RCD testing with isolation transformer
When conducting testing on RCD's if the RCD is being tested on a RCD protected line there is the potential to trip the RCD at the main switch board. This is not desirable. The easiest way to do this is to use an isolation transformer and isolate the earth lead. However the transformer needs to be modified to ensure that the isolation transformer does not interfere with the normal testing.
The purpose of an isolation transformer is to electrically isolate the mains while maintaining a 1:1 voltage ratio. As simple diagram below shows an isolation transformer.
Normal Isolation transformer Modified for RCD testing
As can be seen in the simple diagram the Isolated secondary is completely disconnected from the Primary. As a result of the isolation transformer there will be no imbalance in the primary current and as such the RCD on the primary side will not trip.
In order to test RCD or to have RCD’s function correctly after the isolation transformer the Neutral Isolated must be connected to the Earth line. Other wise there is no way the RCD can function correctly and any subsequent testing will fail or give misleading results.
For continued operation protection the isolation transformer must not be left in place.