Why Did 3 Pole Breaker Trip but 90A Time Delay Fuses Didn’t Blow on 50HP Motor?

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#1 21683279 30 Sep 2019 23:50

Christopher Moya Christopher Moya

Anonymous

Post #1
21683279 30 Sep 2019 23:50

I have an odd confusing situation. This is a industrial size compressor system. The unit is a three phase 480VAC, 80A, compressor with a 50HP induction motor. As soon as the motor is to start, the main breaker in the panel trips.There is a disconnect switch to the compressor with three 90A time delay fuses (one per each phase). My question is why did the breaker trip? If anything, the fuse should of blew first. Should the fuse be fast acting fuses and not time delayed?
Kept looking and discovered there is a phase to phase short in the induction motor, but why didn't the OL trip? So really I have two issues. Some help would be great. thanks
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#2 21683280 01 Oct 2019 01:26

Elizabeth Simon Elizabeth Simon

Anonymous

Post #2
21683280 01 Oct 2019 01:26

What is the circuit breaker rating? What is the normal start-up current of the motor? What is the normal running current?
It sounds to me like the circuit breaker is sized correctly for the load (it tripped when it should have) but the fuses are not.
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#3 21683281 01 Oct 2019 22:21

Christopher Moya Christopher Moya

Anonymous

Post #3
21683281 01 Oct 2019 22:21

The designated three pole circuit breaker is a 175A breaker. The three fuses in the disconnect is 90A fuses. The compressor unit is rated at 80A. I can not tell the start-up current of the motor due to it being a dead short to ground. What i want to know is if there is a motor starter with an overload and three 90A fuses in the disconnect, why did they both fail? How did it fail? You would think the overload would of tripped or a fuse would blow which would of prevented the 175A circuit breaker to trip. Would you agree?
#4 21683282 02 Oct 2019 00:26

Elizabeth Simon Elizabeth Simon

Anonymous

Post #4
21683282 02 Oct 2019 00:26

In my experience, most time delay fuses can withstand twice their rated current for long enough that a faster acting circuit breaker may well trip before the fuse blows.
To more completely answer your question, you need to learn more about trip curves for fuses and circuit breakers. Here are a couple guides.
https://www.c3controls.com/blog/understanding-trip-curves/
http://www.cooperindustries.com/content/dam/p...re/bus-ele-an-3002-spd-selective-coord.pdfThe second one is interesting in that it covers the situation that you have with several protective devices in series.
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#5 21683283 03 Oct 2019 21:45

Christopher Moya Christopher Moya

Anonymous

Post #5
21683283 03 Oct 2019 21:45

Would you recommend fast acting 90A blade end fuses?
#6 21683284 04 Oct 2019 05:38

Elizabeth Simon Elizabeth Simon

Anonymous

Post #6
21683284 04 Oct 2019 05:38

I can't say that a 90A fast blow would work in this application since I don't know the startup current.
I would recommend that you contact the manufacturer of the compressor/motor and ask them for a fuse recommendation.
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#7 21683285 09 Oct 2019 02:04

colin adam colin adam

Anonymous

Post #7
21683285 09 Oct 2019 02:04

same shit happend to me once..
#8 21683286 15 Oct 2019 18:01

FauxN Cotton FauxN Cotton

Anonymous

Post #8
21683286 15 Oct 2019 18:01

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Topic summary

A 50HP, three-phase, 480VAC industrial compressor motor rated at 80A experiences immediate main breaker trips upon startup. The system includes a 175A three-pole circuit breaker and a disconnect switch with three 90A time delay fuses. Despite a detected phase-to-phase short in the motor, the 90A time delay fuses did not blow, and the motor overload protection did not trip, leading to the breaker tripping first. Time delay fuses can tolerate currents up to twice their rating for a duration, allowing a faster-acting breaker to trip before fuse operation. The fuse rating and type may be inappropriate for the motor's startup and fault conditions. Recommendations include reviewing trip curves for both fuses and breakers to understand coordination and consulting the motor/compressor manufacturer for proper fuse specifications. Fast-acting fuses are not necessarily suitable without knowing the motor's startup current and fault characteristics.
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Why Did 3 Pole Breaker Trip but 90A Time Delay Fuses Didn’t Blow on 50HP Motor?

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