How to wire a RCBO/GFCI correctly (in series with the upstream RCBO/GFCI)?
When you introduce a isolation transformer to a supply system you get to choose/define the new local Ground reference.  

The modern day GFCI does not need the ground reference to do sensing any longer (It can be used to test but the test button causes a small current to flow from the one line on the output side to the other line on the input side of the differential current sensor this creating the imbalance).  

The ground line is not the same as the neutral (or other hot) line and must not be carrying any current except for fault currents and in theory should be at the same potential all over a building and connected to a single reference point at the point of supply input.  Cable losses do not create current imbalance but can cause the neutral voltage to rise above the reference earth point though this will not affect a GFCI.

The isolation transformer was a older style protection system used before GFCI became commonplace.  It allows you to contact any point in the floating secondary circuit without fault current to ground.  They can be used together as one prevents fault currents by allowing the supply to float and the other trips if somehow a fault current does flow to a local ground or through the isolation transformer to the supply ground.  The local GFCI would be placed after the isolation transformer and is a good idea if you plan to ground the isolated side, placing it before the transformer would serve only to detect fault currents in the transformer primary and these are already detected by the existing GFCI system in the distribution system.  Typically GFCI units will isolate both incoming circuits as it is not safe to assume which one is live and which is neutral.  Especially in a fault situation it is risky to assume and 2 pin plugs and sockets don't even know which is which and houses, equipment or cables can be cross wired.  Take note that the isolation transformer will isolate the fault current sensing from the primary GFCI the same way that it will prevent the creation of a fault current on the non-ground referenced side.   In the USA the isolation transformer output would be centre-tapped for earth if it is a 220V (110V + 110V) output while in Europe, South Africa or UK it would usually be grounded at the Neutral point and the live would be 230V.

If making a permanent installation you will usually have to comply with local building and wiring codes for wire current carrying sizes, heat and voltage insulation limits and numerous other things.  If this is a isolating portable extension cable for test purposes then it may be cheaper to buy ready made, isolation transformers and GFCI units are available as 'consumer' units with mains plugs and sockets that can be purchased from places like RS Components or similar.

This is not a typical requirement so not all electricians will understand or attempt such a job.  It may be easier to investigate vendors of design, service or equipment in the conditioned power industries.  Sound and video studio, test laboratory, computer centres, cell sites and back up power generation sites will need such services more commonly.  In 3 phase installations the switching of neutral for instance in controversial and the codes vary by locality while in some locations individual phase fuses are still common whereas in other places they are considered a hazard.  GFCI units were developed in South Africa due to generally poor ground conditions and low standard of safety education.







simulate this circuit &ndash; Schematic created using CircuitLab