Re: [RFI] Quiet Shack PC Systems?

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

Jim Brown wrote:
> On Thu, 18 Mar 2004 21:40:58 +0000, Ian White, G3SEK wrote:
>
> > Then feed each side separately through its own
> > *three-wire* mains filter that includes a ground choke.
>
> Hmmm. Are you putting a choke on the ground lead? Doesn't that prevent
> the protection it SHOULD provide for HF transients like lightning and
> power line misbehavior?  I can't imagine that being blessed by an EE
> that can read and write. All codes I know of require a low impedance
> bond between grounded objects. That choke has a high impedance at some
> frequency.
>
> Or do I misunderstand what you are suggesting?
You understand exactly what I am suggesting... but I believe you might 
be misunderstanding the purpose of the mains ground wiring inside a 
building.

As ever, this has to be looked at from three different viewpoints:

1. Protection against wiring and equipment faults at 60/50Hz - which is 
the *only* legitimate purpose of the mains ground wiring inside of the 
building.

2. Protection against lightning - which is achieved only by the 
connection to an earth spike where the mains feed enters the building.

3. RFI - in which the mains ground is often part of the problem, not 
part of the solution.


Your NEC (unless the misunderstanding is all mine :-)  requires a bond 
between your mains ground wiring and an earth spike at the mains entry 
panel. From what I've read, your outside RF ground connection is 
required to be bonded at that point too. Better still is to make this a 
common entry point where all your RF feedlines and other services enter 
the building.

This single-point grounding makes the very best sense from the viewpoint 
of lightning protection, because it keeps the surge currents outside of 
the building, and minimizes potential differences inside the building.

Certainly there should be another cross-connection between the RF ground 
and the mains ground at the shack - but only for the purpose of 
protecting against 60/50Hz faults. The ground chokes in three-wire mains 
filters are rated for that service.

But from the lightning and RFI viewpoints, it is highly *un*desirable to 
cross-connect the mains and RF grounds at the shack. If you do, you are 
creating an alternative pathway for lightning and RF currents - a 
pathway that now goes right *through* the building wiring, and probably 
through your electronics.

As almost always happens, the requirements for lightning protection and 
RFI reduction line up perfectly. High-impedance RF chokes on all three 
mains wires are exactly what's needed for both purposes.

This technique is a perfect complement to single-point grounding. The 
earth connection at the entry panel provides the low-impedance shunt 
path that you want lightning and unwanted RF currents to take - and that 
is outside of the building. The filter chokes are a high impedance on 
the path through the mains wiring, that you want those currents *not* to 
take.

I hope you'd find that an EE who really knows the NEC would understand 
that, and would find no objection to a three-wire mains filter that has 
the correct current rating and compliance markings.


--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek
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