Merits and Demerits

1.    Simple in construction.

2.    Suitable for low breaking capacity operation.

3.    It is indoor type and the control is manual or automatic.

4.    It is used up to 6.6 kV with a breaking capacity of 15 MVA.

5.    This is suitable for repeated operation because medium of arc extinction is air.

6.    Operating control is manual as well as automatic.

Air Blast Circuit Breaker (ABCB) (Or Compressed Air Circuit Breaker)

(I) Cross Blast ABCB, (2) Axial Blast ABCB.

(A) Cross Blast ABCB

                                               

When moving contact separates the arc strikes and the blast of air is directed transversally across the arc and the arc is blown to the arc splitters. These arc splitters are placed in arc chute. Thus, the blast of air removes the heat from the arc and the arc is cooled and also as the length of the arc is increased the arc is interrupted.

Advantages and Disadvantages:

Advantages:

1.    High speed of operation

2.    Short arcing time.

3.    High speed reclosing.

4.    Less weight as compared to O.C.B.

5.    Very less maintenance.

6.    No possibility of explosion.

Disadvantages:

1.    Cost is more

2.    For complete compressor air system installation is required.

3.    These breakers are more sensitive to RRRV.

4.    For operation and maintenance highly skilled persons are required.

(B) Axial Blast ABCB

                                     

Air is admitted in the arc extinction chamber it pushes the moving contact. The air blast takes away the ionized gases along with it. Afterwards arc gets extinguished. High pressure air has higher dielectric strength.The design is such that the air expands into the low pressure (atmospheric pressure zone) zone. The air at high sped removal of heat from arc, thus arc is quenched. Diameter of arc is reduced.

Uses of air blast circuit breaker: As these are suitable for repeated duty, these are used in: () Arc furnace, (2) Traction system.

SELECTION OF CIRCUIT BREAKERS

The various circuit breakers arc quenching medium. Voltage range and breaking capacity are summarized in the following table. Their design features are also tabulated. The table also gives the comparison of various CBs.

Sn	Type	Arcing Quenching Medium	Voltage Breaking Capacity	Construction	Uses Remarks
1	Miniature CB	Air at atmospheric pressure.	400-600V current 5 to 55 Amp	Compact, occupy small space easy fitting.	L.V. circuit Protection Domestic, Commercial, Industrial purpose.
2	MCCB	--- do ----	--- do ----	Molded case contains tripping mechanism and terminal contacts. Thermosetting power is used Long lasting	--- do ---- Long lasting high mechanical strength, high dielectric strength
3	ELCB	--- do ----
4	Air Blast CB	--- do ----	400 V – 11kV, 5-750	Arc runners, splitters, magnetic coils used.	Low and medium voltage power circuit. AC or DC in industries, Current limiting features.
5	Air Blast CB	Compressed air at high pressure 20-30 kg/cm2	66kV-1100 kV 2500-60000 MVA	Auxiliary compressed air system required unit type construction	Fast operating, out-door applications. Arc furnace, also for Extra High Voltage EHV) system.
6	Min. Oil CB	Transformer oil	3.3kV-220kV 150 to 25000 MVA	Small arc control device used. CB chamber is separate.	Outdoor application up to 36kV
7	Bulk oil CB	--- do ----	12 kV, 3.6 kV	Tank fitted with arc control device	HV power system up to 12kV, 500 MVA
8	SF6 CB	Sulphur Hexa-fluoride inert heavy gas at 5 kg/cm2 pressure	3.3-765kV 1000 to 50000 MVA	Puffer system used to quench arc earlier with SF6 gas pressure.	Suitable for medium and high voltage power system. Least maintenance.
9	Vacuum CB	Vacuum	3.3kV-33kV, 250-2000 MVA	Various types of designs, Long life	Variety of uses in the power supply system 3.6 to 33 kV
10	HVD CB	Oil or air-blast	33kV, 2kA	Artificial current zero by switching in capacitors.	Used for metallic return transfer breaker.

Selection of CB

Sn	Rated Voltage	Choice of.	Remarks.
1	Below 1 kV	Air breaker CB, vacuum CB, SF6 CB	------
2	3.3 kV – 33 kV	Vacuum CB, SF6 CB, vacuum CB	Vacuum CB preferred
3	132 kV – 230 kV	SF6, Air blast CB, MOCB	SF6 preferred
4	400 kV – 760 kV	SF6, Air blast CB	SF6 is preferred.

MINIATURE CIRCUIT BREAKERS (MCB)

                                               

Miniature: Circuit breakers area used in residential and light commercial installations. Miniature circuit breakers are commonly used to protect and control the electrical supply to respective electrical circuits of the building. Miniature circuit breakers often are single pole breaker and are installed in a cabinet. MCB typically include an electrical contact mounted on a movable contact carrier which rotates away from a stationary contact in order to interrupt the current path . Such circuit breakers have molded insulated housings of standard dimensions sized to interchangeably plug into or bolt onto panel board. Typically, there is a fixed contact attached to the housing and a movable contact coupled to an operating mechanism. The operating mechanism includes a movable handle that extends outside of the housing the handle has essentially three stable positions : on, off, and tripped. These three positions tell the operator what condition the contacts are in when the handle is viewed. The operating mechanism may be actuated to move the contact into a second, open position. The trip mechanism is automatically releasable to effect tripping operation and manually resettable following tripping operations. The trip mechanism may include a thermal trip capability, which responds to persistent low level over currents, and/or a magnetic trip capability, which responds instantaneously to higher over load currents, the thermal magnetic device includes a ‘bimetal” which heats up and bends in response to persistent over current conditions to unlatch a spring powered operating mechanism which opens the contacts which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. For short circuit protection, an armature, which is attracted by the sizable magnetic force generated in a magnetic core by a short circuit; unlatches the operating mechanism.

In some modern types of MCBs arc fault or ground fault mechanism are operative. The tripping is effected in the event of a line to ground fault or neutral to ground fault. In such case a torridly CT is used for differential current sensing.

Now-a-days a Molded Case Circuit Breaker (MCCB) is available which is the same as MCB and hence the further operation of MCB is explained in MCCB.

MOLDED CASE CIRCUIT BREAKER (MCCB)

Although there are many types of molded case circuit breakers manufactured, all are made up of five main components. There are:

·         Molded Case of Frame

·         Operating Mechanism

·         Arc Extinguishers

·         Contacts

·         Trip Units.

Five Main Components of an MCCB.

The function of the frame is to provide an insulated housing to mount all of the circuit breaker components. The frame is often a glass-polyester material or thermo set composite resign that combines ruggedness and high electric strength in a compact design. The frame is also known as a molded case.

                       

A frame designation is assigned for each different type and size of molded case: This designation is used to describe the breaker’s characteristics such as maximum voltage and current ratings, however, each manufacturer has their own identification system to account for the differences between breaker characteristics.

Operating Mechanism: Increasingly, molded case circuit breakers with conventional thermal magnetic trip units are being replaced by breakers with electronic trips units. These units provide increased accuracy and repeatability. Some units have built-in ground fault protection, removing the need for separate ground fault relays and Shunt Trips. Some units can also provide system monitoring, data gathering and communication to energy management system.

In general, electronic trip systems are composed of three components:

·         A current transformer (sensor) is used on each phase to monitor the current. It also reduces the current to the proper level for a input to a printed circuit board.

·         Electronic circuitry (printed circuit board) that interprets the input and makes a decision based on predetermined values. A decision to trip results in sending an output to the next component.

·         A low power flux-transfer internal shunt trip that trips the breaker. This is typically a mechanical, spring loaded device held in place by a permanent magnet.

When a tripping signal is received from the electronic circuitry, the effects of the permanent magnet are momentarily counteracted by the tripping pulse, allowing the mechanical tripping action to take place. There is no need for an external source of tripping power because the entire tripping system has very low power requirements.