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#69161 - 08/31/06 04:25 PM Motor protected by CB & Fuses
Bill39 Offline
Member
Registered: 11/28/01
Posts: 75
Loc: Indianapolis, IN, USA
Why do many variable freq. drive panels have fuses in series with circuit breakers?

Also which should be first, the fuses or the circuit breaker(starting at the line side)?
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#69162 - 08/31/06 06:05 PM Re: Motor protected by CB & Fuses
renosteinke Offline
Cat Servant
Member
Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country
FUses are still used because:

- They trip / respond / blow a lot faster; and,

- They are available in many more sizes- allowing you to "fine tune" your protection.
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#69163 - 08/31/06 06:40 PM Re: Motor protected by CB & Fuses
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
Yup Reno that is my experience. In the computer biz they use breakers for overload protection and a fast blow fuse at a higher rating for overcurrent protection.
If the motor just bogs down the breaker will hold it for a while but if you get a short the fuse pops immediately, hopefully saving the drivers.
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#69164 - 09/01/06 03:22 AM Re: Motor protected by CB & Fuses
Bill39 Offline
Member
Registered: 11/28/01
Posts: 75
Loc: Indianapolis, IN, USA
I understand the difference between fuses and CBs.

What I'm trying to understand is why the designers of these VFD systems feel BOTH are needed on the 480V side.

The time delay fuses prevent false trips due to high inrush current (but won't the CB trip anyway since it is in series with the fuses).

So the question remains: why is a CB in series with a fuse?
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#69165 - 09/01/06 05:06 AM Re: Motor protected by CB & Fuses
Ron Offline
Member
Registered: 03/13/02
Posts: 582
Loc: White Plains, NY
I've seen fuses provided with breakers (in series), when the manufacturer was trying to achieve a higher AIC rating (SCCR) for the overall assembly.
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Ron
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#69166 - 09/01/06 09:05 AM Re: Motor protected by CB & Fuses
SteveFehr Offline
Member
Registered: 03/19/05
Posts: 1195
Loc: Chesapeake, VA
Regarding the order, I'd hope to see the CB first so that it can be opened while replacing the fuse and add that extra measure of safety.
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