A/C unit blowing fuses

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fracas666

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There is an A/C unit that is blowing a fuse for the feeder disconnect. The feeder disconnect inside the building is fused at 90A. The disconnect at the unit is fused at 60A. The feeder is 120/240 with B phase being a high leg. When B phase is attached to L2 the B phase 90A fuse blows. When B phase is attached to L3 everything works OK. Why?

Hopefully the schematic is attached to this post. If not I'll try again later.

Thanks,
Ron
 

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are you certain it's a 120/240 high-leg system?
(as opposed to a corner grounded or ungrounded system)
 
The schematic implies that the unit is phase agnostic but your experiments demonstrate otherwise. This is a problem. Something is not liking being not centered around ground. The fan is across L2-L3 so this is probably not the problem. This leaves the core heater and the control transformer. Is there a label on the control transformer? The heater could be shorted or the control transformer miswired.

New unit or old. If old then someone has replaced the CT with the wrong unit.

Or, I could be blowing smoke.

P.S. are you sure you are not trying to make the compressor run backwards? Did you rotate the three phases or just reverse L2-L3? If the compressor is stalling in one direction and not the other then, if I recall, that is a water problem. I am without clue. I have a serious lack of compressor foo.
 
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Is fuse blowing immediately upon energizing or after running for some time?

If I understand you are blowing the 90 amp feeder fuse and not the 60 amp fuse at the unit.

What condition is the disconnect containing the 90 amp fuse in? Is there excessive heat from a bad connection or poor switch contact? If that is the case it shouldn't really matter when swapping leads around but maybe there is something being missed otherwise.

When you swap L2 and L3 are you swapping something elsewhere so you don't end up reversing motor(s) rotation?
 
starbolin said:
The schematic implies that the unit is phase agnostic but your experiments demonstrate otherwise. This is a problem. Something is not liking being not centered around ground. The fan is across L2-L3 so this is probably not the problem. This leaves the core heater and the control transformer. Is there a label on the control transformer? The heater could be shorted or the control transformer miswired.

New unit or old. If old then someone has replaced the CT with the wrong unit.

Or, I could be blowing smoke.

P.S. are you sure you are not trying to make the compressor run backwards? Did you rotate the three phases or just reverse L2-L3? If the compressor is stalling in one direction and not the other then, if I recall, that is a water problem. I am without clue. I have a serious lack of compressor foo.
Click to expand...

PPS: Not phase agnostic but neutral agnostic. Phase rotation is still important.
 
HVACR Diag.

HVACR Diag.

Starbolin you are logical in looking at the single phase loads on board and noting where they are landed, but something is amiss here.
The Applied voltage should be the same as you have stated.

Does the fuse blow before any control loads are engaged?
When I have fuses blowing I generally sweep for ground faults right away.
Is the charge tainted?

Is everything really fine after the phase swap? By that I mean is everything on board running and working correctly.
That would mean:
The CCH is drawing normal current with the compressor OFF
The Running current of the compressor and ODF motors are correct with the system RUNNING
The System Gauge Pressures are correct
The measured voltages on the line are correct
The unit has not been miswired by past service procedures
By the diagram this is a split condensing unit and I have to assume that the problem was not always there, so something has changed that is not apparent or necessarily logical.

I have witnessed a number of electrical faults on HVAC eqipment that are not constant, but intermittent....but they are less common.
The problem description sounds very BINARY. Wierd.
Something is being missed. But not every diagnosis is straightforward and or fun. Some will no doubt run you around the bend depending on what day it is.....

On mechanical rotation.
Reciprocating compressors don't care which way they run. However, its better to leave them running the direction that they have been running.
Scrolls have to turn the right direction.

all the best
 
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kwired said:
Is fuse blowing immediately upon energizing or after running for some time?

If I understand you are blowing the 90 amp feeder fuse and not the 60 amp fuse at the unit.

What condition is the disconnect containing the 90 amp fuse in? Is there excessive heat from a bad connection or poor switch contact? If that is the case it shouldn't really matter when swapping leads around but maybe there is something being missed otherwise.

When you swap L2 and L3 are you swapping something elsewhere so you don't end up reversing motor(s) rotation?
Click to expand...

90 amp fuse is blowing approximately 15 seconds after power is completed to the A/C unit. All connections are tight and disconnects are in good working order.

In my initial post I tried to simplify things. I may have confused things in doing so. What was actually done is BL2 was moved to L3. The 90 amp fuse on L2 blows when BL2 is connected to L2 and everything works properly when BL2 is connected to L3.
 
fracas666 said:
90 amp fuse is blowing approximately 15 seconds after power is completed to the A/C unit. All connections are tight and disconnects are in good working order.

In my initial post I tried to simplify things. I may have confused things in doing so. What was actually done is BL2 was moved to L3. The 90 amp fuse on L2 blows when BL2 is connected to L2 and everything works properly when BL2 is connected to L3.
Click to expand...

If it is taking the 90 amp fuse but not the downstream 60 amp fuse, I'd say the problem is likely in the feeder conductor somewhere, apparently something can take 120 volts to ground but not 208, seems strange it doesn't either clear itself or get worse and no longer will hold even with 120 volts to ground. I'd start by megging the feeder conductors and proceed based on whatever results I get from that. I'd leave the conductors connected just open the switches when doing the test so that you include any insulation breakdown in the switch in your reading, then disconnect conductor and test again if you don't get desirable results.
 
Thanks for the suggestions. I was hoping that someone could give me the theory on why this occurred. I appreciate the suggestions on items to check, but, there is no intention on going back to the site as long the installation is operational.

I will continue to monitor this thread to see if anyone can solve this mystery. It might be that this will be another one of electricity's mysteries. I guess that is why electricity is referred to as a theory. We don't always know what it is going to do.
 
Since it is relatively unlikely that the control transformer alone is pulling enough amps to make the difference (Put an ammeter on it to confirm that! ), I am left with three main possibilities:
1. The wiring does not actually match the diagram.
2. The crankcase heater is on all the time, and that load which was originally on the same line as the transformer, combines to overload the line. (Put a meter on that too!)
3. There is a problem with the wiring or contacts of that fuseholder that is raising the temperature and making it more sensitive than the rating would indicate.

PS: Usually it is not that electricity is somehow mysterious and unpredictable, but rather that we are oversimplifying or working from incomplete or incorrect information.
 
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kwired said:
Is fuse blowing immediately upon energizing or after running for some time?

If I understand you are blowing the 90 amp feeder fuse and not the 60 amp fuse at the unit.

What condition is the disconnect containing the 90 amp fuse in? Is there excessive heat from a bad connection or poor switch contact? If that is the case it shouldn't really matter when swapping leads around but maybe there is something being missed otherwise.

When you swap L2 and L3 are you swapping something elsewhere so you don't end up reversing motor(s) rotation?
Click to expand...
I would look at this. I had a friend that wasn't blowing fuses at the unit, but was tripping the breaker. Went over and checked and every thing seemed fine until I started to tighten the screws on the breaker and one phase had probably never been tightened down. Fixed the problem.
 
cowboyjwc said:
I would look at this. I had a friend that wasn't blowing fuses at the unit, but was tripping the breaker. Went over and checked and every thing seemed fine until I started to tighten the screws on the breaker and one phase had probably never been tightened down. Fixed the problem.
Click to expand...
That is part of why I asked if it takes time before it blows or if it blows immediately. If it blows immediately the cause is not going to be a loose connection, and about has to be before the second fuse in the circuit that is only 60 amps (and apparently is not blowing). Any misinformation in some of these details will get us away from a correct diagnosis of the problem pretty quickly.

I am convinced based on information given so far that the problem is not in the AC unit, and is either loose connection somewhere in the disconnect with the 90 amp fuses or is a ground fault in the feeder portion of the circuit. Why it only seems to happen when the high leg is applied to the particular section in question is still somewhat puzzling. Conductors and insulation of other components should be able to handle much more then 208 volts without giving up fault current. I'd expect them to pass 1000 volts meg testing.
 
kwired said:
That is part of why I asked if it takes time before it blows or if it blows immediately. If it blows immediately the cause is not going to be a loose connection, and about has to be before the second fuse in the circuit that is only 60 amps (and apparently is not blowing). Any misinformation in some of these details will get us away from a correct diagnosis of the problem pretty quickly.

I am convinced based on information given so far that the problem is not in the AC unit, and is either loose connection somewhere in the disconnect with the 90 amp fuses or is a ground fault in the feeder portion of the circuit. Why it only seems to happen when the high leg is applied to the particular section in question is still somewhat puzzling. Conductors and insulation of other components should be able to handle much more then 208 volts without giving up fault current. I'd expect them to pass 1000 volts meg testing.
Click to expand...
I agree.
 
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