HVAC tripping

[Kevin15640]

I’m diagnosing an RTU that trips at some point during the running cycle. The voltage is 120 phase to neutral and 208 phase to phase. Each phase is getting 40 amps while running On a three pole 60 amp breaker. Other than some burned insulation on the B phase I can’t see what would be wrong. Any other solutions other than a weak breaker?

 

[augie47]

Can you provide the nameplate data ? (MCA, MOCP)
(The burned insulation on B phase might well be the best clue. Have you checked the buss where the breaker plugs-in or bolts in ??)

 

[ptonsparky]

Loose connection on B . Replace the breaker. Cut wire back to good copper.

 

[augie47]

Loose connection on B . Replace the breaker. Cut wire back to good copper.

I would agree with that solution in 90% of the cases but I would want to look at the buss connection.
With a new breaker on a burnt or damaged buss the problem will likely reoccur don' you think ?

 

[ptonsparky]

I would agree with that solution in 90% of the cases but I would want to look at the buss connection.
With a new breaker on a burnt or damaged buss the problem will likely reoccur don' you think ?

Absolutely.

 

[Kevin15640]

thanks for the info. That’s what I figured it was but wanted to check.

 

[Kevin15640]

Bus bar looks to be in good condition.

 

[kwired]

Bus bar looks to be in good condition.

Make sure it is not discolored. Not burned but discolored may still mean it has been overheated to some extent and is compromised.

It might be fine if output lug is all that overheated though.

Temp check (even with infrared thermometer) might be good idea. If all three lines are carrying same load, then all three connections should be about the same temperature. Be sure to let it run at least a few minutes before checking. Bad connection will start heating up fairly soon under load.

 

[synchro]

I’m diagnosing an RTU that trips at some point during the running cycle. The voltage is 120 phase to neutral and 208 phase to phase. Each phase is getting 40 amps while running On a three pole 60 amp breaker. Other than some burned insulation on the B phase I can’t see what would be wrong. Any other solutions other than a weak breaker?

Does this mean it could possibly be drawing more than the 40 amps at some point when you weren't making the measurement? Or did the breaker trip during your measurements?
Also, are you measuring the current with a true RMS or with an average responding meter? Averaging meters can read as much as 40% low if there is significant distortion from VFDs, etc.

Why is True-RMS So Important? | Rockwell Automation | US

Learn how a true-rms meter can accurately measure imperfect, nonsinusoidal waves compared to an average digital multimeter.

www.rockwellautomation.com

 

[hillbilly1]

Most likely a loose connection, but sounds like it has heat strips for the amperage to be that high. Single stage or two stage heat? Sometimes bad heat strip, but at 40 amps, I would say all elements are working.

 

[StarCat]

For HVAC DIAG. there is a lot of missing information. RTU's do not tpyically utilize a Neutral connection. We do not know if the system is single or 3 phase, OR what mode you are attempting to troubleshoot in. Does " Trips " mean the main breaker is tripping?

 

[kwired]

For HVAC DIAG. there is a lot of missing information. RTU's do not tpyically utilize a Neutral connection. We do not know if the system is single or 3 phase, OR what mode you are attempting to troubleshoot in. Does " Trips " mean the main breaker is tripping?

I’m diagnosing an RTU that trips at some point during the running cycle. The voltage is 120 phase to neutral and 208 phase to phase. Each phase is getting 40 amps while running On a three pole 60 amp breaker. Other than some burned insulation on the B phase I can’t see what would be wrong. Any other solutions other than a weak breaker?

I'm assuming it is three phase. Most likely does not utilize a neutral though.