MCB/MCCB are widely used in electrical distribution system for ON/OFF Electrical supply and it also gives over current and short circuit protection. Selection of MCB/MCCB involves technical as well as Mechanical parameters. Not all the parameters are important but careless observation leads to wrong selection of MCB/MCCB or may increase cost unnecessarily.
Specifications of MCB/MCCB:
Frame Size (Inm): Amp
Rated current (In/ Ie): Amp
Ultimate short circuit breaking capacity (Icu): KA
Rated short-circuit breaking capacity (Ics): % of Icu
Rated voltage (Ve): Volt
Rated Insulation voltage (Vi): Volt
Rated impulse withstand voltage(Vimp): kV
No’s of Pole : SP,DP,TP,TPN,FP
Utilization Category/ Characteristic : B,C or D curve
Rotary Handle: Extended/ Direct
Under voltage Trip
Manual /Auto operation
Protection : Over current / Short circuit
Trip Mechanism: Thermal / Magnetic / Solid / Microprocessor
Trip Mechanism adjustment : Fixed / Adjustable
Reference temperature (if different from 30°C);
Suitability for isolation;
Type of Mounting arrangement;
Electrical Life Cycles;
Mechanical Life Cycles;
Reference Standard: IEC: 60947-1/2, IS: 13947-1/2
Breaker Frame Size indicates the basic framework of the Plastic shell of MCCB that can hold the highest rated current. It is the maximum current value for which the MCCB is designed (upper limit of the adjustable trip current range) and it also a deciding factor for physical dimensions of the device. There are varieties of current rating MCCB for the same series frame Size are available in the market.
For example, DX100 Frame Size MCCB are available in the market for the rated currents of 16A, 20A, 25A, 32A, 40A, 50A, 80A, 100A etc. Also for rated current of 100A MCCB of the frame size DX225 is also available.
It is the current value above which overload protection is tripped and the circuit is disconnected.
For MCB, rated current is fixed, while in MCCB the rated current is adjustable.
Standard rating of MCB is 1A, 2A, 3A, 4A, 6A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 100A.
Ultimate short-circuit breaking capacity (Icu):
Breaking capacity can be defined as the maximum level of fault current which can be safely cleared.
It is the highest fault current that the MCCB can trip without being damaged permanently.
The MCCB will be reusable after interrupting a fault, as long as it doesn’t exceed this value.
It is indicate operation reliability of MCCB
This parameter may increase or decrease the cost, so it should be properly decided. Breaking capacity should be higher than the possible fault level. For domestic application fault level may be 10kA.
Operating short-circuit breaking capacity (Ics):
It is expressed as a percentage ratio of Icu and tells you the maximum short-circuit current if a circuit breaker can break three times and still resume normal service.
The higher the lcs, the more reliable the circuit breaker
It is the maximum possible fault current that the MCCB can clear. If the fault current exceeds this value, the MCCB will be unable to trip and another protection mechanism must operate.
If a fault above the Ics but below the Icu occurs, the MCCB can interrupt it successfully but will need a replacement due to the damage suffered.
The Main difference between Ultimate Short Circuit (Icu) and Service Breaking Capacity (Ics) that Icu (Ultimate Braking Capacity) means Circuit breaker can remove the fault and remain usable but Ics (Service Braking Capacity) means Circuit breaker can remove the fault, but it may not be usable afterwards.
For example, if a circuit breaker has an Ics of 25,000 Amperes and an Icu of 40,000
Any fault below 25kA will be cleared with no problem.
A fault between 25kA and 40kA will cause permanent damage when cleared.
Any current exceeding 40 kA can’t be cleared by this breaker.
Rated working voltage (Ve):
It is the continuous operation voltage for which the MCCB is designed.
This value is typically equivalent or close to a standard system voltage.
In three phase it is usually 400V or 415 V. For single phase it is 230V or 240V.
Rated Insulation voltage (Vi):
It is the maximum voltage that the MCCB can resist according to laboratory tests.
It is higher than the rated working voltage, in order to provide a margin of safety during field operation.
Rated impulse withstands voltage (Vimp):
It is the value of transient peak voltage the circuit-breaker can withstand from switching surges or lighting strikes imposed on the supply.
This value characterizes the ability of the device to withstand transient over voltages such as lightning (standard impulse 1.2/50 μs).
Vimp = 8kV means Tested at 8 kV peak with 1.2/50μs impulse wave.
Number of Poles:
No of Pole for MCCB depends on Single Phase & Three Phase Power Controlling /Protection
Single Pole (SP) MCB:
A single pole MCB provides switching and protection for one single phase of a circuit. It is used for Single Phase circuit
Double Pole (DP) MCB:
A two Pole MCB provides switching and protection both for a phase and the neutral. It is used for Single Phase circuit
Triple Pole (TP) MCB:
A triple/three phase MCB provides switching and protection only to three phases of the circuit and not to the neutral. It is used for Three Phase circuit
3 Pole with Neutral (TPN (3P+N) MCB):
A TPN MCB, has switching and protection to all three phases of circuit and additionally Neutral is also part of the MCB as a separate pole. However, Neutral pole is without any protection and can only be switched. It is used for Three Phase circuit with Neutral
4 Pole (4P) MCB:
A 4 pole MCB is similar to TPN but additionally it also has protective release for the neutral pole. This MCB should be used in cases where there is possibility of high neutral current flow through the circuit as in cases of an unbalanced circuit. It is used for Three Phase circuit with Neutral.