PFC fuses

[karalahana]

Hi, I am hesitating in this issue (POWER FACTOR CONNECT?ON CAPAC?TORS' L?NE PROTECT?VE FUSE RAT?NG )since I am not sure to which criteria we must select the fuses. I mean must we use a motor fuse since the in rush current makes necessary that a delaying type fuse is used. If this is so for sure (I am asking you of this) so what ratio must be between capacitor rated current and fuse rated current? I am confused since there are some sources which says at least 1,5 -1,6 ratio must be between them. Thanks a lot.

 

[karalahana]

Excuse me , a small mistake, as you might understand , it is power factor correction, not connection.

 

[rbulsara]

While the suggestions you are getting seems reasonable starting points for a low voltage system, there may be other considerations.

What is the voltage? Are these independent caps or connected to a motor? If motor O/L and OCPD is protecting the caps, you may not need individual cap protection. Whether or not they have series reactors to limit inrush makes a difference too.

You can check codes applicable to your project, I can only provide NEC reference (Article 460) which only states that for LV PFCC the OCPD (over current protective device) rating shall be as low as practicable (but still able to withstand capacitor inrush currents). Dual element fuse helps in this regard. For medium and high voltage only short circuit protection is required.

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

A quick cheat would be to look at PFC caps that include built-in fusing and see what they use... Not really good engineering practice, but it is expedient.

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[LionelHutz]

There are specific capacitors for fuses up to around 5kV, maybe even higher. The fuse manufacturers also often have guidelines for selecting them.

One thing to keep in mind is that the fuse should protect against the capacitor case rupturing. You might be able to get a capacitor case rupturing curve from the capacitor manufacturer and then plot the fuse curve over top to ensure the fuse protects the capacitor.

 

[karalahana]

excuse me but do not we use fuses against the overheat of the caps feeder? ?f not so how are protected both feeder and capacitor?