Vintage RCD/GFCI teardown with story (weird sensing system)

This old Wylex RCD /GFCI is the simplest I have ever seen. The sensing core is especially unusual.

It's worth mentioning that the unit is marked as an ELCB (Earth Leakage Circuit Breaker), but these days it would be called an RCD (Residual Current Device) in the UK or a GFCI (Ground Fault Circuit Interrupter) in other countries.
The term ELCB was dropped a long time ago due to the risk of confusion between voltage operated units (a coil detects a voltage difference between metalwork) and current operated units that detect a difference between the current flowing out and back through the breaker. The modern current operated versions are much safer as they detect more fault conditions than the old voltage operated devices.

This one was used in a TT electrical system where the house is supplied from overhead lines with a single phase and neutral, but no earth. It relies on an earth electrode at the substation (bonded to neutral) and a local earth electrode at the house to provide a path for fault current. Because the impedance (resistance) of the path through the ground is higher than a direct wired link it requires that the installation be protected by one or more RCDs/GFCIs to ensure that any significant current leakage from live to earth/ground trips out the power for safety.

This unit was retired from use after it failed to trip with a significant live to earth fault that passed 8A continuously (suggesting a 30 ohm earth loop impedance). It was replaced with an isolator and a new distribution board with two separate sections, each with its own 30mA RCD.

I recommend testing RCDs at least once a year by pressing the test button. It emulates a fault and does a full test of sensing and tripping the breaker. If the breaker does not trip instantly or after the designated time delay for programmable units, then do not hold the button in for longer as it may result in failure of the test resistor with unpredictable results.

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