The Miniature circuit breaker consists of two mechanisms

• Thermal mechanism.

• Electromagnetic mechanism.

Thermal-magnetic circuit breakers contain two different switching mechanisms

1. A Bimetal Switch Mechanism

The bimetal serves as a means of handling over currents. The bimetal typically sits behind a trip bar and is part of the current carrying path. Electrical current exceeding the breaker-overload rating heats the bimetal enough to bend it toward the trip bar. As the bimetal bends, it touches and rotates the trip bar to open the circuit. The time the bimetal needs to bend and trip the circuit varies inversely with the current.

2. Magnetic Mechanism

The magnetic portion of the breaker consists of an iron core with a wire coil around it, forming an electromagnet. Load current passes through the electromagnet coils so the electromagnet responds to short-circuit currents. A high-level of current thus makes the electromagnet generate enough field strength to attract a nearby armature. As the top of the armature moves toward the electromagnet, the armature rotates the trip bar to trip the breaker, open the current path, and de-energize the electromagnet coils.

Types of MCBs based on no. of Poles

• Single Pole (SP) MCB: A single pole MCB provides switching and protection only for one single phase of a circuit.

• Double Pole (DP) MCB: A two Pole MCB provides switching and protection both for a phase and the neutral.

• Triple Pole (TP) MCB: A triple/three phase MCB provides switching and protection only to three phases of the circuit and not to the neutral.

• Triple Pole with Neutral [TPN (3P+N) MCB]: a TPN MCB has switching and protection to all three phases of circuit and additionally Neutral is also part of the MCB as a separate pole. However, Neutral pole is without any protection and can only be switched.

AF [Ampere Frame] and AT [Ampere Trip]:

The "Frame" size given for a circuit breaker indicates that it may be used up to a certain current level. A circuit breaker listed as an 800 frame means the manufacturer will use that body to house parts that allow the circuit breaker to be rated for currents say between 250 amps up to 800 amps.

The amps trip is the actual value the particular circuit breaker is rated for. An example is a 400 trip breaker in an 800amp frame. You could not have a 1000amp trip breaker in an 800amp frame.

ICU and ICS:

Icu is really the maximum perspective fault that a circuit breaker can clear (with the fault current being expressed as RMS for ac). This is verified by testing in accordance with the standard and is applicable to a specific set of electrical and environmental conditions. If these conditions change then it may be necessary to derate the circuit breaker. After clearing a fault, the circuit breaker does not have to remain serviceable and could be dangerous to operate. This point is particularly important in circuit breakers when the Ics is lower than the Icu.

Ics is the maximum prospective fault current that the circuit breaker can clear and still remain serviceable. The standard does allow some minor welding of the contacts to take place, so after a large fault, it would still be necessary to inspect the breaker. When specified as a percentage of Ics, the standard proposes ranges of 25%, 50%, 75%, and 100.

Test Connection

Inject AC current (Twice time of the rated current) Using Sverker test kit on each pole of the MCB and check the tripping time according to the tripping characteristics of this MCB Curve type.