Low-Voltage Switchgear and Circuit Breakers:Low-Voltage Circuit Breakers

Low-Voltage Circuit Breakers

Low-voltage circuit breakers that may be found in switchgear, distribution centers, and service entrance equipment are of three types: (1) molded-case circuit breakers (MCCBs); (2) insulated-case circuit breakers; and (3) fixed or draw-out power circuit breakers.


MCCBs are available in a wide range of ratings and are generally used for low-current, low-energy power circuits. The breakers have self-contained overcurrent trip elements. Conventional thermal-magnetic circuit breakers employ a thermal bimetallic element that has inverse time–current characteris- tics for overload protection and a magnetic trip element for short-circuit protection. Conventional MCCBs with thermal-magnetic trip elements depend on the total thermal mass for their proper tripping characteristics. This means that the proper sized wire and lug assemblies, which correspond to the rating of the trip element, must be used on the load terminals of such breakers. Many manufacturers are now switching over from bimetallic elements to power sensor (electronic) type trip elements. Magnetic-trip-only breakers have no thermal element. Such break- ers are principally used only for short-circuit protection. Molded-case breakers with magnetic only trips find their application in motor circuit protection. This arrangement is desirable for smaller motors where their inrush current can ruin a delicate thermal element but where protection for winding failure is still needed. The breaker provides the instantaneous (INST) protection and fault interruption, and other overload devices in the starter handle the long-time overload protec- tion. Nonautomatic circuit breakers have no overload or short-circuit protection. They are primarily used for manual switching and isolation.

Insulated-Case Circuit Breakers

Insulated-case circuit breakers are molded-case breakers using glass- reinforced insulating material for increased dielectric strength. In addition, they have push-to-open button, rotary-operated low-torque handles with independent spring-charged mechanism providing quick-make, quick- break protection. A choice of various automatic trip units is available in the insulated-case breakers. Continuous current ratings range up to 4000 A with interrupting capacities through 200,000 A. The principal differences between insulated-case breakers and heavy-duty power circuit breakers are cost, physical size, and ease of maintenance. Insulated-case breakers are not designed with easy troubleshooting or repairs as the principal feature; whereas, draw-out power circuit breakers are. To partially compensate for this drawback, many manufacturers now offer a variety of accessories for insulated-case breakers that can duplicate the features of their more expensive counterparts. Nevertheless, insulated-case breakers are gener- ally suited to light industry or commercial buildings where frequent or numerous operations are not expected.

Power Circuit Breakers

Heavy-duty power circuit breakers employ spring-operated, stored-energy mechanisms for quick-make, quick-break manual or electric operation. Generally, these breakers have draw-out features whereby individual breakers can be put into test and fully de-energized position for testing and maintenance purposes. The electrically operated breakers are actuated

by a motor and cam system or a spring release solenoid for closing. Tripping action is actuated by one or more trip solenoids (shunt trip coil) or flux- operated devices; generally one for the protective devices on the breaker itself, and another for externally mounted controls or protective devices. The continuous frame ratings for these breakers range from 400 to 4000 A. Some manufacturers have introduced breakers with 5000 and 6000 A frames; how- ever, the long-term benefits and overall reliability of these designs have yet to be proven in the field. Short-circuit interrupting capabilities for these break- ers are usually 50,000–85,000 A root-mean-square (rms) for frame sizes up to 4000 A. Larger designs have approached 100 kA. These breakers can be extended for applications up to 200 kA interrupting when equipped with assemblies or trucks designed to hold Class L, current-limiting power fuses.

Fused Power Circuit Breakers

The trend toward larger unit substation transformers and larger connected kVA loads on such substations has given way to power circuit breakers in tested combination with current-limiting fuses. This is routinely done in order to increase the short-circuit interrupting rating of the switchgear. This combination can be used for all frame sizes. The fuses cause the same prob- lems with single phasing as fuses in the switchboards; however, there are numerous features that compensate for this problem. First, most fuse assem- blies are attached directly to the breakers themselves so fuses cannot be removed or installed unless the breaker is out of service. Most manufacturers solve the single-phasing problem by either an electrical or a mechanical means of blown fuse detection, which in turn causes the breaker to trip immediately after the fuse has cleared. On the largest frame sizes, where the fuses must be mounted apart from the breaker cubicle, the fuse assembly is on a truck or roll-out which is mechanically interlocked with the breaker it serves. It should be noted that the overcurrent protection for overloads is still handled by the breaker’s overcurrent trip devices, and that the fuse is not expected to clear except for the most severe short circuits.

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