Introduction to MCB
The miniature circuit breaker (MCB) is a device designed to isolate a circuit during an overcurrent event without the use of a fusible element. A breaker is a resettable protective device that protects against two types of overcurrent situations
1.Overload 2.Short Circuit |
MCB Construction Details
Current Limiting Breakers use an electromechanical (Thermal /Magnetic) trip unit to open the breaker contacts during a overcurrent event. The thermal trip unit is temperature sensitive and the magnetic trip unit is current sensitive.Both units act independently and mechanically with the breaker’s trip mechanism to open the breaker’s contacts.
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Overload Protection
MCB Overload protection
The thermal trip unit protects against a continuous overload. The thermal unit is comprised of a bi metal element located behind the circuit breaker trip bar and is part of the breaker’s current carrying path.
When there is an overload, the increased current flow heats the bimetal causing it to bend. As the bimetal bends it pulls the trip bar which opens the breaker’s contacts. The time required for the bimetal to bend and trip the breaker varies inversely with the current. Because of this, the tripping time becomes quicker as current increases in magnitude.
When there is an overload, the increased current flow heats the bimetal causing it to bend. As the bimetal bends it pulls the trip bar which opens the breaker’s contacts. The time required for the bimetal to bend and trip the breaker varies inversely with the current. Because of this, the tripping time becomes quicker as current increases in magnitude.
Magnetic Trip Unit ( Short Circuit Protection )
MCB Magnetic trip units
Magnetic trip units (short circuit protection)The Magnetic trip unit protects against a short circuit. The magnetic trip unit is comprised of an electromagnet and an armature.
Components of a Magnetic Trip Unit
MCB Components of a magnetic trip unit
When there is a short circuit, a high magnitude of current passes through the coils creating a magnetic field that attracts the movable armature towards the fixed armature. The hammer trip is pushed against the movable contact and the contacts are opened. The opening of the breakers contacts during a short circuit is complete in 0.5 Milli-seconds.
Arc Runner / Arc Chute
MCB Arc runners / Arc chutes
During an overload or short circuit event,the contacts of the breaker separate and an electrical arc is formed between the contacts through air. The arc is moved into the arch chute by “running” the arc down the interior of the breaker along the arc runner. When the arc reaches the arc chute it is broken into small segmented arcs. The segmented arcs split the overall energy level into segments less than 25V.
Each 25V segment does not have a high enough energy level to maintain an arc and all energy is naturally dissipated.
Each 25V segment does not have a high enough energy level to maintain an arc and all energy is naturally dissipated.
Note:
The arc runner and arc chute limit and dissipate the arc energy during the interruption of an overload or short circuit event.
The arc runner and arc chute limit and dissipate the arc energy during the interruption of an overload or short circuit event.