Neutral-pole switching considerations

[LM2]

Neutral-pole switching on three-phase circuits deserves careful attention, particularly in contexts where it is not commonplace.

Clause 6.9.2.2 in SANS 10142-1 Ed.3 [2020] states:

In the case of a single-phase circuit, the disconnecting device shall disconnect live and neutral. In the case of a multiphase circuit, the disconnecting device shall disconnect all the phase conductors but need not disconnect the neutral conductor in an installation connected to a supply system in which the neutral conductor is earthed direct (see the TN system in annex K).

As such, triple-pole disconnection devices are commonly installed in SA. Nevertheless, when low voltage AC electrical power is distributed in an installation by means of a three-phase, four-wire, system and residual current devices are deployed, caution must be applied so that all the conductors considered to be live are involved in residual current detection, i.e., the neutral conductor is taken into account in the vector summation. This is typically the case when 4-pole RCDs or earth-leakage devices are installed, although there may be instances where users would prefer not to switch the neutral conductor per sé.

As an example that pertains to such a requirement, some triple-pole moulded-case circuit breakers may be equipped with an adaption accessory to permit the combination of a 3-pole circuit breaker and a 4-pole add-on earth-leakage module, such that the imbalance current is taken into account and maloperation of the residual current device is avoided.