Bonding to Earth
Dear Don,
The misprint in the Making a start on the 630m Band article risks safety and needs correcting. The article states “… do not link the transmit earth and the house mains safety earth.” This should, of course, have read ‘do link’ the two earths. The Full Licence Manual page 16 (RSGB 2018) states “...you must take adequate precautions. The most common recommendation is to bond the RF earth to the Main Earth Terminal using cable of at least 10mm2.” Further literature is referenced.
The danger arises with TN-C-S Protective Multiple Earth (PME) supplies. Everything in the house has to be bonded to create an equipotential like a Faraday cage. An open-circuit neutral, not necessarily on the amateur’s own premises, allows this cage to float up to as much as full mains potential. The RF ground is still rooted in ‘Mother Earth’ so possibly 230V now appears between the mains-earthed case of the transceiver and the RF earth terminal coming into the shack from outside.
Don’t be fooled by the reassuringsounding name of the PME system and
don’t follow published guidance that applies to different systems installed in other countries.
I’m lucky in my pre-War suburban house to have old-fashioned TN-S where earth is just about, well, earth. At least it was where grounded at the transformer sub-station end and my house earth is connected to the armour sheath of the distribution cable. You can tell if it’s PME by the absence of a neutral service link. Godfrey Manning G4GLM Edgware ( Editor’s comment: It wasn’t a misprint Godfrey, but you make a good point, specifically where PME systems are concerned. I checked with Peter G3RZP, who wrote an article for PW a few years ago on the subject of PME. He says, “Where you have PME, from a safety viewpoint, the radio system earth MUST be bonded to the mains earth. The danger with that is the common impedance between the two allows noise on the mains earth to be effectively coupled into the antenna circuit. To get around this, the bonding connection (which needs to be a substantial conductor) needs to be threaded through a large chunk of ferrite, especially when considering 475kHz. The TT system, with the mains earth totally separated from the radio earth, and a suitably low earth resistance for the premises safety earth is by far the best.
“As an example, the feed to my house has the neutral earthed at few points, including at the end of the run, to a 4ft earth rod at the base of the overhead feed pole in my garden. The house and radio earths are bonded but the neutral floats at whatever..... A few years back, I was having trouble with the RCBO tripping when putting the kettle on. Different kettle, same problem, plus lights dimmed when putting the kettle on. Called the electricity people at 1400, and a man comes out and measures the mains. Tells me that at 234V it’s OK, at which I told him it wasn’t because it’s usually 239V. Then I put one ring of the electric cooker on and the mains was 180V.... I got him to measure the difference between house earth and neutral with the result that the neutral was 50V above the house earth and that in spite of the earth rod in wet Oxford clay. The man said that “it needs the overhead gang, and they are on a job the other end of Wiltshire!” They turned up at 2200 and eventually found a high resistance joint between copper and aluminium conductors in the neutral up a pole... on a cold pitch-black night. The nearest streetlight is five miles away! Still, at least it wasn’t raining. Had I had a typical PME installation, the return current would have been through mains cables for the rigs!
“Possibly worth noting that this all applies only to the UK, and other countries have different systems and requirements.”
I think, in summary, good advice would be, if you run the earth arrangement as shown in the article, then the whole property you’re in should be protected by an RCD. Without an RCD you should bond the RF earth to the local mains earth to ensure safety in the event of certain mains faults. Take advice from a qualified electrician if in doubt.)