PC-Class vs CB-Class ATS: How to Choose the Right Transfer Switch

Why the transfer switch class decision matters

Facilities such as elevators, fire protection systems, surveillance equipment and banking power supplies cannot tolerate a long outage. A transfer switch provides one preferred feed and one standby feed: when the preferred side fails, the switch automatically connects the load to the standby side. In small systems the standby can be a generator set. The class of the switching device — not just its rated current — determines whether the transfer holds up under fault conditions, whether it switches fast enough for the load, and whether the installation satisfies the local low-voltage distribution code.

PC-class and CB-class at a glance

PC-class: a transfer switch able to make and carry normal and fault currents, but not intended to interrupt short-circuit currents. Its withstand is verified by the short-time withstand current (lcw) and short-circuit making capacity (lcm).

CB-class: a transfer switch fitted with overcurrent releases whose main contacts can make and break short-circuit currents — in practice two circuit breakers form the switching body.

Technical diagram shown at a readable responsive scale.

Mechanism design: why PC-class is more reliable

CB-class units are built from two circuit breakers. A breaker is designed to interrupt arcs, so its mechanism must trip fast; that fast-trip design introduces the risk of latch slip and unreliable re-closure. A PC-class mechanism carries no short-circuit interruption duty, uses a simpler mechanical-plus-electronic transfer lock, and has roughly half the moving parts of a breaker-based unit. For this reason PC-class reliability is generally higher — the same relationship as a load-break switch versus a circuit breaker of similar rating.

CB-class body = two MCCBs, the most complex transfer-switch architecture in common use.

PC-class avoids latch-slip and re-closure concerns, which matters when the standby path sits idle for months.

Clearance, creepage and the two-supply overlap problem

During transfer, the two feeds can momentarily overlap or interact. PC-class designs take this into account: their clearance and creepage distances are typically 180% and 150% of a comparable breaker (standard requirements), giving a safer dielectric envelope around the switching gap. Breaker-based bodies follow circuit-breaker clearance rules, which are tighter because a breaker is designed to clear faults, not to sit between two live feeds.

Contact materials and the oxidation risk

Breaker contacts often pair silver-tungsten or silver-tungsten-carbide to support arc interruption. These materials oxidize more readily, and a standby contact left exposed for a long period can build a stubborn oxide film. When that contact finally carries load, the extra resistance drives temperature up sharply and can destroy the switch or, in severe cases, cause an internal fault. PC-class transfer switches use contact materials and pressure design chosen for the standby position: higher contact pressure keeps the contacts from being blown apart and from welding under overload (the design covers overloads of 20 times rated current and above).

Transfer time decides where each class fits

Transfer time differs significantly between the classes, and for some loads it is the deciding factor. Emergency evacuation lighting, for example, requires the shortest possible changeover and in practice can only be served by a PC-class unit.

Emergency / evacuation lighting: PC-class only — switching time is the hard requirement.

Elevators, security and monitoring: PC-class or CB-class depending on the tolerated interruption window.

General power loads: either class; decide on reliability budget, fault duty and maintenance policy.

Selecting for fire and life-safety loads

Low-voltage distribution design codes (e.g. GB 50054) require that a circuit whose disconnection under overload would cause serious consequences must not be interrupted by overload protection; overload may only be signaled. Where a CB-class transfer switch serves fire-fighting loads, it must be built from circuit breakers providing short-circuit protection only. In practice, fire-fighting loads are usually served by PC-class units to avoid the complexity and to meet the transfer-time and reliability demands.

A common misunderstanding: the short-circuit breaking function of a transfer switch protects the downstream circuit and equipment, not the switch itself. A PC-class unit without short-circuit interruption still performs its transfer function correctly; the designer decides separately whether the system needs an upstream breaker.

Isolation, maintenance and a five-point specification checklist

A transfer switch should offer isolation for maintenance. When the unit has no built-in isolating function, the design must add an isolating switch upstream. In industrial systems the number of isolating switches should be controlled — they take space, add cost and lower reliability; in residential floors they are generally unnecessary.

Five specification points before you order:

PC-class rating: rated current no less than 125% of the calculated (design) current, and withstand of the expected short-circuit current (lcw / lcm).

CB-class rating: select the breaker parameters as you would select a circuit breaker, and verify the exact breaker model inside the unit against the installation duty — many designs only list model, current and poles, ignoring the breaker specification.

Fire-fighting loads: prefer PC-class; if CB-class is used, specify short-circuit-only MCCBs.

Fast-transfer loads (evacuation lighting): PC-class.

Isolation: add an upstream isolating switch when the unit has no isolation function; for fire-fighting supply avoid transfer switches with an intermediate OFF (zero) position.