Lennox CB29M blows 25A fuses

Racer_Joe

I have a Lennox CB29M-65-1G that blew two of the 25A fuses on the incoming 460V legs. I replaced them, the unit ran fine. About 3 days later it blew them again. I again replaced them and this time I checked the current going to the blower motor and it was 2 amps and the incoming lines were also 2 Amps. When shut off, it takes a while to spin down so the bearings look OK. This is a multi speed fan, could one of the other speeds windings be bad? Is there a way to get the unit to change speeds so I can monitor the current? Any other ideas?

nicktech

http://www.zonemaster.com.sg/PDF/50hz_cb29_0296.pdf

Racer_Joe

Thanks NickTech. I already had the manual and per the table the fan motor is pulling the correct current in the high speed mode. I have an ammeter on it and have been monitoring it and it is consistently running at 1.5-2.0 amps. Something must be happening though for it to blow the 25 amp fuses. All of the wiring looks good, nothing looks overheated or chafed.

nicktech

is the unit on an independent circuit? what is the static pressure in the duct? if the the duct work doesnt provide the right amout of resistance then the blower will pull more air resulting in more work and amps. are you sure its 460v? another electrical principle is when voltage drops amperage will increase inversly proportionate. the cause maybe wire gage, seperated circuits wired to the power supply, or even a bad circuity breaker. if the breaker is abnormally hot to the touch, it probably going bad and causing the amp rise. good luck with this job, these are the ones ya need a chair and whole bunch of time to watch and rule what the problem is not, to see what the problem is. good luck!!

Racer_Joe

Yes this unit is on its own circuit. I have not measured the static pressure but no matter what it is it should not blow the 25 amp fuses. I have been monitoring this unit off and on for two days now and it is always between 1.5 to 2 amps. It is on a 460v 3 phase line which measures 484v between phases, a little high. The motor is across two of the phases and all three are used for the electric heat strips. Currently there is no current in that third phase or the motor phases when the motor is off. The unit has been in service about 8 years and still looks in pretty good shape. Wiring all looks like it is sized properly. What ever is blowing the fuses must be a good short for it to be pulling over 12kW and not blowing it self open. Yup, this is a real head scratcher. The last time it blew the fuses was during the weekend when the setback temp was 80 degrees.

Racer_Joe

OK. So I kept my ammeter on one leg of the 480 all last week and periodically I would climb up and check it. Consistently 1.5 to 2 Amps. Well, Friday mid afternoon the system went out and 2 more blown 25 Amps fuses. So back up to the top of the 10 ft ladder, you know standing on top where it says "Do Not Stand" once more looking it over. There it was! A 9 pin Molex connector that the 480 went through, which was tie wrapped in such a way that you could barely see it. The two 480 lines were next to each other and had melted the connector, just between each other, so about once a week they would short and bam there goes 2 more 25 amp fuses. 2 butt connectors and the ladies in the front office are smiling again, very important.

nicktech

hey good job, those are real head scratchers and really make ya look like crap when ya keep having coming back. gotta kick outta the "do not stand" story. bottom line is ya stuck with it and it panned out!

Racer_Joe

Not to rant, but it's always the ones that call the auto club to change their flat tire and when the office gets 78 deg "How do you expect them to work in there?" that say those words when you come down off the ladder from an attic dripping with sweat, it is summer here in Texas, that I hate to hear worse than someone calling my mother names "Maybe we should call someone else who can fix it." And then you can't really charge them for the amount of time you put into it because the fact is, I should have found it the first time.

nicktech

i hear ya, but thats when ya bill'em till ya kill'em