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

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