RTU AC blowing fuses

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MBLES

Senior Member
A customer request service call for nuisance blowing fuse on RTU AC its a 3phase 240 with high leg. The fuses I replaced were ontime fuses that I replaced with slowblow fuses and after few days they blew again this time just one fuse instead of two. I put meter on them and got 38amp each phase and It cycled a few time with no problem. Customer said they changed thermostat but dont see reason why this would effect ac blowing fuses
As much as they have. The compressors were huming pretty bad even during start up. MY GUESS COMPRESERS ARE WORN. Any ideas I will go back tomorrow because I lost day light, but I know I will get some good advise from you guys.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Check for poor connections at fuse clips. Do a FOP or thermal scan. Meg the compressors. Check the contactors for wear on the starters. Check voltage and current while it is online. Check all the controls. I found an A/C unit last summer that was blowing fuses because of problems at/in the thermostat.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I would be suspicious that the compressor may being trying to restart with having time to "bleed off". Could be a compressor problem, or a failure of the "delay" circuit if it is so equipped. Try turning it off and right back on.
Most compressor have a delay circuit to allow the head pressure to stabilize before restarting. If it tries to restart check the circuitry.
 

MBLES

Senior Member
I would be suspicious that the compressor may being trying to restart with having time to "bleed off". Could be a compressor problem, or a failure of the "delay" circuit if it is so equipped. Try turning it off and right back on.
Most compressor have a delay circuit to allow the head pressure to stabilize before restarting. If it tries to restart check the circuitry.

I noticed that the feeder on roof split into directions from jbox one going to each RTU units. I didnt check jbox but it maybe one feeder feeding two ac units. Maybe voltage drop causing fuses to blow when they are each trying to come on at same time? Fuses inside MDP not blow just fuses at disconnect. I probably got bad disconect. I am just looking for different experiences to go on when I get there tomorrow.
 
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