400A Breaker Tripping

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zog

Senior Member
Location
Charlotte, NC
I think it's a good point, but that specific breaker doesn't have rating plugs, the adjustments are just built in to the trip unit that is part of the breaker. These are brand-labeled Cutler Hammer Series G breakers. He didn't say if it was TM or ELT, but most likely its TM with a fixed thermal and adjustable mag trip, 140U-K6D. So the thermal trip element, which is what this would be tripping on in 2 hours, is sealed inside the trip unit.

I thought this was an AB 140U K-frame
 

Pitt123

Senior Member
Here is some follow up info to questions:

This breaker has a 400A trip rating/sensor (In=400A). There are adjustable knobs on each phase for Im which I'm assuming is for the instantaneous setting. These are all at 10XIn which = 4000A. There is no adjustable thermal setting, long time setting, or short time setting. We disconnected breaker a put a megger across each phase of the breaker and saw 0.18ohms on each phase. We confirmed this with a fluke meter as well. This looks o.k.

Measured current on the line side of the breaker and saw about 260A each time we checked. The breaker held for about exactly 13 minutes before it tripped.

The connection on the line side of the breaker is what looks like insulated flat flexible bus bar. (not sure of correct term) This comes from off the vertical bus and connects to the mechanical lugs on the breaker. Not sure of the rating of this cable will have to check. The load side of the breaker has a set of #4/0 AWG coming off each lug and as i mentioned before this is the junction point for this circuit as shown on the diagram tha Jraef posted.

Could it be that this breaker is bad or has a bad trip unit? Or is there something pointing to another problem? There are (3) other of these breakers with the exact same arrangement that appear to be working fine. Should I expect trouble out of them as well?
 
Pitt
I am only guessing at this, but it could also be a couple of other items.

1. Possible high restance ground fault, which could be why it takes 13 minutes.
(take a clamp-on meter and check the equipment ground conductor for amperage)
2. It is also possible that a particular load is spiking at the 13 minute point, and spiking for too long and too high an amperage. Hence your reading is at such a lower amperage.
 

zog

Senior Member
Location
Charlotte, NC
Here is some follow up info to questions:

This breaker has a 400A trip rating/sensor (In=400A). There are adjustable knobs on each phase for Im which I'm assuming is for the instantaneous setting. These are all at 10XIn which = 4000A. There is no adjustable thermal setting, long time setting, or short time setting. We disconnected breaker a put a megger across each phase of the breaker and saw 0.18ohms on each phase. We confirmed this with a fluke meter as well. This looks o.k.

Your minimum megger reading phase to phase and phase to ground should be 100M. Less than 1 ohm is not OK. Or are you saying you meggered from line to load with the breaker closed?? If so neither a megger or a fluke are the right thing to use, you need to use a microhmeter to test the contact resistance.

The only way to know if the breaker is bad is to test it right, have a testing company test it (or ship it to me, I will test it for you, no charge)
 
Last edited:

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Here is some follow up info to questions:

This breaker has a 400A trip rating/sensor (In=400A). There are adjustable knobs on each phase for Im which I'm assuming is for the instantaneous setting. These are all at 10XIn which = 4000A. There is no adjustable thermal setting, long time setting, or short time setting. We disconnected breaker a put a megger across each phase of the breaker and saw 0.18ohms on each phase. We confirmed this with a fluke meter as well. This looks o.k.

Measured current on the line side of the breaker and saw about 260A each time we checked. The breaker held for about exactly 13 minutes before it tripped.

The connection on the line side of the breaker is what looks like insulated flat flexible bus bar. (not sure of correct term) This comes from off the vertical bus and connects to the mechanical lugs on the breaker. Not sure of the rating of this cable will have to check. The load side of the breaker has a set of #4/0 AWG coming off each lug and as i mentioned before this is the junction point for this circuit as shown on the diagram tha Jraef posted.

Could it be that this breaker is bad or has a bad trip unit? Or is there something pointing to another problem? There are (3) other of these breakers with the exact same arrangement that appear to be working fine. Should I expect trouble out of them as well?

So you mention vertical bus bars, is this part of an A-B MCC then? And so is this an A-B factory built Y-Delta starter? If you are measuring current at only 260A and it trips in 13 minutes, then something squirrely is going on. Could be a bad breaker, it happens. Could also be the ground fault or load spike as Pierre suggested; a faulty unload valve on a compressor can do that to you. It could also be a sort circuit fault that takes a while to show up; i.e. something heats up and makes contact with something else, either in the starter, the motor or even the conduit. I think from the sounds of this it warrants a data logger as Len suggested.
 

hurk27

Senior Member
I vote for a data logger on all 3 phase, at the load terminals of the breaker, not knowing what the current is doing on all 3 phases is paramount to knowing what is going on with the circuit at hand, this could be a simple single phasing problem with a bad connection some where that only happens after it heats up, but not monitoring all 3 phases is just guessing in the wind.
 

Pitt123

Senior Member
So you mention vertical bus bars, is this part of an A-B MCC then? And so is this an A-B factory built Y-Delta starter? If you are measuring current at only 260A and it trips in 13 minutes, then something squirrely is going on. Could be a bad breaker, it happens. Could also be the ground fault or load spike as Pierre suggested; a faulty unload valve on a compressor can do that to you. It could also be a sort circuit fault that takes a while to show up; i.e. something heats up and makes contact with something else, either in the starter, the motor or even the conduit. I think from the sounds of this it warrants a data logger as Len suggested.

Yes this is an AB MCC, however the Y-Delta starter was supplied by the compressor manufacturer inside their compressor. I'll have to go look exactly what make it is.

If you are referring to a ground fault then I guess you are saying that it is one that only happens when something heats up. I dont believe this breaker can detect ground fault current, so the ground current would have to to be of a magnitude that would either exceed the thermal sensing or the instaneous setting. Right?

I will put a data logger on this circuit tomorrow to see if anything strange is happening. Should I put the data logger to measure all three pases on the line side of the breaker to see the true current that is going through the breaker?

If I look at current on the load side of the breaker which set of cables should I monitor? I am still confused how one set will show 58% of line current and the other set will show 42% as Jraef mentioned? I would think that both sets would show 58% of the line current. What would happen if I put the CT around both sets of cables at the tap point on the breaker for each phase? Would I see zero current similar to a zero sequence CT?

If i'm using a Fluke 1735 should this be quick enough to catch a possbile fault or transient if sampling time is set to 30s?
 

Pitt123

Senior Member
Your minimum megger reading phase to phase and phase to ground should be 100M. Less than 1 ohm is not OK. Or are you saying you meggered from line to load with the breaker closed?? If so neither a megger or a fluke are the right thing to use, you need to use a microhmeter to test the contact resistance.

The only way to know if the breaker is bad is to test it right, have a testing company test it (or ship it to me, I will test it for you, no charge)

The .18 reading I was referring to was across the breaker contact on each phase (line-load side). We meggered each phase to phase and phase to ground and had greater than 100M. I will use a microhmeter to verify readings

Out of curiosity, what steps would a tesing company take on a breaker like this?
 

zog

Senior Member
Location
Charlotte, NC
The .18 reading I was referring to was across the breaker contact on each phase (line-load side). We meggered each phase to phase and phase to ground and had greater than 100M. I will use a microhmeter to verify readings

Out of curiosity, what steps would a tesing company take on a breaker like this?

Primary current injection to verify trip times per maunfatures TCC's.
 

Pitt123

Senior Member
I will put a data logger on this circuit tomorrow to see if anything strange is happening. Should I put the data logger to measure all three pases on the line side of the breaker to see the true current that is going through the breaker?

If I look at current on the load side of the breaker which set of cables should I monitor? I am still confused how one set will show 58% of line current and the other set will show 42% as Jraef mentioned? I would think that both sets would show 58% of the line current. What would happen if I put the CT around both sets of cables at the tap point on the breaker for each phase? Would I see zero current similar to a zero sequence CT?

If i'm using a Fluke 1735 should this be quick enough to catch a possbile fault or transient if sampling time is set to 30s?


I am finally going to have a chance to measure the current on this circuit tomorrow. Can anyone help me with my question regarding the 58% vs 42% current depending on which set of cable I took measurement.

How about if I measured both sets of cables on the same phase (lets say A phase) with one CT. Would I see the combined current similar to a parallel feed, or zero current since this is not a parallel feed but esentially a delta phase? Or would I see something different?
 

benaround

Senior Member
Location
Arizona
I am finally going to have a chance to measure the current on this circuit tomorrow. Can anyone help me with my question regarding the 58% vs 42% current depending on which set of cable I took measurement.

How about if I measured both sets of cables on the same phase (lets say A phase) with one CT. Would I see the combined current similar to a parallel feed, or zero current since this is not a parallel feed but esentially a delta phase? Or would I see something different?


Pitt, At start up l1,l2,l3 will show 58% motor full load current, because it starts in a wye.

Once the motor is running, now in a delta, the current on each phase or each conductor

of each phase should be the same. Just like a parallel.
 

Pitt123

Senior Member
O.k. heres the update.

I was able to put a power meter on this compressor yesterday to watch the current while running. Monitoring all three phases the current on each phase flucuated between around 225-235A. The highest current that we saw was about 238A. The compressor ran for about 20min before the breaker tripped. The current was around 230A when breaker tripped.

We did not have an IR scan this time, but based upon the temps we saw last time and the fact that the breaker did not seem to be getting hot at all this time, it does not appear to be a heating issue (temps posted above)

So it appears to me that the breaker is a bad breaker or something is wrong with the trip unit. I am gonig to seek a replacement unless there is something else I am overlooking.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I am finally going to have a chance to measure the current on this circuit tomorrow. Can anyone help me with my question regarding the 58% vs 42% current depending on which set of cable I took measurement.

How about if I measured both sets of cables on the same phase (lets say A phase) with one CT. Would I see the combined current similar to a parallel feed, or zero current since this is not a parallel feed but esentially a delta phase? Or would I see something different?

I believe that the 58% vs 42% was a typo. The currents on all the leads going into the motor should be the same.

The key is that you are measuring 'inside the delta'. In a delta connected motor, each phase 'splits' into two branches which connect via coils to the other phases. When you are measuring on the six leads going into your motor is the current after the split. Since these two branches go to different phases, the current flowing in these two branches has different phase angle.

The net current on both leads of a single phase should be 1.732 * the current on a single lead of that phase.

It is certainly worth trying to check all three phases, to confirm that balanced currents are actually flowing.

-Jon
 

benaround

Senior Member
Location
Arizona
Pitt,

What is the nameplate amps for this motor ? Tell me the amp readings without knowing

what it is supposed to draw does not really help.
 
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