Testing Of MCB

Testing Of MCB/Functional Testing of MCB

The Miniature circuit breaker consists of two mechanisms

• Thermal mechanism.
• Electromagnetic mechanism.

Thermal-magnetic circuit breakers contain two different switching mechanisms bimetal switch and an electromagnet. The bimetal serves as a means of handling overcurrents. 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.

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.

What is the difference between AF and AT?

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 a rate 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 800 amp frame. You could not have a 1000 amp trip breaker in an 800 amp frame.

Types of MCBs based on number 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 the circuit and additionally Neutral is also part of the MCB as a separate pole. However, the Neutral pole is without any protection and can only be switched.