Molded Case Circuit Breakers

Molded Case Circuit Breakers are used to efficiently distribute power throughout an electrical system. They provide a means to manually open or close a circuit, and are designed to protect wiring and equipment by automatically opening under an overload or short circuit condition.

In the United States, most Molded Case Circuit Breakers (MCCBs) are listed according to the UL 489 Standard. UL489 stipulates construction and marking requirements for MCCBs. To apply the UL mark, a manufacturer must pass a series of tests which are repeated in front of UL inspectors on an ongoing basis.

MCCBs are available in different frame sizes which correspond to their continuous current rating. The frame determines the overall dimensions of the breaker Within a frame, a breaker may be offered with various interrupting levels. The frame material or internal components of the breaker are often modified to meet the interruption level needed.


Molded case circuit breakers are comprised of six main components:

Molded Case Circuit Breakers
  • molded case (frame),
  • operating mechanism,
  • arc extinguisher,
  • contacts,
  • trip unit,
  • terminal connectors.

Methods of Mounting

  • Circuit breakers are tested as stand alone components, however in practical application they are always used in some type of enclosure.
  • Listed below are ways to mount a MCCB, depending on type and application

Fixed Mount (Bolt On)

  • Used in commercial and industrial applications.
  • The breaker is bolted onto a back pan or panel. The line (supply) side is connected with cable or bus bar The load-side has terminals for connecting cables to the devices fed from the breaker
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Removable (Plug In)

  • Used in Datacenters and Marine Applications
  • The breaker plugs into an adapter block or plate which is connected to panel bus bar. The breaker is front accessible and can be removed and replaced easily by pulling it out of the adapter block.
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Din-Rail Mount

  • Used in Industrial Control Panels
  • A DIN rail is a metal rail that runs across the width of the panel. Breakers with din rail adapters can be mounted close together, and snapped on or off. Due to weight constraints they are offered for small frame sizes up to 100k

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