(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.
April 05, 2017 at 10:15AM
