RE: [TowerTalk] Grounding, portable generators, field day

[Jim Lux <jimlux@earthlink.net>]

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.. )
http://ams.confex.com/ams/pdfpapers/85775.pdf

>  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!
>
> Keith NM5G
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Lux
> Sent: Thursday, January 20, 2005 1:45 PM
> To: towertalk
> 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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