Dec 23, 2017

SELECTION OF MCB / MCCB ( LESSON-1)

SELECTION OF MCB / MCCB

Introduction

MCB or 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 or MCCB involved technical, Mechanical parameters. Some parameters are important but some parameters are confusing and mislead to wrong selection of MCCB. Some parameters are directly affected on cost of MCCB.  

Specification / Name Plate Details of MCB/MCCB:

 Following specifications are required to select appropriate MCB or MCCB.
(A) Current Related:
  • Frame Size (Inm): Amp
  • Rated current (In/ Ie): Amp
  • Ultimate short circuit breaking capacity (Icu): KA
  • Rated short-circuit breaking capacity (Ics): % of Icu
(B) Voltage Related:
  • Rated voltage (Ue): Volt
  • Rated Insulation voltage (Ui): Volt
  • Rated impulse withstand voltage(Uimp): KV
  • No’s of Pole : SP,DP,TP,TPN,FP
(C) Application Type:
  • Utilization Category/ Characteristic : B,C or D curve
(D) Accessories:
  • Rotary Handle: Extended/ Direct
  • Alarm Contact:
  • Shunt Trip:
  • Under voltage Trip:
  • Mechanical interlocking:
  • Manual /Auto operation
  • Motorized Operation:
(E) Protection Type:
  • Protection : Over current / Short circuit
  • Trip Mechanism: Thermal / Magnetic / Solid / Microprocessor
  • Trip Mechanism adjustment : Fixed / Adjustable
(F) Others:
  • Frequency;
  • Reference temperature: (if different from 30°C)
  • Pollution degree:
  • Suitability for isolation:
  • Type of Mounting arrangement
  • Electrical Life Cycles:
  • Mechanical Life Cycles:
  • Dimension: mm
  • Weight: Kg
  • Reference Standard: IEC: 60947-1/2, IS: 13947-1/2

 (A) Current Related:

  (1) Frame Size (Inm): 

  • Breaker Frame Size indicates the basic framework of the Plastic shell of MCCB that can hold the biggest 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 determines the physical dimensions of the device.
  • There are varieties current ratings MCCB for the same series frame Size.
  • For example, DX100 Frame Size MCCB for rated current of 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A.
  • Same DX225 Frame Size MCCB for rated current of 100A, 125A, 160A, 180A, 200A, 225A.
  • In above DX100 and DX225 has two Type of frame Size for rated current of 100A, but the shape and size of breaking capacity of circuit breakers is not the same.

 (2) Rated Current (In /Ie):

  • It is the current value above which overload protection is tripped.
  • For MCB it is fixed while in MCCB the rated current is an adjustable range instead of a fixed value.
  • Standard rating of MCB is 1A, 2A, 3A, 4A, 6A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 100A for MCB.

(B) Voltage Related:

 (3) 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.

(4) 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 Amperes:
  • 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.

 (5) Rated working voltage (Ue):

  • 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.

(6) Rated Insulation voltage (Ui):

  • 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.

(7) Rated impulse withstands voltage (Uimp):

  • 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).
  • Uimp = 8kV means Tested at 8 kV peak with 1.2/50μs impulse wave.

(8) 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.
  • Used: for Single Phase circuit
  • Double Pole (DP) MCB: 
  • A two Pole MCB provides switching and protection both for a phase and the neutral.
  • 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.
  • 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.
  • 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.
  • Used: for Three Phase circuit with Neutral

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