400A Breaker Tripping

[Pitt123]

Troubleshooting a 400A breaker that is feeding a compressor that keeps tripping. Compressor starts and breaker trips after about 20min or so. Measured current going to comprssor and see about 242A which is well under the 400A rating. Breaker appears to be very hot to the touch. If the surrounding enviornment is warm can this cause false trip of these breakers? Is there any way to test this breaker to see if maybe its a bad breaker?

I should mention that tapped of the load side of the breaker are two sets of cables with each set fedding the respecitve connection on a wye-delta starter. We are measuring current at comprssor but then i multiply by 1.73 to come up with current above. Other similar comprssors with similar breakers are running at same amount.

 

[augie47]

Not sure your 1.73 is relative here. I'd check the actual ampere ratings at the load side of the breaker to be sure, but it sounds more like an overheating problem from (a) poor connection at the breaker, (b) poor connection on the line side, or (c) internal problem.
An IR scan would be a great idea.

 

[Pitt123]

Not sure your 1.73 is relative here. I'd check the actual ampere ratings at the load side of the breaker to be sure, but it sounds more like an overheating problem from (a) poor connection at the breaker, (b) poor connection on the line side, or (c) internal problem.
An IR scan would be a great idea.

I mention 1.73 because on the load side of the breaker we are essentially measuring the current inside the delta of the motor windings when measuring a particular cable. Maybe measuring on the line side of breaker would be best?

I will try to do an IR scan.

 

[augie47]

Let us know what you find.
IMHO, sure sounds like a connection problem.

 

[don_resqcapt19]

I mention 1.73 because on the load side of the breaker we are essentially measuring the current inside the delta of the motor windings when measuring a particular cable. Maybe measuring on the line side of breaker would be best?

I will try to do an IR scan.

The current you measure at the motor is the current that is flowing through the breaker (assuming that you are measuring the current on the motor branch circuit conductors).

 

[nakulak]

you already said the breaker was hot, I'm suprised a visual inspection didn't find your problem, but I guess the IR will tell the tale.

 

[Jraef]

Ambient air temperature is not supposed to have much of an affect on breaker trip times, but transmitted heat from a bad connection definitely will. On a standard Thermal-Magnetic circuit breaker, heat from a highly resistive termination will conduct right up the power path directly to the same thermal elements in the breaker that tell it to trip if the current it too high. The sensing elements will not "know" the difference. On an electronic circuit breaker that may not be the case because they sense current directly with CTs (or other transducers) and do a math calculation to interpret the heating effect on the conductors. Actualy physical heat is not seen.

Unless you have some sort of bizarre connection to that motor, you are incorrect about the 1.732 issue. ALL of the current the motor is using will come from that breaker. The OL relay is a different story because it is on the LOAD side of the Y-Delta starter where the circuit splits. But the breaker is on the LINE side (hopefully!).

 

[benaround]

I should mention that tapped of the load side of the breaker are two sets of cables with each set fedding the respecitve connection on a wye-delta starter. We are measuring current at comprssor but then i multiply by 1.73 to come up with current above. Other similar comprssors with similar breakers are running at same amount.

I'm not following the way you fed the starter, to me , the feed is 3 full size conductors

based on motor full load current. Then the conductors between the starter and the motor

(6) are sized to 58% full load current.

 

[benaround]

Pitt,

What is the motors' full load amps, on the name plate ? Compare that to the 242a you are

reading. Also, you know the relationship between phase and motor leads, right.

A phase- t1 and t6
B phase-t2 and t4
C phase t3 and t5

you probally know this, but just trying to help ok

 

[Jraef]

I'm not following the way you fed the starter, to me , the feed is 3 full size conductors

based on motor full load current. Then the conductors between the starter and the motor

(6) are sized to 58% full load current.

If you look at a Y-Delta starter diagram, the line side of the starter splits to feed each side of the circuit. There is no reason (other than convention) why you could not do that split at the breaker load terminals, but regardless, ALL of the current is still coming from that breaker.

 

[Jraef]

Best one I could find to illustrate the point.

Typically, when you buy a Y-D starter made at a factory, the jumpers from L1 to L1, L2 to L2 etc. are already installed, so all you need to do is run 3 conductors to the line terminals provided. But if someone "rolled their own" starter, they could use distribution lugs on the breaker load terminals to cut down on the conductor sizes for the feed to the starter (contactors).

 

[Jraef]

...

Unless you have some sort of bizarre connection to that motor, you are incorrect about the 1.732 issue. ALL of the current the motor is using will come from that breaker. The OL relay is a different story because it is on the LOAD side of the Y-Delta starter where the circuit splits. But the breaker is on the LINE side (hopefully!).

Sorry, I just re-read your original post and see that you said you measured current at the compressor, so assuming that you could only clamp around one set of conductors, the 1.732 factor would indeed apply. However, that's IF you clamped around the correct set of conductors. Basically, 58% goes through one set, the other 42% goes through the other set. If you measured the 42% set, the current you read would need to be multiplied by 2.38. So assuming you read 139.7A (242 / 1.732), then the real current seen by the breaker would be 333A. Still not enough by itself to make it trip, but it could be an added factor in why a little extra heat from a bad connection is giving you trouble.

 

[Pitt123]

Sorry, I just re-read your original post and see that you said you measured current at the compressor, so assuming that you could only clamp around one set of conductors, the 1.732 factor would indeed apply. However, that's IF you clamped around the correct set of conductors. Basically, 58% goes through one set, the other 42% goes through the other set. If you measured the 42% set, the current you read would need to be multiplied by 2.38. So assuming you read 139.7A (242 / 1.732), then the real current seen by the breaker would be 333A. Still not enough by itself to make it trip, but it could be an added factor in why a little extra heat from a bad connection is giving you trouble.

We are measuring current where the number "L1" is shown on the left most contactor. It is my understanding that this is "inside the delta" and thus measuring phases current so we need to multiply by 1.73 to calculate line current seen by breaker.

I dont see what your saying about the 42% set? I thought that meausing inside the delta you would always see 58% of the line current? Can you please explain what i'm missing?

 

[zog]

Is there any way to test this breaker to see if maybe its a bad breaker?

Yep, there are many companies that can test this for you. NETA is a good place to start. A megger test, contact resistance test, and primary current injection test should be done at a minumum. What is the breaker nameplate info, does it have an electronic trip unit? What are the settings?

 

[www.assuredelectric.net]

I once had a similiar problem. The cause was beneath the breaker cover. The manufacturer (Siemans) didn't tighten down a few bolts and was causing the breaker to over heat, and Trip. Id start right there.

 

[Pitt123]

The breaker is an AB 400A 140U-K6. The inst setting is all the way up to 10.

We did an IR scan and saw 140 degrees F on the terminals and the breaker itself. Does this temp seem ok? The same type of breaker feeding another compressor had similar readings.

 

[zog]

The breaker is an AB 400A 140U-K6. The inst setting is all the way up to 10.

We did an IR scan and saw 140 degrees F on the terminals and the breaker itself. Does this temp seem ok? The same type of breaker feeding another compressor had similar readings.

What rating plug?

Lets assume a 400A plug. One issue with this style trip unit is the pots on the trip unit. If you are in between one of the settings, or if dirt is present inside, the trip unit will fail to the minumum settings. The current your breaker will trip is the rating plug times the thermal setting (Ir). If Ir has failed to the default (0.5) with a 400A plug your breaker will trip at 200A, after a failrly long time delay.

 

[Jraef]

What rating plug?

Lets assume a 400A plug. One issue with this style trip unit is the pots on the trip unit. If you are in between one of the settings, or if dirt is present inside, the trip unit will fail to the minumum settings. The current your breaker will trip is the rating plug times the thermal setting (Ir). If Ir has failed to the default (0.5) with a 400A plug your breaker will trip at 200A, after a failrly long time delay.

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.

Pitt123,
140F on the breaker terminals is high, but not overly high. Basic CU/AL aluminum lugs are rated for 75C which is 167F. But if that heat is continuous, as it sounds like it is, then it can transfer up the power path to the breaker sensing elements and cause nuisance tripping. You didn't describe your lug and cable arrangement, but the only usable lug option for that breaker in the way you are using it is this:

K-Frame Multi-Terminal Lug Kit, 6 Wires per Terminal, with Terminal Cover
- 14-2/0 AWG (2...67 mm?)

So assuming you are using this lug and splitting the load as 3 x 2/0 per phase on a 400A circuit, there shouldn't be any problem. But if you have only 2 per phase, or 3 x 1/0, then the heating issue is likely a result of under sized conductors (even with 90C rated wire which would not be appropriate for 75C lugs).

If someone used flex cable such as DLO for the internal connections, there is a really common problem with that. They often read the ampacity of the cable, which is rated at 105C, and think they can use it like that in a panel. But that extra temperature rise is usually beyond the design limits of what that cable is being terminated to. In reality you should use the ampacity charts of 75C rated cable since that is the highest rise you can actually allow. As a case in point; 1/0 DLO says it is rated for 250A, so it appears as though 2 of those per phase will handle a 400A load. But it only handles 250A if the temperature rise is allowed to go to 105C and if you terminate it on something rated for a 75C rise you are in trouble. I have seen a lot of terminal meltdowns as a result of people not understanding this.

Can you post a picture of the starter?

 

[Jraef]

... He didn't say if it was TM or ELT...

Damn, too late to re-edit. I meant ETU, not ELT. I have no idea what an ELT would be...

 

[zog]

Damn, too late to re-edit. I meant ETU, not ELT. I have no idea what an ELT would be...

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