The residual current transformer (core) of an RCD — the installation requirements that determine correct operation

The guide on residual current devices (RCDs) and the guide on RCD types AC/A/F/B cover which residual current waveform an RCD detects. This article goes one step earlier: how the residual current transformer (the toroidal core) of an RCD must be physically installed for it to be able to measure that residual current correctly at all.

The measurement principle: vector sum through the core

An RCD contains a toroidal (ring) current transformer through which all current-carrying conductors of the protected circuit pass — in a single-phase circuit the line and neutral conductors, in a three-phase circuit the three lines (and, depending on the load, the neutral). In a healthy circuit, the vector sum of the currents entering and leaving the core is zero: every ampere that flows out via the line comes back via the neutral. No net magnetic flux then forms in the core, and hence no signal on the secondary winding.

During an earth fault, some current leaves the circuit by a path other than the neutral — for example via earth. The vector sum is then no longer zero, a net flux forms in the core, and this flux induces a signal in the secondary winding that trips the RCD once it exceeds the set rated residual operating current.

Why the PE conductor must never pass through the core

The protective conductor (PE) normally carries no current in a healthy circuit, but that is not the core reason for keeping it out of the toroidal core: if the PE did pass through the core, part of an actual earth-fault current would "return" via that same core, partially or fully cancelling out the very vector imbalance the RCD is meant to detect. The PE conductor is therefore always routed outside the residual current transformer.

Why all current-carrying conductors of the same circuit must pass through the same core

If the neutral of the protected circuit is connected somewhere outside the RCD core — for example a shared or "borrowed" neutral from another circuit — the line and neutral no longer pass through the core as a matched pair. The vector-sum principle then no longer holds: a structural apparent imbalance results, with nuisance tripping or, depending on the current direction, a masked imbalance as a possible consequence.

Earthed cable screens: through the core, not alongside it

For an armoured or screened cable that is earthed at the supply end, a principle applies similar to that for comparable residual current transformers used with earth-fault relays in industrial switchgear (core-balance CTs): the earth connection of the screen/armour must also be routed through the core — in practice often with a loop ("pigtail") of the earthing conductor back through the eye of the core — so that any current returning via the screen is included in the vector sum. If the screen's earth connection is instead routed alongside the core to the earth bar, that screen current does not count towards the measurement: this can cause an apparent imbalance (resulting in nuisance tripping), or, depending on the fault situation, conceal that part of a real fault current is leaking away via the screen.

Note: the exact connection method for a cable screen relative to a residual current transformer can differ per manufacturer and per application (an RCD for a final circuit versus a core-balance CT for an industrial earth-fault relay); always follow the installation instructions of the RCD or earth-fault relay manufacturer for the specific cable type.

Split (add-on) cores for retrofits

When retrofitting an RCD or earth-fault relay to an existing, already wired installation, a split ("add-on") core is often used, which is fitted around the existing cables without needing to disconnect the wiring. Such a split core must be fully and correctly closed/latched after installation: any remaining air gap in the core increases the measurement error and can adversely affect the RCD's sensitivity.

Practical relevance

When investigating an RCD that trips without an identifiable cause (see also the guide on nuisance tripping from cumulative leakage current), it is worth checking, alongside the leakage situation, whether all current-carrying conductors of the protected circuit indeed pass through the same core, whether the PE conductor has mistakenly been routed through the core as well, and — for armoured or screened cables — whether the screen is correctly earthed via the core.

Common mistakes

Routing the PE conductor (also) through the RCD's residual current transformer — this can partially or fully cancel out the detection of a real earth fault.
Connecting a "borrowed" or shared neutral that does not pass through the same core as the corresponding line — this breaks the vector-sum principle and causes a structural apparent imbalance.
Routing the earthed screen of an armoured cable alongside the core instead of looping it back through it, so the screen current is not included in the measurement.
Not fully closing/latching a split (add-on) core after fitting it around an existing cable, leaving a remaining air gap that degrades measurement accuracy.