[ukcol]

I am in complete agreement with the replies so far and would add further comments on the subject of providing a protective earth on AC sets with an isolating type mains transformer (as distinct from an autotransformer). I always remove the twin mains lead on this type of set and fit a twin and earth mains lead with the earth conductor securely connected to chassis somewhere near the mains transformer. Some other earthing may be required, for instance my Murphy A92 has the mains on/off switch mounted on a separate metal plate screwed to the inside of the cabinet, I have added an earth wire from this plate to the chassis to reduce the risk of the on/off shaft becoming live if something disastrous happened to the mains switch.

A protective earth is always more important than a signal earth and I have never had interference problems caused by making the above changes, but I do have a good long wire aerial extending from my house to my workshop which is at the bottom of the garden. I also live in a rural area that is likely to be less prone to mains-borne interference. If you are not so fortunate with your signal to noise ratio you do not necessarily have to choose between a protective earth and a signal earth; depending upon the set design you may be able to have both.

Referring to the attachment showing a modified Murphy A122 front end. In the original circuit both the primaries and the secondaries of the aerial coils had their bottom ends connected to chassis (shown by the ringed dotted lines). These connections are removed and the bottom ends of the primaries connected to the earth socket, a 100k resistor and a wander plug are added as shown and the modification is complete. With the wander plug connected to the earth socket the set uses the protective earth for its signal earth. With the wander plug unplugged a clean earth such as an earth spike can be connected in its place. The 100k resistor is included to prevent static build up on the aerial should the set be left with nothing connected to the earth socket. Its value is not critical and could be anything up to a Meg or two.

Finally an earth spike should not be used as a protective earth, because the earth loop impedance could be too high. For a protective earth to work the impedance of the path from the supply through the fault to earth must be low enough to blow the protective fuse. An earth spike could have an impedance high enough to leave the chassis at mains potential without blowing any fuse.