[Boatanchors] Testing a ground connection

[Steve Uhrig]

Once upon a midnight dreary, James Cundiff <dadofpetie at comcast.net>
pondered, weak and weary:

>     I work for the local utility here (Baltimore Gas and Electric). Two
> years ago when I installed my station grounds I ask the overhead guys
> (linemen and supervisors) for advice. Here is what I was told and/or
> learned. 1.) ALL grounds in the house must be the same potential (tied
> directly together)  It was stressed to me that if the radio station
> grounds and the house grounds were different potential, The station was
> NOT grounded!

This, of course, is true. Points grounded at different places, like a 
ground rod and electric service ground, well may read 20 or 25 volts drop 
across ground.

Tie your Amprobe around various grounds if you have more than one. If you 
see currents not identical, you have a potential problem.

Another thing -- RF grounds are a different breed of cat than 50/60 cycle 
mains grounds. Electricians understand mains grounding, from a safety 
point of view.

For protection of the station, you need to look at the characteristics of 
RF. Calculation presumes lightning is about a megacycle IIRC. Due to the 
tremendous currents induced in a nearby strike, figure what 1000 amps 
will do with 1 Ohm of ground resistance. 5 Ohms? The voltage dropped 
across that ground will eventually find its way back to ground, possibly 
through your equipment. 

If all points are tied to a common ground, there will be essentially zero 
Ohms drop ACROSS ground.

I have a career electrician working for me. Once I was able to break his 
mindset on 60 cycle versus lightning and power grounds and surge 
bypassing, etc. and get him thinking in terms of single point ground, we 
did some marvelous work. Places which died during any electrical storm 
never again failed after being properly protected. Our EOC (next county 
up from Bro. Jim) uses Polyphaser products and installation to my spec. 
We have our UHF and VHF amateur antennas at 285 feet on a 300 foot tower 
which itself is 660 feet ASL, and we've never lost a piece of equipment.

I've written an amateur-specific White Paper on grounding commo systems. 
You can find it here:

http://www.swssec.com/white_paper.html

one of the articles near the top.

Lightning observes skin effect, so you're better with copper strap than 
copper wire or braid. 1.5 inch is standard, though they make it to 6 
inches for long runs to keep the impedance down. Grounds saturate, so 
ground rods (note plural) generally are separated by twice their length. 
More in the article.

Impedance? Yeah. We're dealing with AC, not DC, so it's impedance, not 
resistance. The tiniest bit of reactance raises impedance. Good thing to 
remember. Also, surges and lightning do not turn corners or generally 
cooperate with your way of doing things. Gently radius any turns. 
Wrapping a wire around an eave will be the point at which lightning 
decides it will not make the bend and will shoot straight out.

I know this is way off tangent from the original question, but it's 
pertinent.

We've taken a lightning hit here which has scattered several cords of 
firewood around the yard. None of the radio equipment was affected, but I 
did have to replace the phone line protectors. Cheaper than replacing 
every modem and fax on that line.

Good input, all. It's nice seeing old and new friends on this list. 
Recently I renewed a 25+ year friendship with a former coworker who 
happened to post on this list. And now Jim KB3GFC. Great!

Steve WA3SWS


*******************************************************************
Steve Uhrig, SWS Security, Maryland (USA)
Mfrs of electronic surveillance equip
mailto:Steve at swssec.com  website http://www.swssec.com
tel +1+410-879-4035, fax +1+410-836-1190
"In God we trust, all others we monitor"
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