Type of Tripping Mechanism of MCB / MCCB-(Part-1)


Introduction:

  • Moulded Case Circuit Breakers are electromechanical devices which manually / automatically protect / Open a circuit from Over current and Short Circuit. There is various type of Tripping mechanism to trip MCB/MCCB.

MCCB Tripping Mechanism:

MCCBs have following various Operating Mechanisms.

  • Thermal Trip
  • Magnetic Trip
  • Thermal- Magnetic Trip 
  • Electronic Trip 
  • Microprocessor Trip

 (1) Thermal Trip Mechanism (Inverse-time)

  • The thermal trip mechanism of MCCB works as a delay fuse.
  • It will protect a circuit against a small overload that continues for a long time.
  • In Thermal trip MCCB a bimetal strip is connected in series with the circuit load.

1

  • When normal current pass through bimetallic strip and rise temperature of bimetallic strip and it increase length of bimetallic strip but this expansion rate is not enough for bending movement of strip and the contacts will remain closed.
  • As current of MCCB increase beyond over load current. It heats enough bimetal and thus bimetallic strip bend as per current level and Close contact will be open.
  • The amount of current needed to trip the MCCB depends on the size of bimetallic Strip.
  • The time the bi-metal needs to bend and trip the circuit varies inversely with the current.
  • It has Inverse time characteristics, they allow a long-time delay on light overloads and they have a fast response on heavier overloads.
  • The thermal element will also protect the circuit against temperature increases.
  • It is Sensitive to ambient temperature
  • MCCB must carry 100% of rated current continuously at 40 deg C.
  • At 200% rated current, maximum trip times are

                              Trip Time of Thermal Element @200% current

Amp Rating Max Time @ 200%

0-30

2 min
31-50 4 min
51-100 6 min
101-150 8 min
151-225 10 min
1601-2000 28 min
  • Tripping Action: Tripping Time will depend upon Current. The larger the overload, the faster the circuit breaker will trip
  • Used For: Over Load Protection

 (2) Magnetic Trip Mechanism (Instantaneous-trip)

  • In magnetic trip MCCB an electromagnet (an iron core with a wire coil around it, forming an electromagnet) is in series with the circuit load.

2

  • With normal current, the electromagnet will not have enough electromagnetic field to attraction the trip bar for movement and the contacts will remain closed.
  • As High current (Short Circuit) current through the coil increases the strength of the magnetic field of the electromagnet. As soon as the current in the circuit becomes large enough, the trip bar is pulled toward the magnetic element (electromagnet), the contacts are opened and the current stops.
  • The amount of current needed to trip the MCCB depends on the size of the gap between the trip bar and the magnetic element.
  • On some MCCB this gap (trip current) are fixed and some MCCB are adjustable.
  • Tripping Action: A magnetic circuit breaker will trip instantly when the preset current is present.
  • Used For: Short Circuit Protection
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