Mains Earth/RF Earth

[VK5TM]

I'm having the shed wired shortly (for electricity) and have been having a conversation with the electrician about earthing, both mains and RF.

I have the obligatory seperate earth stake for RF earth, but the electrician believes this will cause problems both in a regulatory sense and RF interference sense in that the house earth stake is some 20m from the RF one.

He wants to put in a special circuit just for the Ham gear using the round earth pin sockets and not having the earth pin wired.
I can see problems where mains powered test equipment (on a normal circuit) is being used on any gear using this special circuit.

I suggested wiring the shed without the earth from the main switch board, but he is not happy to do that.

Any comments based on practical experience on any of the above?

[VK4ADC]

Terry

The 'old' technique was to have a direct RF earth which was to include a high-current plate-style mica capacitor in series to the earth stake. The capacitance value was chosen such that it was a high impedance/reactance at 50Hz but low at the lowest frequency in use (160m ?).   That way there was no issue with corrosion etc due to multiple earth points and distributed 50Hz current.

The problem is two-fold though.  The best style of flat mica sheet capacitors are no longer available - at least new. The second point is that unless that capacitor style goes short-circuit in the presence of a nearby lightning  strike, it won't protect your gear. If it is blasted into smithereens by the initial surge current, where is the next path to ground going to be ? Notice I don't mention a direct strike above - nothing really helps then, except contents insurance !!

I have a 1.8m copper-clad rod just outside my shack building and it is fed via a short-as heavy conductor (thick-ish flat bar is best) through the wall to a continuous strip of aluminium angle screwed along the back edge of my shack benchtop. All the radio gear is then connected via flattened RG213 braid pieces onto 1/4" countersunk screws set upside down into the aluminium angle and tightened down with wing nuts.  It is not a capacitor-isolated RF-earth but I figure that it will outlast me even if corrosion occurs.

The shack coax cable entry mounting strips also get returned to the aluminium angle section - although I do not expect any of it will survive a direct strike. Certainly provides a reasonable grounding effect at HF though.

Doug

[VK3RX]

I think your shed and radio gear should be wired up to "code".

What you do for an RF earth is another matter (what Doug suggests in his para 3 is good) and as the eleco indicated, regularoty-wise the RF earth should probably not be connected to the premises earth.

FWIW the plumbing here is all buried copper pipe, and the switchboard earth goes to an earth stake which itself is connected to the pipe system underground. When setting up my station, initially I had my RF earth connected to a copper pipe that came up out of the ground and into the house via a hole in the brickwork, on the corner where my radio room is.

50Hz hum resulted, so I put in a separate RF earth system along the lines that Doug has suggested.

[VK3PY]

Should one wish to isolate the RF earth from the mains earth with capacitor as Doug suggests, it's quite easy to fabricate a suitable capacitor. I've measured the capacitance of an A4 sheet of aluminium insulated from the roof of my car by a thin sheet of Mylar. The aluminium sheet was held in place by several magnets. The measured value was 6nF. That works out to be 530 kOhm of reactance at 50Hz but just 7 Ohm at 3.5 MHz [This was my very effective, non-invasive earth connection for a HF mobile whip mounted on a roof rack]. It's quite feasible to interleave a few such sheets of aluminium (or copper or brass if you prefer) with Mylar film sheets to achieve proportionally higher capacitance. The main thing is to ensure the sheets are tightly clamped together in order to maximise the resulting capacitance.

Voltage rating is not an issue. As Doug suggests, should a lightning strike occur, it's a good thing for the dielectric to vanish, and in any case, a solid bolt of lightning will destroy anything in its path.

Chas
VK3PY

[VK5TM]

Thanks for the replies so far.

I did think about the capacitor connected earth (and still thinking about it).

"....regularoty-wise the RF earth should probably not be connected to the premises earth."

Yes, that is the case, but my thoughts were about some older gear that I might want to use that has both the mains earth and RF earth bonded to the chassis.

Newer class 2 (double insulated) gear obviously doesn't have that problem, but there's none of that newer stuff to be found in my shack (and recent lottery wins didn't even provide enough for a decent coffee  )

Maybe I should just run the lot off batteries and solar panels   .

[VK2KRR]

How about a read of this from W8JI
https://www.w8ji.com/station_ground.htm

[VK5TM]

I forgot about that one Leigh, thanks.

Reading the Communications Cabling standard (AS/CA S009:2013) shows a method for adding earthing arrangements to various communications equipment by tying into mains earth (including equipotential bonding).

Couple of different setups depending on noise vulnerability etc.

But that depends on several other things needing to have not changed in the latest AS 3000 standard (it's possible the new requirement for all metal framed buildings to be earthed may negate the above).

[VK5TM]

Finally got a copy of latest AS3000, what a delightfull piece of publishing (not).

Anyway, it turns out I can have a seperate protective earth stake installed at the shed for the mains, with just an active and neutral feed from the house switchboard.

I can then have a functional earth system fitted that can be used for RF earth as per AS/CA S009.

Of course, now the electrician is on holiday. Ho-hum.