[R-390] Isolation transformer question

[Paul Galpin]

Certain assumptions are being made about the secondary of the I.T. In fact,
there are several unknowns. What is the secondary live-to-earth resistance,
neutral-to-earth impedance? How do these change when a piece of equipment is
connected? The secondary is floating and could even be standing up at kVolts
of static (very unlikely, but theoretically possible in a very dry climate)

IMHO, it would be best to relate the secondary to earth in a known way.

My suggestion is to connect small 110V globes from each side to earth,
giving visual indication that the supply is now 55-0-55, very safe if you
insist on connecting yourself to the power cables!  If either lamp goes out
and the other goes bright, then you've got a short somewhere between one leg
and earth. You might want to put a shorting switch across one leg to earth
to create this condition, maybe, for certain tests

Of course, down here it would be a 110-0-110 supply, but that's still safer
than the full 220.

Incidentally, ELT's (GFI to you) also go out on a low impedance
neutral-to-earth short.


Paul Galpin
ZS2PG