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 Post subject: Coordination Problem!
PostPosted: Mon Dec 21, 2009 4:01 pm 
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I have attached a pdf of a coordination problem that I have. I have one major boundary condition which is inrush current. My load is 150 HP motor. I know that I can't set my circuit breaker settings low because it would trip upon motor start-up. As one can see in the attachment that I have coordination problems at the instantaneous and overload region of the TCC.

The instantaneous portion is the most important to me. It appears that if a large fault occured that it has potential that all three OCPD's would trip open. Is there a solution to this? Can this be coordinated in any way?

Thanks in advance for any help!


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PostPosted: Mon Dec 21, 2009 10:51 pm 
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Can you attach the PDF in unzipped form?


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PostPosted: Tue Dec 22, 2009 7:00 am 
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bruinfan wrote:
I have attached a pdf of a coordination problem that I have. I have one major boundary condition which is inrush current. My load is 150 HP motor. I know that I can't set my circuit breaker settings low because it would trip upon motor start-up. As one can see in the attachment that I have coordination problems at the instantaneous and overload region of the TCC.

The instantaneous portion is the most important to me. It appears that if a large fault occured that it has potential that all three OCPD's would trip open. Is there a solution to this? Can this be coordinated in any way?

Thanks in advance for any help!


Your TCC looks pretty straight forward. One motor circuit protector, one upstream molded case breaker and one current limiting fuse further upstream.

Yes it does appear that if the fault current is high enough, you could trip all three devices. That is quite common with molded case breakers in the instantaneous region and there is not a whole lot you can do about it. However, how long is the motor feeder?

Typically the curves are cut off at the maximum short circuit current as you have shown. I am assuming this is the current at the MCC or starter. If there is long run of conductor to the motor, then the fault current may not be as high for a fault at the motor – the condition most likely to take out all three devices. However a large fault at the MCC may still be a problem.

Setting the motor breaker lower won’t really help since the mis-coordination is a function of the upstream thermal magnetic breaker’s instantaneous setting.

An electronic trip breaker with Long, Short, time features would help but is economically impractical. Switching to all fuses might help but then, it also might be impractical – and you have single phase problems.

Looks like it is as good as it gets with what you have.

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PostPosted: Tue Dec 22, 2009 8:45 am 
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Thanks for your input. The motor feeder is 150 feet.

I had to zip the file because it was too big to attach. Sorry.


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PostPosted: Tue Dec 22, 2009 9:05 pm 
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Maybe you can dial down the instantaneous setting of the breaker, if it's still at or above where the MCP would trip. (If the MCP holds inrush at that level, so would the branch breaker)

This would give you less overlap with the Class L upstream, for some of those lower level faults.


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PostPosted: Sat Jan 09, 2010 12:04 pm 
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You could install a soft start if this a across the line start motor. Then adjust the ramp time to lower the inrush current. Your TCC curves don't show which is MCC feeder PD and which is the motor PD. You didn't include specifics like cable size, distance, motor rpm, nema rating, starter OL size, and locked rotor code. So I'll make some general assumptions. Right off the bat is why is the 300 amp MCCB inst set for 3000 amps? If the 150 hp motor trips the 300A MCCB on startup at anything less than 3000 amps inst, you have other problems other than a breaker mis-coordination issue. Has winding analysis ever been done on this motor?

What gets you is the locked rotor current until the motor is up to speed. Depending on the KVA/HP rating of the motor (typically a 150 hp motor, 180 FLA has a "G" LRC code). This calculates to 657% LRC (1183 amps). So in this case you could set the 300 amp MCCB (I assume is the motor PD) which looks like its set at 3000 amps inst trip to 1200 amps inst. Now this moves the motor TCC into a better coordination scheme slightly ahead of the main MCCB. It may not be perfect but it provides better protection for an $8,000 motor.

Using a TM type MCCB for a motor protection device in a starter is the worst thing to do from a motor protection and arc flash stand point. I always recommend low peak fuses that open in a 1/4 cycle as the starter short circuit protection which cuts the IE to almost nothing, and use the proper sized thermal OL's for the motor overload protection.

Then I would do a short circuit study of the MCC or distribution system that this motor starter is in and see if the main PD device inst or even the breaker rating are correct and can be increased. Yes, I know this increases the incident energy in an arc flash event. Many times I find "backwards" coordination. Where the main PD device is set too low and the starters are set too high. This always results in the client asking... "Why is a 1200 amp main breaker tripping taking out an entire MCC lineup and not a motor starter 300 amp breaker. The 300 amp MCCB must be defective... right??"

People tend to confuse motor over current protection and motor short circuit protection. They are very different animals. Motor over load protection is done with thermal devices with class 10 or class 20 TCC in the starter, or by I^t setting in VSD's and are sized by motor FLA to prevent winding damage due to overheating. Most motor stators will self-destruct in about 20 second or less at lock rotor current. Then it becomes a stator short circuit issue and the fuses or breaker kicks in. The trick is not letting the motor get to the over load stage in the first place.

I've seen allot of motor stators turned into expensive toaster ovens because the MCCB inst setting are cranked up to the max which does increase the breaker thermal time curve.


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