Breaker Functionality

[RoyIV]

I have a customer that asked me to check out a rooftop HVAC/Heatpump unit feeder supply. Apparently the feeder trips when it is cold outside (We're in CA...so..."cold" = about 40F out here...quit laughing, please!). At any rate, the breaker would not trip while I was present (I waited about 1 hour) and the ambient temp was about 68F while I was present. I logged voltages ph-ph(215v, 215v 216v) & ph-gnd (216v, 214v 215v) -acceptible values. I meggered the wires ph-ph & ph-gnd & also achieved acceptable values (almost infinity in all cases). The feeder = 50A 3pole breaker & #8Cu wires (less than 125 lineal feet of conduit run)...The owner had previously called a HVAC contractor to come & troubleshoot the unit, The HVAC service-call paperwork had information written on it...(all though it was chicken-scratch). The notes stated that while the unit was operating -with heater strips on- that the unit was pulling "over 40A" ( yes... the HVAC service-call representative DID NOT write the actual value, he/she simply wrote "over 40A"). The HVAC service-call representative diagnosed that the feeder breaker was warm to the touch & was bad. Although the heater strips were not "on" while I was taking data- I witnessed no "warm" ckt breaker (ph-ph amps @ unit disconnect = 19.0A, 19.6A, 13.5A). This HVAC unit is quite old & I cannot find any nameplate data on the unit. I found a heater assy sticker that reads= Allied/Carrier model 88EM0100EA00...so...I'm guessing that the unit is a Carrier50QQ. This is a complete guess. What I'm letting you know is that I dont know what Max OC protection or min ckt amps should be-according to the manufacturer (all the stickers & nameplates are gone). At any rate, I got to thinking about the big picture. Quite often I'll hear a client, a general contractor, or a member of another construction trade state that "the breaker is bad" or "the fuses are bad". I have to say that it has been my experience that 99% of the time...the breaker/fuses were working exactly as intended. I realised that -technically speaking- my knowledge of breaker internal operations is pretty limited....

My question (Finally, I know....): If a 50A 3pole breaker is subjected to a constant load of lets say....45A (well in excess of 80%) would it be normal expected operation for the breaker to trip after a given time period?

 

[Dennis Alwon]

If the unit is suppose to draw 45 amps then I would install a 60 amp breaker on it. Per NEC we can go 175% of the fla assuming there is overload protection in he unit. 125feet is a long run for #8 at 45 amps.

 

[jim dungar]

My experience is that the grease in the HVAC unit gets 'stiffer' when it gets cold. This means the unit draws startup current for a longer period of time causing the breaker to trip. Once the hold of the grease is broken, current readings return to normal and all efforts at troubleshooting are in vain.

 

[Dennis Alwon]

My experience is that the grease in the HVAC unit gets 'stiffer' when it gets cold. This means the unit draws startup current for a longer period of time causing the breaker to trip. Once the hold of the grease is broken, current readings return to normal and all efforts at troubleshooting are in vain.

I thought those units had heaters to keep the oil warm. No?

 

[kwired]

I thought those units had heaters to keep the oil warm. No?

The colder they are expected to operate the more likely. They can fail also.

I would suspect the breaker is tripping during a cold start, once it gets running there is no problem.

 

[ron]

http://www.geindustrial.com/publibrary/checkout/Time Current Curves|GES-6113|generic

Every breaker is slightly different, but from the curve show on the GE Website above, it will stay on at least 1000+ seconds.

 

[Strathead]

My question (Finally, I know....): If a 50A 3pole breaker is subjected to a constant load of lets say....45A (well in excess of 80%) would it be normal expected operation for the breaker to trip after a given time period?

I overhauled and tested breakers for years. The standard tolerance of a circuit breaker is plus or minus 10%. That means that theoretically a low end breaker could trip at 45 amps, it may also never trip at 54 amps. That said, as long as the breaker is operating correctly, it would be far less likely for the breaker to trip due to such close to tolerance amperages, because the trip element is thermal, and when the amount of heat rise is that closed to tolerance, the outside air temperature would have a greater affect on it. Also, the trip time for a 2-3 amp overdraw, would likely be in the range of 24 plus hours of continuous load. We used to test a thermal breaker at 10% over and it could take all day to trip.

So the simple answer is no. Given California labor rates, the cheapest method of troubleshooting this problem would probably be repalceing the breaker assuming 208 volts and 10KAIC.

 

[templdl]

1. 80% really hasn't anything to do with it so it should be ruled out.
See if you can get a copy of the breaker's TM trip curve which should give you a good handle on when the breaker should trip. Should you happen to be on sight when the breaker trips knowing the time it took the breaker trips and comparing it to the trip curve should give you an idea as to what current the breaker is seeing.
Also, again, if you are on sight when the breaker trips if the breaker trips on thermal it is most likely that you would not be able to reset the breaker right away as the thermal elements would have to cool off some before resetting. A breaker that trips instantaneously (magnetically) can be reset and closed immediately after a trip but is is highly recommended that you should never close a breaker back into a fault as bad things could happen.
In addition, breakers do get warm. Remember often times when something is expected to be cool to the touch and it's warm is misjudged judged to be hot when actually hot means you can't hold your hand on it very long before it is uncomfortable. Yes, breakers do often get warm which is normal.
Did you check the terminations? Loose line side terminations generate heat which is conducted into the breaker heating up the thermal elements which causes the breaker to derate. The additional load caused by a longer start may cause the terminals to heat more.
It is important not to overlook the obvious being the cause.
The best thing that I could suggest is to get a TM trip curves for the breaker so that you aren't flying blind and guessing.