[UK-CONTEST] shack design

Tue Mar 28 14:52:38 EST 2006

Hi Ian and all, I guess many will know much of this anyway but without 
getting too technical it might be usefull to some. On cables in conduits 
etc, my  initial thoughts about this was confirmed when I checked with the 
ECA and IEE. Any mixing of cables of different bands in a conduit or 
trunking system no matter where in the system is deemed to be against the 
16th edition. However bunching of flex's etc in a non permanent cable tidy 
arrangement,(cable tied together or spiral wrap etc ) is usually within the 
regs. This could be a tv with av amp, video etc plugged into a 13A socket 
outlet with the usual rats nest of cables behind. This said I would still 
like to see the seperate bands kept together etc.but practically very 
difficult to achieve. This assumes the correct flex and cabling is used for 
those services of course.
On Earthing, the big mistake is when two or more earthing systems are used 
in the same property and the risk that a lethal potential difference between 
these could occour. For example , you run power into a shed at the bottom of 
the garden, your main house earth is under the stairs via the supply 
authortiy , ( there are different systems   tns, tncs etc all forms of pme.) 
You plug your rig into the 13A socket outlet, perhaps your aerial is an end 
fed wire which is tuned against ground. You have an earth cable connected to 
the earth lug on the back of the rig  and connected to ground via an earth 
spike just outside the shed. Here is where it all goes horribly wrong, that 
earth spike could cause a lethal potential difference to occour, it could 
effect dissconnect times of the supply voltage. The standard advice is to 
have only one earthing system in your property and take only earth 
connections from that point. ( This is often not achievable especially with 
this shed model ) and probably useless anyway in terms of  RF because of 
impedience issues. There are ways to achieve this with the use of a TT 
system  rcd's and earth spikes TIED together but all works of this nature 
will effect electrical safety and must be undertaken by properly qualified 
people and tested to the current regs.with the appropriate test certificate, 
I hope this is of help. 73 Paul G6PZ
----- Original Message ----- 
From: "Ian White GM3SEK" <gm3sek at ifwtech.co.uk>
To: <uk-contest at contesting.com>
Sent: Tuesday, March 28, 2006 10:09 AM
Subject: Re: [UK-CONTEST] shack design


> 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
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