At least here, 240 volt single phase is from the centre point of a Y connected
transformer. The outside ends of the windings all have 415 volts between them
i.e. if you wire them a to a delta load, the load is a three phase 415 volt
load, and it seems the system in Chile is similar, but the voltages are
different. Here distribution is at 400kV, 275kV, 132kV, 66kV, 33kV, 11kV and
there may still be some places with 6.6kV or 3.3kV. It's now mainly 11 and 33kv
distribution in rural areas. Our local expert is G4LDL, who is this week
commissioning and in charge of the testing of a 90MVA 132kV primary transformer
- rather bigger than what we are all accustomed to! I gather that in the US,
the 'Pole Pig' is a single phase transformer with a primary connected between
one phase and ground, which is also the star point of a Y connected transformer
secondary. The secondary is then a 240 volt winding, with grounded centre tap,
giving 120 - 0 - 120.
I presume the Thai 4 wire system is three phases and a Y neutral. By
transforming to 240 or 220 or whatever and not grounding one side, what
prevents the line floating up? - for instance, you could have one wire at 300
volts above ground and the other at 540 volts above ground. I would expect that
measuring the volts from each side to a good ground will show one pin somewhere
around a few volts above ground and the other 240 or so volts higher. Having
typed all this and checked what has come in, I see Manfred has exactly the
same thought.
Ideally, Charly would use a HV tester to test the insulation between input
wires and chassis (with the amp unplugged) and from the transformer primary
terminals to chassis. It should be very high. As the amp doesn't use the
chassis as a return for anything connected to the AC line, then the fusing of
one side shouldn't make any difference although the main fuse should then feed
other fuses so that fault tolerance is built in. What does need doing is to
ensure a good external ground is connected to all the station equipment, and
then it won't matter which way round the amp is plugged in.
73
Peter G3RZP
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