|To:||email@example.com, "'towertalk'" <firstname.lastname@example.org>|
|Subject:||RE: [TowerTalk] Grounding, portable generators, field day|
|From:||Jim Lux <email@example.com>|
|Date:||Thu, 20 Jan 2005 14:44:45 -0800|
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 problems.
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 ground. http://www.crh.noaa.gov/pub/ltg/crh_boltblue.php
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 down!
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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