Breaker in Switchgear for Sub tripping randomly

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MaybeMe

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Hey guys,

Having an issue in a factory with two 200a sub panels fed thru a 3" EMT with Cu 250s. Both breakers trip randomly, sometimes with no load on them. Conductors are not abnormally hot, and I've tested for dead shorts between phases and ground.

The panels are only for feeding many computers and servers, all of which are 208 loads, no neutral.

Is it possible harmonics are causing spikes in voltage? I'm stumped and not sure how to proceed.

Thanks for reading!:thumbsup:
 
DRGREG said:
What is the current draw? Are the loads 3 phase?
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Loads are 3 phase. Multiple 208v loads spread evenly across all 3 phases of the two sub panels.

About 110A per phase.

What temperature are the breakers supposed to trip at? They have gotten up to 120 degrees fahrenheit, measured with a laser thermometer, right after they trip.

Connections are tight, wires are at a steady 95 degrees, as are all the breakers in the subs.

I can try swapping breakers and see if they hold, but this has really got me stumped.

Thanks for the help
 
DRGREG said:
Are there adjacent breakers that are hot? Those breakers should be running cool, the wire should be cool. Are you feeding a UPS?
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Adjacent breakers up and down are a little cooler, but only a couple degrees. Wire is the same temperature. The panels are feeding a cryptocurrency-mining set-up, lots of non-linear loads.

gesparky said:
What kind of breakers, molded case, or drawout type?
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Siemens molded case 200a breakers.
zog said:
Local testing company can do it. What type of breakers are they?
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Same as above, I will try swapping with a breaker known to work, and just order another if that's the case.
 
MaybeMe said:
.... The panels are feeding a cryptocurrency-mining set-up, lots of non-linear loads. ...
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100% non-linear loads, harmonic current could easily be 30-40% of the fundamental current but not seen by your meter. That could be causing the thermal elements in the breakers to trip prematurely (from the perspective of fundamental current values). I predict that swapping breakers will likely result in the same thing happening. If so, you may need to look into harmonic mitigation.
 
Jraef said:
100% non-linear loads, harmonic current could easily be 30-40% of the fundamental current but not seen by your meter. That could be causing the thermal elements in the breakers to trip prematurely (from the perspective of fundamental current values). I predict that swapping breakers will likely result in the same thing happening. If so, you may need to look into harmonic mitigation.
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That's what I was originally thinking and I brought it up a few days ago to the owners since they set-up isn't running at 100% capacity, only about half of what they intend to have. I don't know enough about harmonics aside that they are invisible and can be dangerous for equipment and people. Are there still harmonics if there is no neutral?
 
MaybeMe said:
That's what I was originally thinking and I brought it up a few days ago to the owners since they set-up isn't running at 100% capacity, only about half of what they intend to have. I don't know enough about harmonics aside that they are invisible and can be dangerous for equipment and people. Are there still harmonics if there is no neutral?
Click to expand...
Yes there are still harmonics without a neutral, just not 3rd order harmonics because they will cancel each other out in a 3 phase distribution system. So you will have 5th, 7th, 11th, 17th, etc. etc.; all odd numbers that are not divisible by 3. A typical spectrum from non-linear loads, such as all of the power supplies for these bitcoin servers, will be heavy on the 5th, 7th, and a little 11th, but all of the rest, cumulatively, will add up as well.

Image7.jpg

So when I say that the harmonic current may be 30-40% of the fundamental, what that means is that if your fundamental current is 110A, the actual total current flowing through the breaker will be 154A, but a simple clamp-on ammeter or panel meter fed by a CT will not see all of that. So it APPEARS as though the current is only 110A, but the thermal sensing unit is seeing 154A. In addition, there is what's called the "Peak Factor" in harmonic currents that is known to cause simple bi-metal thermal sensing elements to heat more for a given amount of current flow, resulting in them tripping at lower than expected values. A few companies are now making harmonic compensated bi-metal trip elements for things like Overload Relays, but I don't think that concept has made its way down into Thermal-Mag breakers yet. What most breaker mfrs have done is to compensate their ELECTRONIC trip units in circuit breakers to account for harmonic currents and ignore the Peak Factor issues. That may be another way to go with this; replace your thermal-mag breakers with the more expensive ETU breakers. But before doing that, talk to Siemens about which of their ETU breakers will work in your panel. Some of the older ones (i.e. the old ITE Sentrons) don't have this compensation yet.
 
Jraef said:
Yes there are still harmonics without a neutral, just not 3rd order harmonics because they will cancel each other out in a 3 phase distribution system. So you will have 5th, 7th, 11th, 17th, etc. etc.; all odd numbers that are not divisible by 3. A typical spectrum from non-linear loads, such as all of the power supplies for these bitcoin servers, will be heavy on the 5th, 7th, and a little 11th, but all of the rest, cumulatively, will add up as well.

View attachment 18838

So when I say that the harmonic current may be 30-40% of the fundamental, what that means is that if your fundamental current is 110A, the actual total current flowing through the breaker will be 154A, but a simple clamp-on ammeter or panel meter fed by a CT will not see all of that. So it APPEARS as though the current is only 110A, but the thermal sensing unit is seeing 154A. In addition, there is what's called the "Peak Factor" in harmonic currents that is known to cause simple bi-metal thermal sensing elements to heat more for a given amount of current flow, resulting in them tripping at lower than expected values. A few companies are now making harmonic compensated bi-metal trip elements for things like Overload Relays, but I don't think that concept has made its way down into Thermal-Mag breakers yet. What most breaker mfrs have done is to compensate their ELECTRONIC trip units in circuit breakers to account for harmonic currents and ignore the Peak Factor issues. That may be another way to go with this; replace your thermal-mag breakers with the more expensive ETU breakers. But before doing that, talk to Siemens about which of their ETU breakers will work in your panel. Some of the older ones (i.e. the old ITE Sentrons) don't have this compensation yet.
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Great, thanks for the info! Still trying to wrap my head around it haha. Is there any way to see what the load "really" is? Or is it calculated?
 
MaybeMe said:
Great, thanks for the info! Still trying to wrap my head around it haha. Is there any way to see what the load "really" is? Or is it calculated?
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There are harmonic measuring meters, yes. Anything costing under $1,000 is a waste of money though.

If you are really curious, find someone who does power quality studies in your area, they will (should) have a meter capable of that. With a 100% non-linear load like this, I HIGHLY recommend it.
 
Jraef said:
There are harmonic measuring meters, yes. Anything costing under $1,000 is a waste of money though.

If you are really curious, find someone who does power quality studies in your area, they will (should) have a meter capable of that. With a 100% non-linear load like this, I HIGHLY recommend it.
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Okay, sounds reasonable. I will swap breakers and if that isn't it, I will call a local company.

Thanks for the help!
 
1. Measure current
2. Perform a FOP
3. Check THD
4. Determine if you need to upsize or replace based on the above.
 
MaybeMe said:
Hey guys,Having an issue in a factory with two 200a sub panels fed thru a 3" EMT with Cu 250s. Both breakers trip randomly, sometimes with no load on them. Conductors are not abnormally hot, and I've tested for dead shorts between phases and ground.
Click to expand...

This sounds like you used the continuity function of your meter. If you haven't megged them, that is the very first thing I would do before I even thought about pulling breakers and sending them off for testing.
 
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