Fuse and Circuit Breaker Selection Method 1

[PowerSynch]

Hello All,
What is the method for selection of fuses and circuit breakers when they are used in the same circuit. For axample we have a 75KW motor on a 415V system, If we choose a 160A circuit breaker, what should the fuse size be in the same circuit to buck up the C/B?
Is there a ratio between fuse and C/B that must be followed.

Thank you in advance.

 

[wareagle]

The NEC does not list 415 volt motors but a 460 volt 75 HP has a FLA of 96 amps. If you ratio the voltages a 75 HP at 415 would have 112 FLA. If thats true I would use a 225 amp
breaker. Bussman lists a 125 amp fuse as a backup protector.
Again this is for a motor with 96 FLA. I would suggest you contact Bussman for some assistance. Why do you think you need the extra fuse?

 

[waross]

hello PowerSynch
Your voltage looks like an IEC voltage. I am not sure if you are using the breaker and/or the fuses for overload protection or overcurrent (Short circuit) protection.
NEC standards for motor running protection are:
115% or 125% of rated current for motor running protection, depending on the service factor and/or the rated temperature. This would apply to fuses or a breaker with adjustable thermal trips, or an overload relay. (We would treat a breaker with adjustable thermal trips as a manual motor starter in NEC or CEC jurisdictions.)
For overcurrent (short circuit) protection, the following apply
Non time delay fuses, 300%
Dual element, time delay fuses, 175%
For instantaneous breakers, 800%
For inverse time, (thermal) breakers, 250%
There are exceptions that allow you to increase some of these ratings in the event that a motor can not be started.
respectfully

 

[itsmoked]

I might note that fuses as motor protection are pretty bad and often result in burnt out motors.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[waross]

Right itsmoked.
I should have added that the fuses for motor protection are dual element, time delay fuses. Fuses for overload protection is one of those things you see in the code book but not in the field. Non time delay fuses will not protect a motor from overload.
respectfully

 

[rbulsara]

what waross said...

 

[PowerSynch]

Thanks for your thoughts.
We are using CBs for overload protection and Fuses for short circuit protection. Of course CBs also provide short circuit protection, but for some important drives and motors fuses are also used as additional back up protection.
The problem is that in case of a short circuit, CB and fsue will be racing to clear the fault. Proper coordination between them, should provide a condition that fault is cleared in the most appropriate manner.

 

[itsmoked]

Why don't you just use the standard mag breaker and a motor starter with an electronic overload? No fried motors.


Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[powerjunx]

Powersynch, you are up to the "METHOD for selection of fuses and circuit breakers when they are used in the same circuit"

We usually refer to the GRAPH on pre-arcing and arcing time of the FUSE. A good rule is " a FUSE backs up the CB" (your correct), else a faulty design and miscoordination exists.

In order to prevent the race between FUSE and CB, we always limit the CB not to "overlap the pre-arcing time, if not, hinders it to reach on arcing time" of the FUSE, otherwise racing and blowing of fuse couldnt be prevented. Eventually, this would have a sound coordination.

But this time, we always to recommend to use a CB having a AT and AF ratings (manufacturer's provided you data you need) which cause even higher costs. Thus, ease to use a fuse.