when using an oscilloscope to make measurments across components to a mains connected device, is it better to put the isolation transformer on the scope or the device being tested?

 

Put it on the device being tested. Be aware, however, that many commercial isolation transformers carry the earth ground through to their outlet connector. The POWER section of the isolation transformer output is isolated since there is not Hot/Neutral where the Neutral is also referenced to ground. The output is free to float.

 

BillB3857 means that the secondary of the transformer should not be connected to ground at all. Some isolating transformers have one end or a centre-tap connected to ground.

Be aware though, that if you equipment has a mains filter it will have capacitors connected between the supply and ground. Therefore it will no longer be completely "floating".

Great care is needed for safety. The use of an isolation transformer will not prevent electric shock risk. In fact it might increase the risk because you will no longer be protected by the building's RCD (if it has one).

 

Use of an isolation transformer to connect your appliance -any appliance that uses a switch mode power supply is a MUST DO before connecting your test equipment(scope etc)....it does mean you still need to be aware that although the appliance as far as the scope is concerned is mains isolated you can still receive a shock if you are not careful......