Re: Isolator Transformer Query
The two scenarios identified by @julie_m are well explained and the final statement is a very good one-line summary. However, in my view, neither of those two scenarios encompass all the main reasons that I would plug an electronic device into my bench isolating transformer. Two common ones would be:
* To reduce the risk of shock to earth via casual contact with live functional parts that have been made accessible for maintenance purposes. I.e. the parts are intended to be live in operation but not intended to be exposed to the touch.
* To enable test equipment to be connected to the unit in such a way as neither conductor of the supply cable would be at or near earth potential. For example, earthing the negative of full-wave rectified mains.
In both of these cases, the secondary must be isolated. In @julie_m's scenario 2, the other bench-based use-case, identical potentials to earth will exist on the various parts once the test-gear earth is taken to the DUT, regardless of whether the transformer secondary is earthed. Therefore, somewhat contrasting with the picture painted by that description, the 'default' situation for a service-bench isolating transformer is for a floating secondary.
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