Earthing and the radio amateur

In reply to David Stone I have this to say. I was initially considering an external outbuilding as a radio shack. I have seen several garden sheds or old wash houses used as an amateur radio enthusiast's radio room. Ships' radio rooms may have confused me not being called shacks.  Now I understand that a radio shack can be an internal room in a house.


The items mentioned like radiators, storage radiators or metal pipes may or may not be extraneous-conductive-parts within a house. It depends on how well they are connected to earth. If plastic pipes are used a central heating radiator may not be connected  with earth at all, or very poorly. An electric storage heater will be earthed via its circuit protective conductor, but if the house is TT earthed then that is not a problem. So many new properties are supplied by plastic main pipes these days, such as gas and water so no main equipotential bonding within the house will be required. 


A whole house can easily be converted to TT earthing from PME earthing if required. That would just need the PME earth conductor disconnecting and a separate earth electrode installed in a suitable location and connected to the house main earthing terminal. RCD protection then is essential on all circuits.


With PME earthing that is why electricians are required to install 10mm2 main-equipotential-bonding

conductors to extraneous-conductive-parts in a house and other locations, to mitigate potential shock risks in case of a lost neutral on the incoming supply cable. And that does happen as has been reported on the IET electrician's forum.


I know little about amateur radio so am just speaking from the point of view of an electrician and BS 7671.


C.

Thanks for all those that have replied. It is all very interesting. It's clear to see there are lots of different opinions on the subject. I have read the RSGB leaflet.


If you were going to right a list of do's and don'ts regarding earthing for the radio amateur what would it contain?

I have to take significant difference with Clive Brittain on this one, simply because he does not appear to understand the difficulties with TT should there be other extraneous parts (such as central heating pipes or radiators, storage heaters etc.) in the shack. It is simple to TT an outbuilding but not part of an internal installation in a house. I think the worry about lost neutrals is excessive, there are many more serious significant dangers with high power radio equipment, and often much higher potentials available, as well as seriously dangerous RF potentials. The word shack does not mean shed, it is a technical term for the radio operating space! It is possible to TT the whole building installation, but this is not always wise where there are buried metal services which are extraneous to the building, or everything around the site is a TNC-S installation which must always be assumed. RCD protection in line with BS7671 2018, is suggested whatever the installation type. Unfortunately advice from average electricians about this kind of installation is very unlikely to be satisfactory, most struggle with normal TT installations in my experience. Items and the external earthing system may be separately bonded to an Earth marshaling terminal if desired in the shack, and thence to the MET. RF voltages on the house electrical system are very undesirable, and could be a fire risk, hence a series choke.

Even if one has beam antennas on a tower or mast the best solution is undoubtedly a good external earth system, and it is worthwhile from an operating point of view if any unbalanced antennas are used. As I mentioned low radiation resistance is important and makes the antenna pattern of directional aerials much more predictable, which can only be a good thing. TVI is also likely to be less likely. Burying a length of copper wire, pipe or tape the length of the lawn may be more rewarding than expected, and is likely to be much more effective than the normal earth electrode. It will also reduce the danger from lightning, by controlling the potential surrounding the antenna system. I have no difficulty bonding this to the MET, perhaps with a high current RF choke, in TNC-S installations, again serious mains voltages are much less likely than RF ones, which can cause both shocks and severe burns to the unwary. The household should be aware of this danger.


I have a good copy of Dud Charmans Aerial Circus DVD available which you may like to borrow as there is a lot of good antenna (and earthing for RF) material discussed. PM me if interested. The one on youtube is obviously ex 6 generations of VHS and useless! I made this many years ago for Dud, with the Bristol TV group, so as good as we could manage at the time, the youtube one is an unauthorised rip-off, I hold the copyright.


Regards

David CEng MIET G8FNR.

The Radio Society of Great Britain has a publication entitled "Earthing and the Radio Amateur".

http://rsgb.org/main/files/2012/11/UK-Earthing-Systems-And-RF-Earthing_Rev1.3a-.pdf


C.

TT earthing for an outbuilding is quite easy. You just do NOT export the earthing system from the TN-C-S of the house supply. TT is good for outdoor supplies such as caravans or farms etc. The supply is just Line and Neutral to the outbuilding, with say an earthed armouring of a supply cable from the house. At the outbuilding the earth is insulated back and does NOT continue to exposed conductive parts or socket earth terminals. The outbuilding is then earthed via a local earth electrode (rod) A 30mA double pole R.C.D. is normally used to provide shock protection. The earth fault loop impedance Zs can be up to 1666 Ohms. (Table 41.5 B.S. 7671) But anywhere around a stable 200 Ohms is good.


TT earthing of the outbuilding is good as no metalwork can be raised to a dangerous Voltage by an external fault on the supply cable to the house.


Exporting a TN-C-S earth to an outbuilding can introduce potential shock risks and should be avoided if possible, especially if there is metalwork that is in contact with the ground, true earth.


C.

Whilst these points might look great on paper, broadly follows BS EN 50310 practice we use for large electronic installations in buildings (e.g. computer rooms and data-centres etc.), and would certainly make things safe (electrically) in the shack, there are a couple of considerations:

1. The PME broken-neutral danger is moved from the shack to accessible earthed parts of the antenna outdoors (similar to the outside tap requiring a plastic insert if fed with metal pipes connected to PME earthing terminal) - the lower the antenna earth rod resistance, the lower the touch voltage and the smaller the risk.

• No-one has mentioned surge protection at all yet ... but if the main installation has SPDs, there should be a separate SPD on the feed to the shack ... and perhaps  query whether a separate SPD scheme is provided in the shack itself.
  
  (Or alternatively, whenever thunderstorms are forecase, simply disconnect and separate the antenna connector from the equipment and earthed / bonded parts in the shack if you want to save cash.)

Thanks for all the input. It is much appreciated. Personally I've always used dipoles and loops so no need for earth rods to be a part of my station! 


Clive I'm a bit ashamed to say I am a qualified electrician!! As I said in my opening post I have never given PME's and radio much thought. In 20 years I've never had anything to do with TT installations.


The main reason for all the questions is I have volunteered myself to give a talk at radio club on earthing in the shack. In my ignorance I thought it would be straight forward. Obviously it isn't straight forward as David pointed out. I can see on the night I am going to be met with one of my favorite lines  "It's been like that for years and never been a problem". I'll carry on with my questions on here and all being well when the time comes in April I'll be an expert.


One point that does need getting over is how dangerous a PME can be in fault conditions where a radio shack is in use.


A few thoughts on a shack that has got an antenna using an earth rod, in a property that is supplied by a PME

1.  An earth bar is installed in the shack all equipment is bonded to the earth bar and a 10mm2 back to the MET along with a 10mm2 from the earth rod to the MET

• Bond the piece of equipment that is connected to the earth rod antenna system where it enters the shack with a 10mm2 back to the MET?

• Supplementary bond with a 4mm2 across all the gear and back to the main earth terminal.

• Rely on the coax braid to keep at the metal gear at the same potential

I don't know if it's me making this be more complicated then it is but it certainly appears to me that the more I think about earthing in a radio shack the more complicated it becomes.

Purely from an electrician's point of view, a P.M.E. supply to a radio shack that has earthed equipment with conductive parts such as earthed cases that can be touched can present shock risks. The P.M.E. earth is connected directly to the neutral of the supply cable at the main intake meter position in the house. The neutral can attain a Voltage above true earth, as the neutral carries current and supply conductor cables' resistance cause a Voltage rise on the neutral. So, a P.M.E. earth terminal in the house can attain a Voltage above true earth, and this Voltage can appear on earthed equipment in the radio shack.


If you have conductive parts in the shack that are earthed via an earth rod at the shack, there can be a Voltage appearing between TT earthed equipment and P.M.E. earthed equipment. This is undesirable, and could be dangerous.


If the supply neutral was broken due to a fault before the house, the P.M.E. earthed equipment could reach a potential approaching full mains supply Voltage of 240 Volts in the shack. That coule be very dangerous.


TT earthing  is the safest method for a radio shack using a suitable earth rod and R.C.D.s. Normally an earth rod (electrode) would be at least 4 feet long, or two may be joined together to extend the length. Alternatively two rods may be used spaced at least the length of a rod apart.


It would be a good idea to get a qualified electrician to advise you and undertake the work.


If we bond metalwork in an outbuilding, such as a metal radiator or metal water pipe which are extraneous-conductive-parts, to comply with B.S. 7671, we would run a 10.0mm2 green and yellow cable back to the main earthing terminal in the house.


Generally it is best to keep TT earthed installations completely separate from P.M.E. earthed installations. So a fully TT earthed shack is ideal if it meets your radio requirements.


Test any R.C.D.s regularly using the test button to confirm their operation.


C.