What is MPCB Motor Protection Circuit Breaker

What is MPCB Motor Protection Circuit Breaker:

Before talking about any electrical power circuits in the industry you must know the basic idea about the following protecting equipment,

  1. MPCB
  2. MCB
  3. SFU
  4. SDU
  5. MCCB
  6. CB
  7. Relay unit

What is meant by MPCB:

MPCB: Motor protection circuit breaker.

It is the Primary protection of the motor/any power unit. It will protect our equipment from short circuit, equipment overload, single phasing and sometimes from single phase to earth fault. This is additional protection or backs up protection for our motor. It is available from 0.1 Amps to 630 Amps. It does not have any range selection option for short circuit protection. We had used up to 400 Amps MPCB for our one of the 160 KW motor. It is costlier than other primary protection. It is used for indoor application (inside the factory or home premises) purpose only.

Example: 45 – 63 A Siemens make MPCB cost around Rs 45,000 INR ($750 USD)

How to select MPCB(Motor Protection Circuit Breaker):

Based on the range:

MPCB Range Calculation= (KW/1.732*Volts*pf) *160 % (For Step 1 upgradation)


Calculate your current requirement. Consider 7.5KW, 3 phase, 415 V, calculated current is 14 Amps (7.5 KW=10 Hp=> 14 amps, 1 Hp= 1.4 Amps in 3 phase), Then you have to choose the MPCB range between 13 A-22 Amps. If you are doing any electrical design work, it should be in one step upgrade. By selecting the MPCB range between the 13 Amps to 22 Amps you have two powerfull advantage,

  1. MPCB does not load more than 10 % of its full load. So that you can avoid frequent failure of MPCB.
  2. You can use the same circuit for 11 KW (Full load Amps = 21 Amps) motor also.

Based on make:

  1. Siemens make
  • In smaller range, it does not have any trip indications. We cannot find any difference between OFF and Trip.
  • Rough use
  • Less wear and tear against short circuit
  • Less failure of mpcb accessories like resetting mechanism, on/off nob and side NO/NCs
    1. ABB make
  • It has same advantage like Siemens but the disadvantage is frequent failure in resetting mechanism. I have replaced around 25 number of ABB make mpcb for over two years due to resetting mechanism failure and break down.

Better you choose Siemens make.

MCB: Miniature circuit breaker:

It will protect from short circuit. It does not protect from earth faults. If you want earth fault protection you need to install ELCB (earth leakage circuit breaker) or RCCB (Residual case circuit breaker)at the range of 100 mA or more.

Where to use:

Use it for the Lighting circuit, control circuit protection (control transformer input and output), Welding machine power distribution and some indoor/outdoor temporary power distribution.


Also see: The Full details about MCB:What is MCBs & How To Select MCB & How to Avoid Failure MCB

Also see: How Welding machine Works:

SFU: Switch Fuse unit:

It consists of fuse section and Fuse base section

Most Widely used indoor, outdoor and domestic power distribution application. It protects our circuit from Short circuit and overload, not from earth fault. It is considering of 3 number of fuses (at your required range), open and close mechanism.

If any short circuit situation arises in the circuit, then the particular phase’s fuse will blow. Then circuit will be protected. The fuses blowing speed is purely depending upon the MOC (Material of construction) of the fuse filament. There are many type of fuses is available in the market, select them (base on your application).

Also see: Relation ship between Line Voltage, Line Current, phase voltage, and Phase current

  1. HRC fuses

High Rupturing Capacity fuses, it is also called as general purpose fuses. It can be used in heater circuit, lighting circuit, a control circuit with non-electronics etc.

  1. HBC fuses

High breaking capacity fuses. It can be used in input power distribution of power electronics’ types of equipment like AC VFD drives, DC centrifugal machine drives, any small voltage AC to DC convertor soft starters etc. this is faster than HRC fuse.

  1. Semi-conductor fuses

Its main job is to protect the circuit from any fault on semi-conductor’s failure like transistor, thyristor, diode, electronics control board etc. it is faster than HBC type fuses.

Fuse rating Calculation:

I fuse = (KW/(1.732*Volts*pf)) *130 % for three phase

I = (KW/(Volts*pf)) *130 % for single phase

Fuse base Selection:

Selection of fuse base much higher than the fuse rating.

Fuse base= fuse * 150 %

Example: For 400 A fuses, you should select 630 Amps fuse base.

MCCB: Molded Case Circuit Breaker

It is the second option for MPCB/SFU. It has some advantages over MPCB

Also see: What is MCCB, How to select MCCB, How to avoid frequent failure of MCCB

CB: Circuit breaker & Relay unit

It is used for high short circuit current and high voltage application

Also see: Why Capacitor use as High Voltage Protection 

Use it for more than 400 Amps application.

Types of circuit breaker: depending upon the medium used

  • ACB

Normal air is used as breaking medium. Low Voltage application (LT), upto 4000 Amps can be used

SF6 Gas use as breaking medium. It absorbs free electron which property uses at here.

  • Vacuum circuit breaker

Vacuum does not allow free electron which property is used here.

Relay unit:

Relay unit is the primary decision maker for open or closing any circuit breaker.

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