Breaker Coordination

K

kjroller

Member
Location
Dawson Mn
Occupation
Master electrician
I have a 2500 amp main, a 800 amp feed to a mcc from switch board, and 500 amp inverse time breaker bucket feeding a 200 hp motor. These are all in series with each other out to the motor. Recently we have a lightning storm I had to come in because the main had tripped upon further investigation the 800 amp feeder and the 200hp bucket had also tripped all at the same time. My first question is can lightning create a surge tripping everything downstream of the device it struck?
My second question is this motor is has very high iduction and the inrush from the chart my plc graphs based on the PLC shows when we tried to start the motor back up we were drawing almost 800 amps. Long story short when operators tried to restart the motor the main, mdp and bucket tripped again simultaneously. Why do they trip simultaneously? If it gets to the main and the main clears because they are not coordinated properly why would the mcc feed and the bucket also trip?

(I adjusted the instantaneous trip settings the second time we tried again and it started up but the windings were warm and it only drew 650 amps starting up and then leveled off to 150 and stayed level and is now working but if anyone has some insight I would appreciate any feedback)
 
This happens more often than it should. You are looking for selective coordination which is rarely ever included as part of new construction specifications.

A voltage surge does not, by itself, cause tripping however it can lead to things such as a momentary blip in the power grid which causes all motors to restart as they are trying to spin down.
 
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Ive been reading up on it and it sounds like what Ron said about tcc overlapping is whats happening the minimum trip time to maximum clearing time must be over lapping on all 3 breakers causing each one to trip since the fault wont clear before it hits minimum trip time. What sucks is we just had a company come in and change all the settings on the breakers so they wouldnt do this so either the report is wrong and they put in wrong settings or they missed it
 
kjroller said:
What sucks is we just had a company come in and change all the settings on the breakers so they wouldnt do this so either the report is wrong and they put in wrong settings or they missed it
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Was it their job to actually implement the changes? I have performed many studies and have no idea it they were ever acted on.

Reducing Arc Flash Incident Energy levels often makes selective coordination difficult. This is one reason arc flash reduction switches are now required by the NEC, so they are available for use during maintenance.
 
What is starting method for this 200 HP motor? Across the line, electronic soft starter, auto transformer, part winding, wye-delta? Variable frequency drive? VFD and electronic soft starter likely not to give you as high of a surge of starting current unless there is something wrong with it or some parameters are not set correctly. I could see a surge of around 800 amps being considered normal for some the old school reduced voltage starting methods, is still a fair amount less than what you likely would see with across the line starting.

Maybe the lightning event took out something that effects the starting method? This kind of presuming you never had this starting problem before the lightning event.
 
Ive worked there a year so I have previous knowledge of this instance in particular but we have many othwr ones like it and it is on a soft starter it has a HIM but not used as a VFD.
kwired said:
What is starting method for this 200 HP motor? Across the line, electronic soft starter, auto transformer, part winding, wye-delta? Variable frequency drive? VFD and electronic soft starter likely not to give you as high of a surge of starting current unless there is something wrong with it or some parameters are not set correctly. I could see a surge of around 800 amps being considered normal for some the old school reduced voltage starting methods, is still a fair amount less than what you likely would see with across the line starting.

Maybe the lightning event took out something that effects the starting method? This kind of presuming you never had this starting problem before the lightning event.
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We do have surge protection at the Main as well but it is possible. My other question is on those TCC charts when you have a fuse when it hits the unlatch trip point will they automatically blow or do they have a span as well like a breaker? (Given the time requirement has been met) Im wondering cause if the trip curves overlap and the breaker feeding kicks before the fuse blows will the fuse still blow
 
jim dungar said:
Was it their job to actually implement the changes? I have performed many studies and have no idea it they were ever acted on.

Reducing Arc Flash Incident Energy levels often makes selective coordination difficult. This is one reason arc flash reduction switches are now required by the NEC, so they are available for use during maintenance.
Click to expand...
It was there job to do so our management tends to go with there opinion and they recommended it as part of there 5 year arcflash study
 
kjroller said:
Ive worked there a year so I have previous knowledge of this instance in particular but we have many othwr ones like it and it is on a soft starter it has a HIM but not used as a VFD.

We do have surge protection at the Main as well but it is possible. My other question is on those TCC charts when you have a fuse when it hits the unlatch trip point will they automatically blow or do they have a span as well like a breaker? (Given the time requirement has been met) Im wondering cause if the trip curves overlap and the breaker feeding kicks before the fuse blows will the fuse still blow
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Answered my own question it has a 5% band so once it reaches its mark it will unlatch instantly normally less then 50 milliseconds
 
kjroller said:
Ive been reading up on it and it sounds like what Ron said about tcc overlapping is whats happening the minimum trip time to maximum clearing time must be over lapping on all 3 breakers causing each one to trip since the fault wont clear before it hits minimum trip time. What sucks is we just had a company come in and change all the settings on the breakers so they wouldnt do this so either the report is wrong and they put in wrong settings or they missed it
Click to expand...
Even if they did an awesome coordination study and set the breakers per the study, many breakers in the common power path will overlap in the instantaneous region and cannot be selectively coordinated unless selected very carefully during installation.
 
ron said:
Even if they did an awesome coordination study and set the breakers per the study, many breakers in the common power path will overlap in the instantaneous region and cannot be selectively coordinated unless selected very carefully during installation.
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Yeah ive noticed that the problem is i dont think the overlap that is tripping us out is taking place in the instantaneous region i think its in the short time pickup
 
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