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 Post subject: Motor Contribution and Fault CurrentPosted: Mon Jan 21, 2008 4:07 pm
 Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 510
Location: New England
Arriving at the actual Arcing Current and determining breaker clearing time is the key to Incident Energy. High Fault Current values produce larger arcing currents, but lower value arcing currents may shift the response of the breaker and take longer to clear and result in more Incident energy - hence we calculate the 85% value of calculated Arcing Current and then recalculate Incident Energy to see which case is worse.

For several of the equations you are asked to input a KAIC value for 3 phase bolted fault to calculate Arc Current. When doing a 'fault current evaluation' you normally include motor contribution when dealing with installations that have significantly motor loads. So this could add 20% to the transformer let through fault value. One could say to add in the motor contribution and make the fault current value higher. Another to argue that an electrician could be working at a time when most of the motors are off and hence there is no motor contribution.

Seems to me we have to perform a multitude of cases to arrive at the one that will produce the worse Incident Energy.

A.) Secondary transformer fault current with infinite bus primary with motor load contribution
B.) Secondary transformer fault current with finite bus primary with motor load contribution
C.) Secondary transformer fault current with infinite bus primary without motor load contribution
B.) Secondary transformer fault current with finite bus primary without motor load contribution

I am often coming up with motor contributions of 15 to 23% in many of my buildings. So I am concerned about just using the transformer impedance.

How do you suggest dealing with 'motor contribution' of fault current and is there a customary way to deal with the finite versus infinite bus debate. I personnally prefer to use infinite bus, as then I don't have to worry about changing made off site by the utility, and or upstream of the building transformer.

I am thinking the two worse cases would be with the highest and lowest value of fault current. So I would calculate:
1) Highest Fault Current = Infinite Bus primary + full motor contribution
2) Lowest Fault Current = Finite Bus + no motor contribution (althought if the primary is by the Utility I don't really have control of it, so I might just assume a 10% reduction from the infinite bus value as a worse case.

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 Post subject: Posted: Tue Jan 22, 2008 6:48 am
 Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
Haze

Your worst case for purposes of arc flash should be with motor contribution, regardless of whether you use infinite bus or use utility contribution.

This is because the motor contribution (on the same bus) should have no effect on breaker operating time, because the breaker will not see it, assuming the fault in below the clearing breaker. The motor contribution will increase the arcing current, however, causing the energy level to increase.

As for infinite bus vs. finite bus, I prefer to go with the most accurate calculation method. Because I use SKM, though, it is pretty easy to change the parameters and rerun the study for my facility, though. That being said, at my facility I have changed the utility contribution in order to calculate differing scenarios, and at this point it has had little to no effect, because the majority of the high voltage transformers are saturated with the utility contribution all ready.

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 Post subject: Posted: Tue Jan 22, 2008 9:12 pm
 Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 510
Location: New England
I'm thinking of an Industrial layout where you have switchgear feeding MCC line ups. Electrician is working on the MCC when fault occurs. Switchgear breaker would not see the motor contribution current but the electrician would, and since you calculate Incident Energy at the fault, you would need to use transformer contribution and motor contribution. Clearing time would be based on transformer contribution only.

I get the feeling that you need multiple scenerios for worse case. Worse fault current would be transformer + motor contribution and assume infinite bus. Worse clearing time would be transformer only and assume finite bus.

Then calculate Incident Energy for both and see which is worse. Then take 85% value of transformer only finite bus fault current and calculate IE.

This would then give you the highest fault current possible, and the most limited fault current with longest clearing time.

Would this be a good approach.

If you wanted to be even more technically accurate, you would have to also realize that motor contribution will only last one cycle for induction motors, and you could apply some reduction factor based on how many cycles to clear.

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 Post subject: Posted: Wed Jan 23, 2008 8:25 am
 Sparks Level

Joined: Fri Nov 09, 2007 3:27 pm
Posts: 54
Location: Texas
haze10 wrote:
I'm thinking of an Industrial layout where you have switchgear feeding MCC line ups. Electrician is working on the MCC when fault occurs. Switchgear breaker would not see the motor contribution current but the electrician would, and since you calculate Incident Energy at the fault, you would need to use transformer contribution and motor contribution. Clearing time would be based on transformer contribution only.

I get the feeling that you need multiple scenerios for worse case. Worse fault current would be transformer + motor contribution and assume infinite bus. Worse clearing time would be transformer only and assume finite bus.

Then calculate Incident Energy for both and see which is worse. Then take 85% value of transformer only finite bus fault current and calculate IE.

This would then give you the highest fault current possible, and the most limited fault current with longest clearing time.

Would this be a good approach.

If you wanted to be even more technically accurate, you would have to also realize that motor contribution will only last one cycle for induction motors, and you could apply some reduction factor based on how many cycles to clear.

This is where a three dimensional fault curve would be helpful to allow at least one of the two variables - motor contribution and utility contribution - to be modeled.

To alleviate the above problem some manufacturers offer breakers with SERVICE switch that turns down the instantaneous setting to minimum to help squeze down the available fault energy. Of course this creates a host of other problem, such as automatically starting large motors, but it is not a perfect world that we live in .

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 Post subject: Posted: Sun Mar 23, 2008 7:42 pm
 Sparks Level

Joined: Thu Oct 18, 2007 7:10 pm
Posts: 263
Location: NW USA
This is a reflection of the open-ended-ness of arc flash calculations. As NFPA 70E 130.7 (C) (9) (a) FPN No. 1 states: Both larger and smaller available short cuircuit currents coudl reslt in higher available arc-flash energies.

Motors on other branches can contribute to the fault current sensed by the protecting relay, but that contribution would usually diminish quicker than needed by Time Overcurrent for sensing; instantaneous should see it. I believe most software packages consider motor contribution as a continuous current.

My personal opinion is that arc flash exposure will be much affected unknowns such as arc resistance, which might be determined by unknown quantities such as ambient temperature and humidity. Also, there is no practical way of knowing how strong the utility is operating at every time, which parallels they have open, etc. So we use the best software we can, do the calculations as accurately as feasible, knowing that we have improved the work environment even though the answers are not black and white. It's a way of thinking not a yes/no answer. (unfortunatley a trial jury might not see it that way).

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