[PJL]

There are conflicting posts on the use of isolating transformers and we need any advice here on safety to be correct. It might be a good idea to create a sticky on the subject so here is my understanding but we need consensus that we are giving correct advice so please correct me if I am wrong.

1) The purpose of an isolating transformer is to completely isolate the equipment under repair from the incoming mains. That means there should be no direct electrical connection between the mains supply and the supply to the equipment under repair and that includes earth.
2) Using an isolating transformer, an operator can touch a single point on the equipment under repair, HT or AC, and not get an electrical shock as there is no return path to the ground that the operator is standing on.
3) Using an isolating transformer and touching two points on the equipment under repair, such as HT and chassis, will result in electrocution and could be fatal. In this case the current flow is contained within the equipment under repair and an RCD on the supply will NOT provide protection.
4) Any mains powered test equipment used for the repair must also be powered from the isolated supply if isolation is to be maintained.
5) If the earth lead of a scope probe is connected to the equipment under repair and the scope is powered from the mains, then the single touch protection no longer applies, however, if the supply is on an RCD circuit there will be some limited protection.
6) Connecting the neutral and earth together on the isolated socket is advisable as it ensure the earthed parts of the equipment under repair are not floating. This will also cause the RCD to trip if an earthed scope probe or similar is connected to the earthed parts of the equipment under repair.

It is important to note from point 4) that using an isolating transformer with valve equipment does not protect you from shocks from the dangerous potential differences in the equipment under repair.