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Use of Molded Case Circuit Breakers in Single Phase Applications

Per UL 489 molded case circuit breaker poles in multi-pole breakers are to be constructed and tested individually.

Typically, in the system where the breaker is installed, the number of breaker poles of the breaker in question matches the number of phase conductors in use.  However in cases where a multi-pole breaker is specified to serve a single-phase load, and the breaker contains more power poles than will be used in the protection scheme, use of a multi-pole breaker in a single-phase installation is acceptable provided breaker voltage and interrupting ratings are compatible with its planned use.

Eaton multi-pole molded case circuit breakers are built with a common trip mechanism.  This means that an overcurrent event in one breaker pole sufficient to cause that pole to open or trip, thus protecting the connected load, simultaneously causes all breaker poles to open, thereby eliminating the possibility of single-phasing any portion of a connected load.

Furthermore, if any one pole of a multi-pole breaker is left totally unconnected, as might be the case when a multi-pole breaker is used as the disconnect/overcurrent protector for a single-phase load, this is a permissible connection scheme within the voltage operating parameter for the breaker (more below).  In this scenario the circuit breaker overcurrent protective operation remains unaffected, for those poles that are used in the application.

For operation, molded case circuit breakers are current-sensitive devices, only. Due to their independent pole construction technique, if current flow through any one pole exceeds the calibrated trip point for that pole then all breaker poles will trip open, disconnecting the load served by all breaker poles in use.

Safety factors in specifying a circuit breaker for use in an electrical delivery system are the voltage, number of phases, and number of wires of the system in which the breaker will be installed, and the (system) method of connection or connection scheme in use.

Factory testing plays a role in determining whether a breaker is compatible with prospective single-phase use.  Some breakers are labeled “Suitable For Use On Single Phase AC circuits.”  These breakers may be applied at their full labeled voltage (generally 600VAC), using one or two poles of the breaker.  For 600VAC rated breakers that do not bear this label, these breakers may be applied safely at up to 480VAC, using one or two poles of the breaker.

The circuit breaker voltage rating, and interrupting rating at that voltage, along with the intended connection scheme should be considered when selecting the breaker, so that the breaker is not applied outside its design operating parameter.  Should it be called upon to clear a catastrophic fault in the system in which it is installed, the selected breaker should be rated to clear overcurrent conditions to which it is subjected without damage to itself or surrounding equipment, and by extension should not pose the threat of injury to nearby personnel.