The miniature circuit breaker
(mcb) has a contact system and means of arc quenching, a mechanism and
tripping and protection system to open the circuit breaker under fault
conditions.
Most mcbs are of single-pole construction for use in single-phase circuits. The complete working system is housed within a plastic moulding, typical external appearance being shown in Fig. (1). A section showing the principal parts of the mcb is shown in Fig. (2). The contact system comprises a fixed and a moving contact, and attached to each is a contact tip which provides a low-resilience contact junction to resist welding.
Modern mcbs are fitted with arc chutes consisting of metal plates which are held in position by insulating material. The arc chute does not necessarily surround the contact; in some designs arc runners are provided to pull the arc into the arc chute.
MCBs are designed and tested according to the requirements of IEC 898.
Most mcbs are of single-pole construction for use in single-phase circuits. The complete working system is housed within a plastic moulding, typical external appearance being shown in Fig. (1). A section showing the principal parts of the mcb is shown in Fig. (2). The contact system comprises a fixed and a moving contact, and attached to each is a contact tip which provides a low-resilience contact junction to resist welding.
Fig (1) Miniature circuit breaker, external view
Fig (2) Principal parts of a miniature circuit breaker ABB
Modern mcbs are fitted with arc chutes consisting of metal plates which are held in position by insulating material. The arc chute does not necessarily surround the contact; in some designs arc runners are provided to pull the arc into the arc chute.
The tripping mechanism
usually consists of a thermal-magnetic arrangement. The thermal action
is provided by a bimetal with, in some cases, a heater. For ratings in
the range 6–63A the bimetal forms
part of the current path, the heat generated within the bimetal itself
being sufficient to cause deflection. The deflection is then used to
activate the tripping mechanism. The characteristics of the bimetal are
chosen to provide particular delays under certain overload or fault
currents according to the required time–current characteristic. A
high-resistance bimetal is used for low-current devices and a lower
resistance bimetal for high-current devices. In very low-current mcbs a
heater may be incorporated around the bimetal in order to generate
sufficient heat to deflect it.
High breaking capacity mini circuit breaker 32A
High breaking capacity mini circuit breaker 63A
The magnetic tripping element usually
consists of a coil which is wrapped around a tube, there being a
spring-loaded slug within the tube. Movement of the slug operates the
tripping mechanism to open the mcb. It can also be used to assist in
opening the contacts by locating the coil close to the moving contact.
When a fault current flows, the high
magnetic field generated by the coil overcomes the spring force holding
the slug in position; the slug then moves to actuate the tripping
mechanism and forces the contacts apart by striking the moving contact
arm. For low mcb ratings the coil is formed from thin wire with many
turns; for higher ratings the wire is thicker, with fewer turns.
The magnetic trip is set by the
manufacturer according to the required characteristics. These
characteristics are defined in the standard and form ‘types’ which are
shown in
Table 1.
MCBs are designed and tested according to the requirements of IEC 898.
Table (1) Magnetic trip settings for
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