High leg delta/high leg heating up and tripping the breaker

[Crazyhorse2882]

Ok I have a question that hopefully someone can answer. I'm an electrician and I recently had a service call where a hotel had a 175 amp 3 phase breaker feeding an ac unit with 4 compressors and 2 blowers that kept tripping. I first checked all the compressors and blowers to see if they were grounded or open and all were good. I then turned on the unit and all 4 compressors ran for about an hour and then the breaker tripped. Once it tripped I felt the 3 phases and phase C which happens to be the high leg was very hot so that leg is the culprit which tripped the breaker. I amp probed all three phases while they were running and it was a little off balance. The C phase was drawing around 120 amps, B phase 115 amps and A phase 105 amps. The compressors are 7.5 hp and draw approximately 22 amps each at 240 Volts. Does anyone have any idea why this one leg could be getting hot and tripping the breaker? Could it be that the motors are wired for wye connection and the delta service is the problem? Any suggestions would be helpful. Thank you in advance

 

[NascentOxygen]

You found one of the lines to the circuit breaker was heating up? With the breaker rated at 175A, then presumably each line would be rated for something like 250A continuous current?

This is not a new installation, so it has been working happily for years? So you can conclude a fault has recently developed? I take it you didn't find or fix the fault?

I'm wondering whether that cable has a loose or corroded connector, and this is causing local resistive heating. Can you feel whether the cable in that line is heated all along its length on both sides of the breaker?

I don't know whether it's possible for a breaker to be tripped by having one of its terminals subjected to local heating?

 

[Crazyhorse2882]

The supply is a delta high leg 3 phase. The high leg is 208 phase to ground, the other two phase are 120 to ground and it's 240 phase to phase. The load was a little off balance. A phase was drawing 105 amps, B phase 115 amps and C phase 120 amps

 

[Crazyhorse2882]

The supply is 120/240

 

[jim hardy]

I don't know whether it's possible for a breaker to be tripped by having one of its terminals subjected to local heating?

yes looseness can cause heat at the connection that'll travel along the conductor both directions, into the breaker and away from it via the wire, so your advice about feeling the wire was good
when ants move in and build a nest inside the breaker it blocks airflow and an infested breaker can overheat

 

[Crazyhorse2882]

I found the problem, it was a high resistance ground fault. I found a nick in the wire just inside the flex connector. Thanks for your help anyways

 

[jim hardy]

Wow ! Good find.

Nothing burned up ?

 

[anorlunda]

Thanks for the reminder. Step two to troubleshooting an electrical problem would be to think about it from a circuit analysis view, and maybe post a question to a EE forum.

Step one to troubleshooting an electrical problem should be a very thorough visual inspection.

Look before you think sounds overstated but ...

 

[Crazyhorse2882]

The ground fault was just inside of a flex connector that I had no way if seeing. The company I work for didn't have a meg ohmmeter to check the wires so they insisted on changing the breaker to a fused disconnect which I thought was fooling because I knew that the breaker was not the problem. So when I pulled the old breaker box off the wall I then pulled the wire out of the flex and visually inspected it and I found the ground fault. There were some burn marks on the inside of some reducing washers that were being used with the flex connecter

 

[Crazyhorse2882]

Foolish not fooling

 

[Windadct]

I would also have checked the Phase-to-Phase and Phase-to-Ground voltages - in your case it may have shown the phase with the fault to have a lower voltage, but ALSO if the utility's line voltage was out of balance, the 3 phase induction motors would draw excessive current.

 

[jim hardy]

There were some burn marks on the inside of some reducing washers that were being used with the flex connecter

i'd guess it was intermittent fault that tripped the breaker before welding itself into a permanent fault ? Else something should have got really hot...?

 

[Crazyhorse2882]

The voltage was fine but each leg had a little difference in current, not much though. It would run for an hour and then trip. So the only possibility that I can see is that the fault wasn't in direct contact with the can but was so close that just a tiny bit of fault current was leaking through and it took an hour to heat up the leg to the point of tripping the breaker. I called a friend of mine who's also an electrician and he called it a "high resistance" ground fault. Meaning that there was enough resistance between the leg and the can so that it didn't trip immediately but in time it would heat up and trip the breaker. In this case it was taking an hour almost to the minute every time

 

[dlgoff]

... I knew that the breaker was not the problem. So when I pulled the old breaker box off the wall I then pulled the wire out of the flex and visually inspected it ...

Now that's being a "dedicated troubleshooter".

 

[jim hardy]

How'd the currents look afterward?
I'm still wondering where all that excess energy went for an hour.

But - you saw what it looked like and i didnt.

Glad it's running smooth for ya' now.

old jim

 

[Crazyhorse2882]

My last measurement showed each leg around 115 amps. Everything was balanced. I'm guessing the excess energy was in the form of heat in the one leg that was heating up.