Fuses Blowing on Hermetic Refrigerant Motor-Compressor??

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fraserjim

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Denver, CO
Customer has two HVAC systems. Each system has two compressors. Stage 1 and Stage 2.

We got called in to help troubleshoot Buss LPJ-30SP 30amp. fuses blowing on Stage 1 of both systems. The systems runs at 480 volt (measured 485V). We measured 24.2 amps while the Stage 1 compressor was running.

Manufacturer's datasheet states: RLA of 27.6 at 460volts.

Each system has a Control Panel with a "Power Block". Control Panel is part of the system. The Power Block feeds the two fuse blocks, single-phase monitors, and contactors.

Last month a HVAC service tech. measure 29amps. running and reduced the refrigerant charge by 2lbs. Still blowing fuses.

Vendor has asked if they can upsize the fuses. Manufacturer states that UL will not allow them to upsize.

Since the Control Panel / Fuse Blocks / Fuses are integral to the system, I do not know what ULs rules are for sizing.

Also, I have never seen a compressor pull that close or over its RLA.

Can someone tell me about how UL sizes fuses and any other things to look for??

Thanks,

FraserJim
 
It really depends whether the unit has overload protection or not. Generally with smaller HVAC systems the fuses are sized 175% of the nameplate FLA. The unit should have the max fuse allowed. If the unit is blowing the max fuse then I would say there is a good chance there is an issue with the compressor.
 
I don't think changing the fuse size is in the cards as this is a factory engineered setup and they probably have thousands of them in the field just like it. Sounds like there is something uniuqe to your system. Do they blow on startup or while it has been running for a while? My guess would be possible voltage imbalance or system charge.
 
fraserjim said:
Customer has two HVAC systems. Each system has two compressors. Stage 1 and Stage 2.

We got called in to help troubleshoot Buss LPJ-30SP 30amp. fuses blowing on Stage 1 of both systems. The systems runs at 480 volt (measured 485V). We measured 24.2 amps while the Stage 1 compressor was running.

Manufacturer's datasheet states: RLA of 27.6 at 460volts.

Each system has a Control Panel with a "Power Block". Control Panel is part of the system. The Power Block feeds the two fuse blocks, single-phase monitors, and contactors.

Last month a HVAC service tech. measure 29amps. running and reduced the refrigerant charge by 2lbs. Still blowing fuses.

Vendor has asked if they can upsize the fuses. Manufacturer states that UL will not allow them to upsize.

Since the Control Panel / Fuse Blocks / Fuses are integral to the system, I do not know what ULs rules are for sizing.

Also, I have never seen a compressor pull that close or over its RLA.

Can someone tell me about how UL sizes fuses and any other things to look for??

Thanks,

FraserJim
Click to expand...

I see conflicts, if RLA is 27.6 I would expect to see a minimum suggested fuse of at least 40 amps, and probably even more likely to be 45 or 50 amps.

Can you give us the unit nameplate data, particularly minimum circuit ampacity and maximum overcurrent protection?
 
I agree with the others. Appears to be a incorrect application. You might try to talk with a Bussmann rep and then the factory.
 
kwired said:
I see conflicts, if RLA is 27.6 I would expect to see a minimum suggested fuse of at least 40 amps, and probably even more likely to be 45 or 50 amps.
Click to expand...
Unless, as Dennis suggested, there is no OL relay and the fuses are being used as the running OL protection for the motor, not just the OCPD. It's a bad idea, but it is technically acceptable and unfortunately done all the time in the HVAC industry.
 
fraserjim said:
Customer has two HVAC systems. Each system has two compressors. Stage 1 and Stage 2.

We got called in to help troubleshoot Buss LPJ-30SP 30amp. fuses blowing on Stage 1 of both systems. The systems runs at 480 volt (measured 485V). We measured 24.2 amps while the Stage 1 compressor was running.

Manufacturer's datasheet states: RLA of 27.6 at 460volts.

Each system has a Control Panel with a "Power Block". Control Panel is part of the system. The Power Block feeds the two fuse blocks, single-phase monitors, and contactors.

Last month a HVAC service tech. measure 29amps. running and reduced the refrigerant charge by 2lbs. Still blowing fuses.

Vendor has asked if they can upsize the fuses. Manufacturer states that UL will not allow them to upsize.

Since the Control Panel / Fuse Blocks / Fuses are integral to the system, I do not know what ULs rules are for sizing.

Also, I have never seen a compressor pull that close or over its RLA.

Can someone tell me about how UL sizes fuses and any other things to look for??

Thanks,

FraserJim
Click to expand...

If you can post any other service data on the equipment along with the model # or name plate data would help. There are several other reasons for the fuse blowing other than the electrical line side, such as ambient temperature, dirty coil, overcharge, bad contactor etc.
 
Thanks for all of the feedback.

Next site visit is on Monday 9/24.

We have a Fluke 1656 power meter at the fuse block logging for this week.

Some responses.

1. Fuses blowing up while running, not on startup.

2. During our initial testing with the 1656 meter, while running, we logged:

VTHD = 5.49%
Voltage imbalance = 0.64%
ITHD = 3.8%
Current imbalance 5.0%

3. Will try to get all unit nameplate data.

4. Ambient temperature probably in mid-70s.

Additional information / comments / questions

Impact of Voltage imbalance?

They has a catastrophic failure of one Stage 1 fuse block. They replaced with Ultrasafe holder so we can't get temp readings on fuses.
We did scan Stage 2 fuse block and it was reading in the 180 degree range and at about 24amps.

I see NEC 440.52 (A) (1) says overload relay at 140% of RLA. So that wouldn't explain why the fuses are 30amp. if being used as OL relay.

NEC 440.52 (A) (3) calls out for sizing at 125%. So an RLA of 27.6 x 125% = 34.5. Do they have to round down to 30amps. or can they round up?

Bad contactor. Since this is happening on multiple systems, a bad contactor is less likely.

I am still questioning why the RLA is so close to the actual measured running current. The manufacturers service techs. don't seem to be concerned. Is this normal to run at almost 90% of the RLA?

I am still wondering about UL vs. NEC in determining fuse sizing?

Thanks again.

Jim
 
fraserjim said:
Thanks for all of the feedback.

Next site visit is on Monday 9/24.

We have a Fluke 1656 power meter at the fuse block logging for this week.

Some responses.

1. Fuses blowing up while running, not on startup.

2. During our initial testing with the 1656 meter, while running, we logged:

VTHD = 5.49%
Voltage imbalance = 0.64%
ITHD = 3.8%
Current imbalance 5.0%

3. Will try to get all unit nameplate data.

4. Ambient temperature probably in mid-70s.

Additional information / comments / questions

Impact of Voltage imbalance?

They has a catastrophic failure of one Stage 1 fuse block. They replaced with Ultrasafe holder so we can't get temp readings on fuses.
We did scan Stage 2 fuse block and it was reading in the 180 degree range and at about 24amps.

I see NEC 440.52 (A) (1) says overload relay at 140% of RLA. So that wouldn't explain why the fuses are 30amp. if being used as OL relay.

NEC 440.52 (A) (3) calls out for sizing at 125%. So an RLA of 27.6 x 125% = 34.5. Do they have to round down to 30amps. or can they round up?

Bad contactor. Since this is happening on multiple systems, a bad contactor is less likely.

I am still questioning why the RLA is so close to the actual measured running current. The manufacturers service techs. don't seem to be concerned. Is this normal to run at almost 90% of the RLA?

I am still wondering about UL vs. NEC in determining fuse sizing?

Thanks again.

Jim
Click to expand...


Do you happen to remember the brand of equipment, Trane Carrier, York etc? Tonage, 10, 12, 20, etc?
 
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