Breaker Design Correct? 1

[mp08]

We had an incident where we shutdown parts of plant by tripping one of our main breakers.

Maintenance went to switch a 70A breaker that tripped which caused the breaker. We know the 70A breaker tripped because of a ground fault when we found some wire with no insulation on it.

My question is that between the 70a and main breaker, there were two smaller breakers. One was for the full panel that the 70A breaker is. The other was on the main distribution panel before the main breaker. Both were set for 225 A, which according to our documents and calculations, they are set correctly at.

We had one person comment that maybe those two breakers were designed for over current and in the case of a sudden trip, like a ground fault, they wouldn’t trip. Does this sound correct and is there anyway to prove our system was designed this way?

I’m a chemical engineer and we have no electrical resources here, so I thought posting might be worth a shot since it’s helped in the past.

Thanks.

 

[wbd]

The best way is to look at the time current curves for the breakers to see how they line up. Also knowing the magnitude of the SLG fault would help and that can be plotted on the TCC. You state that the breakers are correctly set, so you had a study done? Also you don't mention if the main breaker has ground fault settings on it.

 

[waross]

Sounds like too much guessing.
This puzzles my:

My question is that between the 70a and main breaker, there were two smaller breakers............... Both were set for 225 A,

In my world 255A is greater than 70A??

. We know the 70A breaker tripped because of a ground fault when we found some wire with no insulation on it.

When trouble shooting, some of us see a subtle difference between a breaker tripping because of a ground fault and a breaker tripping because of the excess phase current caused by a ground fault.
Tripping on a ground fault may imply that the breaker has ground fault monitoring and tripping capability.
This is rare on a 70 A breaker.
The vast majority of North American breakers in frame sizes from 70 Amps to 225 Amps are thermal/magnetic.
The thermal trips have an inverse time characteristic.
Tripping is not instantaneous, but the higher the overload, the shorter the tripping time.
The magnetic trips are instantaneous and operate on fault currents, called over-current in some codes.
Note:
Overload; Too much load applied.
Over-current: A fault condition.
Instantaneous trips in that range of frame sizes are typically set to operate at from 500% to 1000% of rated current.
The instantaneous trip setting is adjustable from 5X to 10X in many breakers.
Given that, the maximum fault current before a trip on a 70 Amp breaker is 700 Amps or 10 times 70 Amps.
The minimum fault current before a trip on a 225 Amp breaker is 1125 Amps, or 5 times 225 Amps.
There is no overlap here.
There is no basis to question a 70 Amp breaker tripping before a 225 Amp breaker.
Do any of these breakers have ground fault monitoring?
You have not indicated this.
If any of the breakers have ground fault monitoring, then you must compare the trip settings, not the breaker frame sizes.
Or, compare the ground fault trip setting on one breaker with the instantaneous setting on a breaker without ground fault tripping.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Nice synopsis Bill.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Thanks Keith. I value your praise.

Bill
--------------------
"Why not the best?"
Jimmy Carter