[UK-CONTEST] shack design

[Ian White GM3SEK]

Al wrote:
>In a message dated 27/03/06 10:13:21 GMT Daylight Time from uk-contest,
>paul at prolectric.co.uk writes:
>
>
>> Please be carefull about earthing, concideration about PME and seperates
>> for RF can cause problems and in some cases  be lethal.
>
>Indeed.
>
>Although the UK is not a major lightning zone in comparison to the states for
>example, it's worth incorporating a bulkhead approach to incoming RF cables
>so that they are bonded together and grounded such that any damaging currents
>flow past, rather than through, your radios.  I believe Ian 'SEK has 
>dealt with
>this matter in the past through his monthly Radcom column

June 2003, Jan 2005

>and also W8JI has a
>whole bunch of postings re the subject and his personal experiences on the
>Towertalk reflector at Contesting.com.  No need to subscribe as there's a
>searchable archive too.

W8JI and others point out that mains safety earthing, RF earthing and 
lightning precautions are three different things.

Mains safety is achieved by the green/yellow wires in the mains leads. 
It is always a good idea to bring these back to a single point, where 
you (or a breaker) can switch off the whole shack.

In the shack there is no such thing as a true RF ground. The best you 
can manage is RF *bonding* to bring everything to the same RF potential. 
This doesn't interfere with the mains safety earthing for each piece of 
equipment, but it cross-bonds everything on the table together at RF. 
The main requirement is very low inductance, so some kind of wide strip 
like PC board is in order [1].

The tricky thing is what to do with your RF ground connections coming in 
from the outside. Certainly plan to gather them together and cross-bond 
them to your single mains entry for the shack. But as Paul points out, 
this is actually an extra mains earth, so legal regulations apply, not 
only for PME but also for other types of earthing. However, while 
complying with the regs it is also important to avoid introducing RF 
into the mains, or mains-borne noise into the shack. It's about time for 
an update to G3RZP's article on PME, to include these EMC considerations 
and also other types of mains earthing.

Roughly speaking, whatever is good for both mains safety and RF 
grounding/bonding will be good for lightning protection as well - 
particularly the single-point entry and cross-bonding between mains and 
RF grounds. We're not talking about industrial-strength protection 
against a direct strike, but precautions against the much more common 
surges coming up the mains and phone lines.

Regarding conduit, it would be good to use at least three separate runs: 
mains, RF  and computer/control. The regs for conduit being used as a 
"cable tidy" downstream of the 13A plug may be more relaxed than for 
conduit in a fixed mains installation (perhaps Paul could comment?) but 
for EMC reasons it's a good idea to keep mains and other wiring separate 
anyway.

For conduit being used as a "cable tidy" it is worth looking at the 
comb-type stuff available from CPC in a variety of sizes:
http://tinyurl.com/p9729
Small wires can be threaded out through the combs, or you can break out 
a few teeth for the larger cables. Cables can also be tied on below the 
conduit.



[1] Looking for some wide, long strips of PC board for my own new shack 
too: about 6ft long by at least 6in wide, condition immaterial. Anybody 
got anything within reach of Stirling next weekend, or the M6 the 
weekend after?



-- 
73 from Ian GM3SEK