In three-phase systems with neutral, a broken or high-resistance neutral is one of the most dangerous failure modes: the neutral point drifts, and single-phase loads can see voltages far above nominal, destroying equipment before any overcurrent device trips.
A 4P breaker switches all three phases and the neutral together. When it opens, every live conductor — including the neutral — is fully disconnected, which eliminates back-feed and makes maintenance safe. A 3P+N device may leave the neutral connected or only partially rated, leaving the circuit exposed to exactly this scenario.
CNMIBO ATQCB2 smart breakers are available in true 4P configuration with independent neutral switching, combined with zero-line (neutral-loss) detection that alarms the moment the neutral degrades. For tenant sub-metering, PV and generator-backed systems, true 4P is the safe specification.