[TowerTalk] station grounding question

[Ian White GM3SEK]

Eric Scace K3NA wrote:
>
>
>on 05 Nov 23 08:03 Ian White GM3SEK said the following:
>> [...snip...]
>>
>> Don't worry - INSIDE the station, you are not expecting large lightning
>> currents in the RF ground bonding, so the grounding bolts on the backs
>> of the rigs are already quite big enough.
>>
>> The purpose of ground bonding inside the station is to keep all the
>> equipment at the same potential as the panel, and you have already done
>> the right thing by using a wide, low-inductance strap for almost the
>> whole length of the run. What you use for the last few inches as a
>> jumper can hardly matter.
>
>    Unfortunately, most residential buildings are nearly transparent to 
>lightning-induced currents
>--  or any other kind of electrical activity.
>
>    Envision your home without any of the non-metallic materials. 
>Since most of us do not live in
>an enclosed metal shell/screen (i.e., a Faraday shield), all the 
>metallic bits and pieces of the
>building, plumbing, wiring, computers and radio equipment are simply 
>hovering in the space just
>above the ground, as far as lightning and RF are concerned.
>
>    Our buildings keep the rain out and moderate the climate -- but do 
>essentially nothing to
>protect us against electrical activities.
>
>    With this mental picture in mind, one can more easily grasped how 
>exposed our equipment and
>wiring is to electric-magnetic fields of all kinds, including those 
>produced by nearby lightning.
>

While not disagreeing with the facts that Eric states, I do think it's 
putting a possible problem ahead of the main Big Idea of lightning 
protection.

The Big Idea is to create a protected environment INSIDE the station, 
where voltages and currents caused by lightning are many orders of 
magnitude lower than in the big bad world outside... low enough to give 
the electronic equipment inside the protected zone a very good chance of 
survival.

As we know, this is normally achieved by three precautions:

1. the single-point entry panel, which ensures there is no other entry 
or exit point that would allow currents to pass through the shack; AND

2. good grounding outside of the shack, including the entry panel; AND

3. good bonding inside the shack, which ensures that all the equipment 
stays at the same potential (even if the local earth potential suffers a 
severe "ground bounce").

The possible problem that Eric identifies is if lightning current 
outside of the shack (either in the strike itself or in an outside 
ground conductor) induces a current surge directly into the shack. A 
similar situation created by electric fields would be a side-strike that 
drills straight in through the wall.

But beefing up the bonding conductors inside the shack is not the 
answer. It doesn't address the real problem - which is that your 
lightning precautions have been bypassed by a sneak current path passing 
  through the shack and your equipment. If you think this could happen, a 
much more effective solution is to prevent it, by re-routing the 
conductors inside and outside the building to reduce the magnetic (or 
electrostatic) coupling between them.

My main point was that if the Big Idea of lightning protection is 
working correctly, and your shack is a protected zone, it should never 
be necessary to use outdoor-sized busbars inside the shack as well. The 
priority inside the shack is not to carry big currents, but to provide 
low-inductance bonding back to the entry panel. For that you need large, 
flat conductors, or groundplanes on the operating benches - but it 
doesn't matter how thin they are.

It worries me to see big, solid copper busbars used for ground bonding 
inside ham shacks. They won't do any harm... but they almost certainly 
mean that somebody hasn't quite grasped that Big Idea.


(I'll be on the road tomorrow, and won't be back until Saturday UTC.)



-- 
73 from Ian GM3SEK