Friday, 31 March 2017

RadCom asks - me? - to help them out!

Last month, RadCom ran a second article about earthing an amateur radio station.

None of the following constitutes advice.  It is for you to engage qualified people to examine and install your system.

In this latest contribution, there was text, supported by a graphic, that showed, in essence, each piece of radio equipment attached to an earthing bus, connected to a stake/rod stuck in the ground outside.

Now, all amateurs are meant to have passed an examination which should mean they understand earthing to a reasonable level.  But, visit many shacks or listen to on-air QSOs, and you will quickly realise there are an awful lot of people who have absolutely no idea why they are earthing their equipment, or how to do it properly (i.e. safely.)

Unfortunately for RadCom and its author, who seem not to have thought an awful lot before publishing, they failed to include instructions on how to properly earth a shack that is served by what is actually a pretty common system in the UK - Protective Multiple Earth (PME).

The PME earthing layout, very common in the UK.

Setting up a shack with a ground rod attached to equipment in a PME-served household is potentially extremely dangerous.  The earth wire at the incoming mains is not a separate connection back to some remote earthing point, but one that is completed via a connection to the neutral wire.  If the neutral breaks during a storm or other failure, you could end up with a whole neighbourhood's supply trying to use your equipment and its questionable earthing to complete a ciruit.  Fire is the most likely outcome.

Solving this problem is not difficult.  You do the same as RadCom published - i.e. connect each equipment to a ground stake, but also add a separate wire that is connected directly to the stake, and runs back on its little own to the consumer unit. The wire needs to be a minimum of 10 square mm to handle the potentially high current that might try to use it, down to your earth stake.

Even then, the job may not be good enough.  I found that the very short earth wire between the incoming mains and the consumer unit was very thin - probably way below what modern regulations stipulate.  That was installed by the supply company about 40 years ago, so it's entirely their fault.  If your wire is similarly inadequate, you will need to ask the supply company to inspect and change the wire, hopefully without charge because it doesn't conform to their own regulations.

I pointed all this out to RadCom and the author of the article.  After a month, I received a sheepish e-mail from the editor of the magazine that said many people had been in touch about this issue, and would I consider writing an article to set the record straight?  I politely declined the offer, saying that they had better get someone with current electrical engineering qualifications to do so, and maybe pay more attention to proof reading and quality control in future.

So, if you have PME, do consult the document that RadCom and its author seemingly didn't, freely available here:

None of the foregoing constitutes advice.  It is for you to engage qualified people to examine and install your system.

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