For one of my projects, I need to add a RCBO/GFCI for the protection of the users. I know that there is a RCBO somewhere up the chain, therefore I am facing a confusion: there is a non-zero resistance between Earths which can be very similar.

Doesn't that mean that the current will be limited or won't be able to flow at all? Or maybe that the current forces the two Earths to be different by R*I? What's the correct way of chaining RCBOs?


simulate this circuit – Schematic created using CircuitLab

Bonus question: the customer requires an isolation transformer to be inserted between those two elements, "just in case", and I have no idea why. They don't seem to know either.


Is that correct now?


simulate this circuit

  • 2
    \$\begingroup\$ An isolation transformer can be used for a number of things, the first two reasons usually being that they can eliminate ground loops by isolating the neutral from any ground reference, and they reduce noise that can be carried on a ground conductor. Ground is a very noisy place. \$\endgroup\$
    – R Drast
    Commented Apr 9, 2015 at 16:23
  • \$\begingroup\$ Looking good. The isolation transformer case grounding needs to be connected to the supply ground to gain the protection from the supply side ground and GFCI. The local ground should be connected to a good ground reference (or the supply ground) at the point where the Ground and neutral are connected. If local ground is floating (not ideal) then bleed resistor is recommended. Product does not usually need another GFCI. \$\endgroup\$
    – KalleMP
    Commented Apr 12, 2015 at 10:21

1 Answer 1


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 – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ Brilliant answer, thanks a lot. I work in Europe, so we have 230-240VRMS line-neutral 50Hz. I've updated my post, is that correct now? \$\endgroup\$ Commented Apr 10, 2015 at 9:56

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