MCC Breaker Tripping - Troubleshooting Help

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bindem

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Location
Westlake, LA
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Engineer
I have a MCC with a 600A MCCB breaker with a 310 digitrip unit which powers 3 motors: 100Hp, 150Hp, 200Hp.

We can run the 100 with the 150 no issues. When we try to start the 200, the 600A breaker trips pretty quickly.
We can run the 100 with the 200 no issues. When we try to start the 150, the 600A trips pretty quickly.

Looking at the TCC, it doesn't seem like the locked rotor current plus the FLA of the running motor doesn't get anywhere close to reaching the trip threshold.

Looking for help digging into this issue. I am a new engineer and not really sure where to start. Thanks in advance
 
Is this a new issue with an existing installation (i.e., has it worked properly in the past)? If so, try looking for what might have recently changed. Often, new issues can be traced back to recent maintenance, upgrades, replacements, and other changes that, at first glance, seem totally unrelated and unlikely to be the cause.

If this is a brand new installation, take a look at any testing or commissioning that was performed after installation.

Just out of curiosity, have you tried running the 150 and 200 together, then tried starting the 100?
 
charlie b said:
Is this a new issue with an existing installation (i.e., has it worked properly in the past)? If so, try looking for what might have recently changed. Often, new issues can be traced back to recent maintenance, upgrades, replacements, and other changes that, at first glance, seem totally unrelated and unlikely to be the cause.

If this is a brand new installation, take a look at any testing or commissioning that was performed after installation.

Just out of curiosity, have you tried running the 150 and 200 together, then tried starting the 100?
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To my knowledge, this issue has existed for several years. I am unaware of any recent changes, but will look more into this to be sure before ruling it out.

As far as sequence of startups, the two larger motors are unable to be ran in any event at the same time. The only scenario that works is running 100hp with (3823 OR 3825).
 
bindem said:
As far as sequence of startups, the two larger motors are unable to be ran in any event at the same time. The only scenario that works is running 100hp with (3823 OR 3825).
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Has the breaker ever been injection tested or serviced.
In the back of my mind I recall something about Digitrip plug and corroded contacts.
 
I'm not sure of the specific breaker model of course, but the TCC I whipped up makes it look awfully close.

It depicts the 200HP starting with the other two running. How long does it take to trip the 200HP? The point they come together is about 5 seconds, but since you're also having issues starting the 150HP I'd bet it's a little faster than that?

1750358692606.png
 
jim dungar said:
Has the breaker ever been injection tested or serviced.
In the back of my mind I recall something about Digitrip plug and corroded contacts.
Click to expand...
What about a digitrip plug
HarrySlother said:
I'm not sure of the specific breaker model of course, but the TCC I whipped up makes it look awfully close.

It depicts the 200HP starting with the other two running. How long does it take to trip the 200HP? The point they come together is about 5 seconds, but since you're also having issues starting the 150HP I'd bet it's a little faster than that?

View attachment 2578274
Click to expand...

Are you using ETAP to model this? If so, would you be able to teach me how to run this scenario? What is the amperage that it is intersecting at?

The information I've been give is it trips pretty "instantaneously" after mashing the start button. But that is relative. No recent startups for me to personally confirm an exact timeframe. For extra info this should be the specs on the trip unit:

1750359654744.png
 
bindem said:
As far as sequence of startups, the two larger motors are unable to be ran in any event at the same time. The only scenario that works is running 100hp with (3823 OR 3825).
Click to expand...
Now you have me confused. From your first post, I inferred that the trip happens when you have two motors running and you attempt to start the third. For example, you said that the 100 and the 150 can run together, but a trip happens when you start the 200. Now you are saying that the 150 and the 200 can never run at the same time.

Please clarify.
 
Sorry it's SKM - the intersection was at ~1870A and ~5s.

That was using 6.5 * FLA for inrush, then adding an extra running load of 300A for the other two
 
charlie b said:
Now you have me confused. From your first post, I inferred that the trip happens when you have two motors running and you attempt to start the third. For example, you said that the 100 and the 150 can run together, but a trip happens when you start the 200. Now you are saying that the 150 and the 200 can never run at the same time.

Please clarify.
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I don't believe in any case the 150Hp can be running and successfully start the 200Hp. and vice versa. regardless if the 100hp is already running
 
bindem said:
According to the TCC I'm finding for the breaker, 1800A wouldn't trip until 40 or so seconds
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Yeah I had a different breaker than you at first. It looks like there's room to start either of them, and since you can't even start the 150HP while the others are running the breaker is suspicious.
 
Note that there is a difference between LRC / Starting current, and the instantaneous magnetic inrush current when you first energize the motor. Inrush can be VERY high, as in 20x FLC in theory, albeit of very short duration. But when playing "close to the bone" like this, it might be the straw that is breaking the camel's back.

Are all three starting Across-the-Line? Maybe that's not such a good idea?
 
Jraef said:
Note that there is a difference between LRC / Starting current, and the instantaneous magnetic inrush current when you first energize the motor. Inrush can be VERY high, as in 20x FLC in theory, albeit of very short duration. But when playing "close to the bone" like this, it might be the straw that is breaking the camel's back.

Are all three starting Across-the-Line? Maybe that's not such a good idea?
Click to expand...
How can I get an accurate measurement of the inrush value and duration? Thank you for pointing this out as I was under the impression that inrush was the same as LRC
 
bindem said:
How can I get an accurate measurement of the inrush value and duration? Thank you for pointing this out as I was under the impression that inrush was the same as LRC
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Locked rotor current occurs when the motor rotor is not turning.
Inrush current occurs when the magnetic field is being created allowing the hunk of metal to become a motor.

LRA can be measured by simply turning on the motor then jamming the rotor so it doesn't turn. This is the value on the nameplate.
Inrush current can be measured but it cannot easily be repeated. This current depends on the power supply (including source, conductors, and magnetic properties of the motor) so what happens in the factory is not likely to be what happens in the field.
The motor nameplate may show a Code Letter which might be used to compare inrush current range.

Peak inrush current happens very very quickly and should only be seen by the Instantaneous portion of the breaker curve.
 
jim dungar said:
Locked rotor current occurs when the motor rotor is not turning.
Inrush current occurs when the magnetic field is being created allowing the hunk of metal to become a motor.

LRA can be measured by simply turning on the motor then jamming the rotor so it doesn't turn. This is the value on the nameplate.
Inrush current can be measured but it cannot easily be repeated. This current depends on the power supply (including source, conductors, and magnetic properties of the motor) so what happens in the factory is not likely to be what happens in the field.
The motor nameplate may show a Code Letter which might be used to compare inrush current range.

Peak inrush current happens very very quickly and should only be seen by the Instantaneous portion of the breaker curve.
Click to expand...
Having a hard time finding info differentiating between inrush and LRC as far as calculations. Typically use 6xFLA for LRC, is there a good rule of thumb for IRC? Is a MCCB able to react to the inrush current since it is typically only 1/2 cycle?
 
bindem said:
Having a hard time finding info differentiating between inrush and LRC as far as calculations. Typically use 6xFLA for LRC, is there a good rule of thumb for IRC? Is a MCCB able to react to the inrush current since it is typically only 1/2 cycle?
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In the few times i have had to guess at inrush I have used 20X for 1/2 cycle.
Yes some OCPDs can react to inrush currents, because they look like bolted faults. This is one reason the NEC allows motor, and transformers protection to be sized up to 250% full load amps.
 
bindem said:
I have a MCC with a 600A MCCB breaker with a 310 digitrip unit which powers 3 motors: 100Hp, 150Hp, 200Hp.

We can run the 100 with the 150 no issues. When we try to start the 200, the 600A breaker trips pretty quickly.
We can run the 100 with the 200 no issues. When we try to start the 150, the 600A trips pretty quickly.

Looking at the TCC, it doesn't seem like the locked rotor current plus the FLA of the running motor doesn't get anywhere close to reaching the trip threshold.

Looking for help digging into this issue. I am a new engineer and not really sure where to start. Thanks in advance
Click to expand...
Just a retired commercial electrician but here is my 2¢. The numerous 100 HP 4 Pole plain Jane NEMA frame 480 volt motors that I installed & PM'ed had 124 amps FLA So a 100, 150'& 200 HP 480 Volt three phase motors would have a combined ampere of 558 amps if my old brain is correct. That is too high of a current for a 600 amp circuit breaker. If all three motors were on VFD'S with at least a 20 second ramp up time and maximum Hertz reduced somewhat ( if your system can deal with reduced running speed ) you should be able to get away with it until a larger circuit breaker along with larger wire are installed.
 
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