[Caperodder]

Hello Folks,

Someone has given me a variable transformer with isolation. I tested it to make sure it was....

So now it's safer to work on the radio as it is completely isolated from mains. Question is this: With the radio plugged into the isolation transformer, does the test equipment also need to be plugged into the transformer, or should it be plugged in to a standard mains outlet?

Thanks.

[morzh]

Yes, plug it in mains.

Otherwise it is as if you did not use the transformer. That is, safer for you but still unsafe.

[Caperodder]

Thanks much!

I also read somewhere that, while testing a radio that is connected to an isolation x-former, that you should also connect a 1M Ohm resistor from the radio's chassis to mains ground. That keeps the isolated ground from floating up too high, and creating a high voltage situation

True?

[morzh]

That fights capacitive coupling and yes will keep the two at the same potential, but: the currents and the charge are so small they pose no danger, and if there is real breakdown 1M will not help at all. So, yes, true and, yes, useless for radios.
For laptop computers with isolated supplies connected to a grounded devices, especially in Europe with its 220V, yes, useful - keeps USB and RS232 ports from blowing up.

[codefox1]

Always remember that even test equipment with transformer power supplies usually have a capacitor and sometimes resistor from line to ground. I've rewired my AC operated stuff to 3 prong plugs used with a known correct power strip. Now then, a set you are working on may not have this mod yet, but if it is also transformer operated and powered by an isolation transformer, the chassis can almost certainly be linked to the mains ground through a resistor, or directly. Anyway the object is to not get little jolts or worse whilst connecting and using test equipment or creating other problems as already mentioned.