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MCCB Selection for APFC Panels

MCCB Selection for APFC Panels
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For any APFC panel, MCCBs are required for short circuit protection, overload protection and for isolation of capacitors. MCCB selection for capacitive load is tricky because of heavy inrush current, high overload capacity and continuous full load. These inherent traits of a capacitor, complicates the selection process. The selection should be such that the MCCB should not nuisance trip during inrush current and should withstand continuous flow of overload current.


Whenever we use MCCB in an APFC panel, proper measures need to be taken against the ill effects of the inrush current. Normally the inrush current (more than100 times the capacitor rated current) will remain for a few micro-seconds and will not be sensed by the MCCB. However the contacts of MCCB may repel and bounce because of the current limiting feature, causing micro-arcs between the contacts of MCCB. This multiple bounce can result in premature failure of MCCB contacts. In order to reduce the magnitude of the peak inrush current, MCCBs must be used along with capacitor duty contactors or inrush current limiting reactors.


The maximum permissible current in a capacitor branch is 1.46 times the rated current. This factor is comprised of the following:


  • Harmonics overload and over voltage - 30%
  • Capacitance tolerance - 10%
  • Frequency variation - 2%


The branch MCCB must be capable of carrying this current continuously.

The following table** shows the selection of MCCBs and Capacitor switching contactors for an APFC panel:

kVAr Rated Current (A) Derating Factor Permissible Capacitor O/L (A) Recommended Contactor MCCB* Thermal Setting (A) Magnetic Setting (6 to 10*Ir)
In Ir (100% In)
10 13.12 1.46 19.16 MO C10 DU / DN / DZ 19 19 171
15 19.68 1.46 28.74 MO C15 DU / DN / DZ 29 29 261
20 26.24 1.46 38.32 MO C20 DU / DN / DZ 38 38 342
25 32.80 1.46 47.90 MO C25 DU / DN / DZ 48 48 432
30 39.37 1.46 57.47 MO C30 DU / DN / DZ 58 58 513
50 65.61 1.46 95.79 MO C50 DU / DN / DZ 96 96 864

* Selection principle is same for other family of MCCBs

**The table is valid only for E&A switchgear and capacitors at 440 V


The rated current of a capacitor can be calculated as

Rated Current Capacitor

For any capacitor bank, permissible overload is 1.46 times rated capacitor current. Hence, MCCB should be rated to carry continuous over current as given in the above table. 


Above selection chart is valid only if MCCB is used along with capacitor duty contactor. Please consider the above table purely as a guideline for selection. Actual selection needs to be done based on considerations of connected load and the electrical network properties.

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About the Author

author

Sourav Dasmodak,

Product Management & Marketing (Powergear - ACB)

Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).