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Earthing and the radio amateur

I've recently joined the IET forums. I replied to a  topic regarding amateur radio and PME on the old forum. Unfortunately I cant access the old forum anymore so I thought it would be best to start a new topic.


So. On the bench there is a transceiver with a metal case. Next to the transceiver there is a antenna tuning unit which is connected to a antenna system which is using an earth rod. Am I right in saying there is a chance of a potential difference between the two metal cases of the equipment regardless if it is a TT, TNS or TNCS?


What would be the ideal solution?  Put the shack on it's own TT supply and bond all the radio equipment back to the MET of the TT and if a earth rod is used for the antennas connect that back to the MET also?


If the TT system was not an option what would be best if the supply was a PME and you didn't want to use balanced antennas removing the use of an earth rod? 


To be honest I've never really given PME's and amateur radio much thought which in hindsight was probably a mistake. I do remember when I did my training an earth rod was recommended for the radios.


Apologies for being so random.


Stewart M0SDM
  • Yes, you are correct that regardless of whether the installation earthing arrangements are TN or TT, the earthed side of the antenna connected to its own earth electrode is classed as an extraneous-conductive-part, and should be main bonded.


    If the shack is a separate outbuilding, and there are no simultaneously-accessible extraneous-conductive-parts or exposed-conductive-parts of both installations, you can provide a separate earth electrode for the mains and provide a separate TT system ... or use the antenna's earth electrode if it meets the requirements for protective earthing.


    If it's an attached building, then it may also be possible to provide a separate TT system provided again there are no simultaneously-accessible extraneous-conductive-parts or exposed-conductive-parts of the two installations, but depending on what materials are used for the construction and foundations, there could be undesirable effects in some cases, where the main installation is a TN system, under some circumstances of distribution network faults.


    It's recommended that the antenna electrode and the electrode of the TT system are kept at a suitable distance from buried metalwork connected to the MET or protective conductors of the main installation - the guidance comes from Figure 16 of BS 7430, and indicates a separation distance of between 3.5 and 8 m ... although at distances less than 8 m you may need to take into account ground potential rise and reintroducing touch voltages in some circumstances.
  • Stuart

    Whilst the basic information given by Graham is perfectly correct, in a real situation you do not have the possibility of a (significant) potential between the Tx and ATU because the TX output coax screen connects the two directly unless it is of a curious design where there is deliberate isolation of the two items cases, perhaps by RF transformer. Connecting the two items to the RF earth electrode does provide an additional current path to earth for mains currents, but unless you manage a particularly good RF earth system, cannot carry very much current whatever the mains is doing. All of the shack equipment and the earth electrode are inherently bonded unless you deliberately decide to isolate them in some way, the only real question is the size of this bonding conductor. Depending on the type of aerial system you are using, long wire or dipole for example, there will be differing RF earth currents, and from the RF point of view the conductor to the earth electrode should be as short and large as possible. Unless your earth rod resistance to the MET is very low (measure it carefully, there may be a potential present, if so use an AC ammeter to calculate the resistance), the possible current will be able to be carried by the normal house wiring without problems. As you do not want to pass RF into the mains wiring, you may consider a suitable RF choke in the earth to the MET from the shack.


    If you have a transceiver with an external PSU it may be possible to isolate all the radio equipment from mains earth fairly easily, in which case nothing accessible connected to mains earth must be accessible when in contact with the radio equipment, as again there may be a potential between them, which under fault conditions may be dangerous, even if the fault is in the next street! Under these conditions it would be wise to make the whole shack a TT "island" with its own RCD and no MET connected earth conductors present at all, although this may be difficult if there is any pipework, radiators etc present which may be connected to the MET.


    The discussion is not so simple is it? Mains earths are also noisy when connected to long wire type unbalanced antennas, and verticals without radials, because the earth resistance is connected in series with the radiation resistance and has mains borne RF currents induced in it (in the HF range, 160 - 10m) particularly if the antenna feed resistance is also low at that frequency. BS7671 is not the ideal reference for HF radio installations because the RF requirements are also important. However the full range of safety aspects must be considered, along with the range of possible interference sources to make a satisfactory installation.


    Regards

    David G8FNR
  • 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.


  • 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.
  • 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.)

  • 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.
  • 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.

  • Clive Brittain:

    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.




    Agreed




    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.




    As long as the installation earth electrode is separated by a suitable distance from buried metalwork connected to the PME earth. See Figure 16 of BS 7430




    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.




    Agreed


  • Clive Brittain:

    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.




    Some of that needs to be read rather carefully - e.g.



    It used to be the case that all extraneous-conductive–parts in a house with  TN-C-S needed to be bonded, but with the recent changes to the IET Wiring Regs if you have a supply totally fed from RCD or RCBO protection devices, this requirement is no longer mandatory.



    could be very misleading (it's not obviously within the context of supplementary bonding within bathrooms),


       - Andy.

  • 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.