Electronic – Selecting residual current for RCBO

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}

What would be the safety approach for the secondary side? Can we say that the transformer already isolates the earth so an RCD is not necessary both at secondary and primary sides?

Yes, but only until the first earth fault occurs. Until that happens the circuit is isolated and theoretically touching either wire will not result in electric shock. In practice, capacitive coupling between the primary and secondary may cause a small current to flow.

The problem is that there is no monitoring of the circuit so the user will not be aware of the first fault grounding one of the output wires thereby making the second one live.

Or is there still some benefit to have an RCD before the isolation transformer.

It will protect the user if the transformer chassis were to go live.

I mean if the secondary in above diagram is earthed accidentally the isolation will be defeated;

Yes, but you won't get a big fault current as you would by accidentally shorting the L wire to earth. This can be very useful in fault finding, for example.

Clarification 1: If you plug a defective piece of equipment into the mains and there is an L-E fault a very large fault current may flow. With the equipment plugged into the secondary no fault current will flow. This gives you time to trace the fault.

... in that case would the RCD in primary side will still trip if someone touches the live chassis of the load?

No. The L + N current in the primary would still sum to zero so the RCD would not trip.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Float monitoring.

By adding a couple of very low wattage (high resistance) lamps or secondary voltage-rated AC LED indicators the secondary voltages will be pulled to centre around earth voltage (0 V). For example, if the secondary voltage is 200 V then the lamps will cause the secondary to be like a split-phase 100 - 0 - 100 secondary. Both lamps will glow at half voltage.

If an earth fault occurs on one 'phase' that lamp will go out and the other will go to full brightness.

This may be useful in your application.

From the comments:

1. "Yes, but only until the first earth fault occurs." Yes to what?

See Clarification 1.

2. About having RCD before primary first you write "It will protect the user if the transformer chassis were to go live" then you write "No. The L + N current in the primary would still sum to zero so the RCD would not trip."

The transformer itself has a metal core and may have a metal case. The RCD would protect the user should s/he touch the live core or case.

RCDs work by monitoring the live and neutral wires by passing them both through what is a small current transformer. If everything is OK the current in on the live wire returns on the neutral and cancels out exactly resulting in a sum of zero. If there is an earth fault some of the current returns to the supply via the earth path, the neutral current is reduced and now the sum of live and neutral currents is non-zero, the RCD detects this and trips in milliseconds.