Type of Tripping Mechanism of MCB / MCCB-(Part-2)


(3) Thermal-Magnetic Trip mechanism (inverse-time & instantaneous-trip)

  • Thermal-magnetic circuit breaker (TMD) is most common use for over current and short circuit protection.
  • It is a combination of Thermal Circuit breaker and Magnetic Circuit Breaker.
  • It contain two different switching mechanisms, a bimetal switch and an electromagnet
  • The thermal Property (Bimetal Strip gets elongated when heating) is used to sense the overload and Magnetic Property (Magnetic Flux / induction) is used to sense the short circuit.
  • Characteristic:
  • In Thermal-Magnetic Circuit Breaker both Thermal element (Bimetallic Strip) and Magnetic element (Electromagnet) are connected in series with load.
  • In normal Load a bimetallic element is heated by the normal load current, the bimetallic element does not bend, and the magnetic element does not attract the trip bar.
  • If the temperature or current increases over a sustained period of time, the bimetallic element will bend, push the trip bar and release the latch. The circuit breaker will trip.
  • If the current suddenly or rapidly increases enough, the magnetic element will attract the trip bar, release the latch, and the circuit breaker will trip.
  • Thermal Trip gives inverse time characteristic and Magnetic Circuit Breaker (instantaneous-trip circuit breakers) gives instantaneous-tripping.
  • MCCB Rating: 10 A to 1600A
  • Operating Time: 4mili sec.
  • Application:
  • For residential Load
  • For to heavy industrial loads.
  • For higher level (short circuit) over currents,
  • For motor-circuit protection.
  • AC/DC power distribution.
  • Electrical machines
  • Protection for transformers, motors, generators.
  • For Protection of capacitor.
  • Protection Range:
  • The adjustable overload protection is from 70% to 100% of the nominal current (0.7 to 1xIn)
  • Short circuit setting from 5 to 10 times of the rated current is possible.
  • For example: A 100 A thermal-magnetic circuit breaker will trip within a short time if it is subjected to a current of 400 A, but a 100 A instantaneous-trip circuit breaker will carry that overload indefinitely, if the adjustable trip is set above that level.
  • Instantaneous-trip circuit breakers are circuit breakers that have a magnetic trip function but not a thermal trip function. They are designed for one very specific purpose, that being to provide branch-circuit short-circuits protection for motor circuits.
  • Advantage:
  • economical, tried and tested technology
  • Disadvantage:
  • The operating characteristics of the breaker may vary depending on the ambient temperature.
  • It needs particular time to trip ( heat up the metallic strip > open circuit the holding coil > opens the contacts
  • Provide less flexibility of adjustment than electronic releases.

  (4) Electronic (Static) Trip Mechanism:

  • A coil, placed on each conductor, continuously measures the current in each of them.
  • This information is processed by an electronic module which controls the tripping of the circuit breaker when the values of the settings are exceeded.
  • Both the overload trip action and the short-circuit trip action of breakers with electronic trip units are achieved by the use of current transformers and solid-state circuitry that monitors the current and initiates tripping through a flux shunt trip when an overload or a short circuit is present.
  • MCCB Rating: 20 A to 2500A
  • Operating Time: 4mili sec.
  • Protection Range:
  • The adjustable overload protection is from 60% to 100% of the nominal current (0.6 to 1xIn)
  • Short circuit setting from 2 to 10 times of the rated current is possible.

 

  • Advantage:
  • The operating characteristic of the breaker is independent of the ambient temperature. 
  • More accurate and more flexible settings
  • Becoming standard for larger size breakers
  • Ground fault easy to provide
  • It has wide flexibility to takes care of future increases in load capacity of an installation and ensures better planning at an optimum cost
  • Disadvantage:
  • It is costly compare to TMD Type MCCB.
  • Application:
  • Electronic trip breakers are generally applied for applications where high levels of system coordination.
  • Electronic trip breakers can provide superior protection and coordination as well as system alarms and diagnostics, monitoring and communications.

(5)  Microprocessor Trip Mechanism:

  • In Microprocessor type tripping mechanism release, sensing and tripping executed by Microprocessor by use of CT or current sensing resistor
  • It gives the very faster response than TMD Release.
  • MCCB Rating: 20 A to 2500A
  • Operating Time: 4mili sec.
  • Protection Range:
  • The adjustable overload protection is from 60% to 100% of the nominal current (0.6 to 1xIn)
  • Short circuit setting from 2 to 10 times of the rated current is possible.
  • Advantage:
  • System Diagnosis is possible as it stores the Trip history within the internal memory.
  • Trip current indication is also available for understanding of type of fault and set-up programming at site. 
  • High repeat accuracy and High reliability.
  • Provide coordination, Interlocking to other MCCB.
  • High Flexibility
  • Disadvantage:
  • It is costly compare to TMD Type MCCB.

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