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.


About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

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

  1. Makarand Joshi says:

    Your article on floating neutral is informative. Just would like to know in case of such fault, under what condition voltage between phase and neutral (floating) will go as high as 415 V? In my opinion this would happen only if neutral opens and gets connected to other phase. In case of highly unbalanced load, it should race to say 350 V. Please comment

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