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Motor protected by CB & Fuses #69161
08/31/06 07:25 PM
08/31/06 07:25 PM
B
Bill39  Offline OP
Member
Joined: Nov 2001
Posts: 75
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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Re: Motor protected by CB & Fuses #69162
08/31/06 09:05 PM
08/31/06 09:05 PM
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
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.

Re: Motor protected by CB & Fuses #69163
08/31/06 09:40 PM
08/31/06 09:40 PM
G
gfretwell  Offline

Member
Joined: Jul 2004
Posts: 9,298
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.


Greg Fretwell
Re: Motor protected by CB & Fuses #69164
09/01/06 06:22 AM
09/01/06 06:22 AM
B
Bill39  Offline OP
Member
Joined: Nov 2001
Posts: 75
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?

Re: Motor protected by CB & Fuses #69165
09/01/06 08:06 AM
09/01/06 08:06 AM
R
Ron  Offline
Member
Joined: Mar 2002
Posts: 582
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.


Ron
Re: Motor protected by CB & Fuses #69166
09/01/06 12:05 PM
09/01/06 12:05 PM
S
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,213
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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