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Earthing and the radio amateur

Stewart Mason
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
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    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.



    Although PME is often held up as a worst case example - there are similar dangers from having any two unconnected earthing systems within reach of each other - regardless of which type they are.


    With any TN system a fault between a line conductor and true earth (but not a c.p.c) can result in the star point being pulled a long way from zero volts - which is then imposed on the DNO's earth equally in TN-S and TN-C-S systems.


    Likewise standing leakage currents in a TT installation can raise the MET and everything connected to it significantly above zero volts - no more than 50V if the installation is in accordance with BS 7671 and all RCDs are functioning correctly, but another TT installation could be at held at a different voltage again. In the worst case if the other installation is on another phase, you could have as much as 50V √3 = about 87V between them.


    Then there are a whole raft of faults (L-PE on any TN system, plus L-N on TN-C-S systems) that can temporarily raise the voltage on earthed conductors - most installation ones should clear within 5s, but DNO ones could take longer.


    In short, try not to have any two different earthing systems within reach of each other, whether one of them is PME or not.


       - Andy.
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  • 0

    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.



    Although PME is often held up as a worst case example - there are similar dangers from having any two unconnected earthing systems within reach of each other - regardless of which type they are.


    With any TN system a fault between a line conductor and true earth (but not a c.p.c) can result in the star point being pulled a long way from zero volts - which is then imposed on the DNO's earth equally in TN-S and TN-C-S systems.


    Likewise standing leakage currents in a TT installation can raise the MET and everything connected to it significantly above zero volts - no more than 50V if the installation is in accordance with BS 7671 and all RCDs are functioning correctly, but another TT installation could be at held at a different voltage again. In the worst case if the other installation is on another phase, you could have as much as 50V √3 = about 87V between them.


    Then there are a whole raft of faults (L-PE on any TN system, plus L-N on TN-C-S systems) that can temporarily raise the voltage on earthed conductors - most installation ones should clear within 5s, but DNO ones could take longer.


    In short, try not to have any two different earthing systems within reach of each other, whether one of them is PME or not.


       - Andy.
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