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For any APFC panel, fuses are required for faster short circuit protection and overload protection of capacitors. Fuse selection for capacitive load is critical because of heavy inrush current, high overload capacity and continuous full load. These inherent traits of a capacitor complicates the selection process. Hence the selection should be such that even during these permissible abnormalities, the fuse should not blow.
Normally, the maximum permissible current in a capacitor branch is 2 times the rated current. This factor is comprised of the following:
The fuse used for branch protection must be capable of carrying this current continuously. Hence the fuse should be sufficiently rated so as to avoid the deterioration of the fuse element. Further the fuse should not blow during switching of the capacitors because of the inrush current (more than100 times the capacitor rated current). In order to prevent nuisance operation of fuse, its time-current characteristic should cover the peak inrush current of the capacitor. Hence for the optimal selection of fuses, the inrush current must be limited by using either capacitor duty contactor or inrush current limiting reactor.
The following table** shows the selection of Fuses and Capacitor switching contactors for an APFC panel:
| kVAr | Rated Current* (A) | Derating Factor | Allowed O/C for Capacitor (A) | Recommended Contactor* | Fuse Rating (A) |
|---|---|---|---|---|---|
| 10 | 13.12 | 2 | 26.24 | MO C10 | 32 |
| 15 | 19.68 | 2 | 39.36 | MO C15 | 40 |
| 20 | 26.24 | 2 | 52.48 | MO C20 | 63 |
| 25 | 32.80 | 2 | 65.60 | MO C25 | 80 |
| 50 | 65.61 | 2 | 131.22 | MO C50 | 160 |
* Selection principle is same for other family of fuses
** The table is valid only for E&A switchgear and capacitors at 440 V
The rated current of a capacitor can be calculated as.
For any capacitor bank, permissible overload is 2 times rated capacitor current. Hence, fuses should be rated to carry continuous overcurrent as given in the above table. In case Thyristor Switching Modules are used instead of capacitor duty contactors, it is highly recommended to use High Speed Fuses (semiconductor fuse).
Above selection chart is valid only if fuses are 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.
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).
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