure the coax is destroyed, but nobody dies)
Maybe there are easy things to do that will significantly make things
better, if only some thought is given to it.
As for the "if a storm approaches, take it down" approach... a
significant number of lightning fatalities in the US are from what are
called "bolts from the blue". These are lightning strikes where the path
has a long lateral extension from the cloud and then turns down to the
In a recent study (2005, Lengyel, et al), even though overall lightning
casualties are decreasing, some 54% of the lightning victims were
classified as having little or no warning of the approaching threat of
cloud or lightning. For instance, the stroke causing the injury or death
might have been the first stroke of a newly developing storm. About 40% of
the casualties were attributed to "poor judgement" (i.e. not getting out of
the storm.. )
> but few
>clubs have the people and other resources to put one together in a short
>amount of time. If you are operating FD and see a storm approaching, shut
>From: towertalk-bounces at contesting.com
>[mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Lux
>Sent: Thursday, January 20, 2005 1:45 PM
>Subject: [TowerTalk] Grounding, portable generators, field day
>Gosh... there haven't been any long threads on grounding in the last few
>hours, so here's another one...
>Here's the scenario:
>You're at field day, running off a generator. You've got extension cords
>strung all over the place to get AC power to your operating tents. You've
>got tower trailers or temporary towers up.
>What's the grounding approach?
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