Intermittent short in 3 phase

[4x4grey88]

I am a HVAC tech and am having issues with a 208/240v 3Ph rooftop Heat pump system. Installed about three years ago and ran issue free until lately. First call was "no power" to thermostat. So I looking into why I had no 24v control voltage. Found that the 30A "B" fuse was bad and Breaker tripped, so I replaced the fuse and check the power. System stated okay and I checked the amp draw (12.6/12.8/12.7 on start up) and dropped to 7.8/ 7.7/7.7 Run. So I departed. three days later same issue. again replaced fuse and about the same amp draw as before. I tightened the wire connection and replaced the breaker. three days later again same issue. tried rotating the phases equally (kept rotation). Three days later again.

I am at a loss because I am really not seeing the reason that the one (same) phase keep dropping out without showing were the extra power draw in pulling.

checked compressor (normal), Blower fan (normal), and condenser fan motor (normal) amp draw...

Anyone have a input ???

P.S. This unit is one of five and the others are running normal without issues.

 

[ATSman]

My first thought is that you have an intermittent short to ground on B phase down stream of the 30A fuse. I would first disconnect the wire at the 30A fuse and pump disconnect end and megger the wire to ground with 500v to see if the short shows up. If you don;t have a megger then disconnect the 3 wires at the heat pump disconnect and energize the circuit (unloaded.) If no fuses blow after a week then you know that the short has to be in one of the 3 motors. Then connect the motors one at a time and run them until you find the shorted motor.

 

[LarryFine]

Did the blown fuse change location with the wire swap?

Is the load-side run short enough to pull wires and look at them?

 

[Another C10]

I am a HVAC tech and am having issues with a 208/240v 3Ph rooftop Heat pump system. Installed about three years ago and ran issue free until lately. First call was "no power" to thermostat. So I looking into why I had no 24v control voltage. Found that the 30A "B" fuse was bad and Breaker tripped, so I replaced the fuse and check the power. System stated okay and I checked the amp draw (12.6/12.8/12.7 on start up) and dropped to 7.8/ 7.7/7.7 Run. So I departed. three days later same issue. again replaced fuse and about the same amp draw as before. I tightened the wire connection and replaced the breaker. three days later again same issue. tried rotating the phases equally (kept rotation). Three days later again.

I am at a loss because I am really not seeing the reason that the one (same) phase keep dropping out without showing were the extra power draw in pulling.

checked compressor (normal), Blower fan (normal), and condenser fan motor (normal) amp draw...

Anyone have a input ???

P.S. This unit is one of five and the others are running normal without issues.

It does seem like B Phase is the culprit, especially being that the duration has become pretty routine, not likely a weather or rodent issue, its a specific condition based on duration that appears to create the overload. The best way I've found to accurately troubleshoot is to simulate the problem at will, it would be great if you could use a different nearby source to confirm the issue is within the equipment as opposed to a structural wiring issue. I'm not literate enough with AC systems so I'd be more concerned with the equipment provided power source. you'll get it just isolate all the elements until the issue becomes obvious.

 

[ptonsparky]

Check for VD across the fuse holders. Line to load each phase. Look for overheated circuit connections.

 

[oldsparky52]

If megging doesn't find anything maybe use an IR meter to check for a hot connection on a contactor pole?

 

[synchro]

Check for VD across the fuse holders. Line to load each phase. Look for overheated circuit connections.

I agree this would be a good test. If the same fuse location is still blowing after shifting phases (as Larry asked) then resistance at the fuseholder contacts might be heating the fuse and causing it to blow even with normal current levels.

 

[hillbilly1]

Is this a two stage unit? If it is, can you force the second stage on while you are there? I have seen bad second stage compressors happen all the time. Especially at 480 volts (I know, yours is 208) In some areas of the country, the second stage rarely runs, and apparently locks up. Also, since it’s a heat pump, does it have resistance heat as backup? Could be a bad element that is shorted when the backup heat is activated.