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Posts: 110
Location: Chicago area - USA
Joined: September 11, 2005
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Most of us here are unfamiliar with UK wiring practices, so you should consult somebody there. But, I will make a few comments that may (or may not) help. The original 120-V ST70 sold in the U.S. did not have a grounded chassis (just a two-prong hot and neutral plug, unpolarized). For newly manufactured equipment sold in the U.S., that wouldn't meet present-day requirements. Either the chassis would have to be grounded via a 3-prong plug, or there would need to be double insulation between the line voltage and the chassis or other exposed metal parts. Such requirements are not retroactive, and an old ST70 is permitted to be used in most U.S. jurisdictions and situations. (Maybe not in a hospital operating room!) Grouding the chassis is likely to cause a hum and/or troublesome ground-loops, particularly if a turntable or preamp is connected. I have a modern, kit-built ST70 that came with a gounded line cord. Due to hum, I replaced it with an ungrounded cord. Here, getting shocked with 120V isn't comfortable but most people survive. (Probably the B+ plate supply is more hazardous.) Now, with your 230-V supply, things are a bit more dicey. Even with an ungrounded chassis, you can achieve a measure of electrical safety by using a receptacle protected by what is called in the U.S. a "ground-fault interrupter." In Europe, I believe they are called "residual current devices." They sense when there is a tiny mismatch between the currents carried by the two power conductors supplying the appliance. If there is a mismatch, there is a ground fault, possibly due voltage on the chassis passing through your body - and the power is automatically tripped. I am not familiar with the "disconnecting network" that you describe. In the U.S., at least, I don't think that would cut the mustard as far as a achieving a safety ground. In the U.S., modern 120-V residential circuits have three conductors: hot, neutral, and ground (often bare). Your term "earth ground" isn't commonly used. Both the neutral and ground conductors are connected together at the neutral bus in the main distribution panel which is, in turn, connected to a ground rod driven into the earth for the house. It is not kosher to try to run a separate ground wire from, say, an appliance to perhaps a water pipe. All three conductors have to be run together inside a conduit or bundled together in an insulated cable.
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