MCC Breaker Tripping - Troubleshooting Help

B

bindem

Member
Location
Westlake, LA
Occupation
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).
Click to expand...
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.
Click to expand...
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.
 
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