MCCB Working Principle MCCB Rating and MCCB Current Setting

What is MCCB:

MCCB is Another Most popular Panel protection electrical device like MCB and MPCB. MCCB is a short term of molded case circuit breaker. It does not require any additional relay set up to trip the power circuit. It will protect from over current, short circuit, instantaneous and earth fault.


How to select MCCB:

While choosing an MCCB you have to look some important tips:

  1. Full Load current (written on the MCCB with some number ending with A)

Calculate approximate full load current, take 170 % of MCCB rating. Suppose if your load current is 32 A and then you have to choose (32 A X 170 % = 55 A.) 63 Amps MCCB. As rate comparison 63A MCCB is slightly higher than 32 A MCCB. So that you can purchase 63 A MCCB. By this We can avoid MCCB loading factor. Practically, MCCB do not load more than 55% of its full load to ensure the safe operation.

  1. Short circuit Current Carrying  Capacity:
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1.5–4 times of its full load current you can set.

  1. Instantaneous protection:

It does not have any time or current setting. It works based on IDMT (Inverse Definite Mean Time) curve.

  1. Earth fault:

By default, manufacturer does not give this future to his product, if you want, you need to spend some additional cost.

    1. Spreader link:

By default, they do not give. This link is used to connect multiple cables in one terminal. This is not necessary; if you want you have to purchase it.


  1. On/OFF handle:

This gadget is used to operate MCCB from outside door. If you are a panel builder, you must purchase it. To fix this we need some additional skill. Normal maintenance engg. does not require to purchase.

  1. MCCB status feedback: (trip Feedback)

If you are operating your starters start/stop through DCS/PLC with separated control supply, this future must be enabled (added), because if suppose due to some problem MCCB got tripped, due to separated control supply the starter contactor will be in holding position only. Then PLC/DCS will consider motor/load in still running condition. Then the associated with interlock does not work. Major breakdown will happen.

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To avoid this, break down, you must purchase this future.

  1. Cost depending upon make and future:

Siemens > ABB > Schneider > Telemechanic > L&T

    1. User support:

Siemens > Telemechanic > ABB > Schneider > L & T

  1. Quality of usage: (high life time)

Telemechanic > ABB > Siemens > Schneider > L&T

How to avoid frequent failure of MCCB:  Why why analysis of MCCB failure:

  1. Loose contact:

 At the power terminal side which leads to insulation damage and terminal connection bolt failure. To avoid this, at least 6 months once you have to check the tightness of the bolt at both incoming and outgoing side of the MCCBs. To perform this use appropriate spanner or Allen Key. By this you can avoid 80% of MCCB failure.

  1. Frequent tripping:

Frequent tripping leads to damage the inner bimetallic strip, without proper investigation on fault history, do not switch it on. Get result of 20 % MCCB failure free.

  1. Loading factor:
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Do not load any MCCB more than 80 % of its full load. Always maintain 50-70 % of its full load which results good performance and lifelong free from failure. Result of 10 % failure free.

  1. Improper Power cable lugging:

Use exact size lug during cable termination. If your cable is 35Sqmm copper, then use 35 sqmm copper lug only. Do not use low size or higher size lug. If you do so, which leads to loose contact at cable to lug terminal.

  1. Improper lug crimping:

While lugging of your cable use standard size crimping bit. For 35 sqmm cable use 35 + 25 bits. This combination gives 99.5 % success rate.

  1. Using improper lug:

For Copper cable use copper lug. For Aluminium cable use Aluminium lug. Do not interchange it. If you do so, after 5-7 years again you have to rework on it.

Also see: How to Do proper Lugging

  1. Falling Power Cable loads on MCCB terminal:

Give proper mechanical support to your power cable.

These why! Why! analysis really helpful do you….

Thanking You!!! Have a Nice day……….  



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