A/C Unit Breaker fuses blowing....

[Din]

I have 2 a/c units.....1x 5ton and 1x7.5 ton a/c unit.

My 7.5 ton a/c unit that cools the VHR's and the HP bed fuses are blowing. Has 3x30A fuses in the breaker box. 2nd time that the fuses have blown. There was a power failure yesterday and when i came in the a/c unit is not working...guessing it's the fuses again. The first time it happened, there was no power failure.

What could be causing the fuses to blow on my breaker box for the a/c unit. 3x30A cartridge Type "D'' fuses in the box.

My electrician is coming to replace the fuses again but says that i would have to call the a/c guy to check it out further as to WHY it's blowing fuses.

Any ideas? As me and my a/c guy had a falling out....something about him delaying the construction of our salon by 5 months which is why we opened late and lost the entire busy season..... If i see him, he ain't making it far...

Please help....

[Jim Fisher]

It sounds like your fuses are too small.. Look at the data plate on the unit and see what the max fuse size should be and the min. Also make sure that the fuses are tight in the disconnect box because if they are loose they will over heat and fail because they are loose. Cuttler Hammer disconnect boxes are cheap if that is what you have and they are famous for failing fuses. What size breaker is on the unit at the panel downstairs?

[Din]

It says 30A......and that's what we have in there. Any other suggestions?

If the filters need servicing....would that blow the fuse.

But hold on....the a/c unit was NOT in use/on overnight. So if the power failure occured, the a/c unit should not have been affected unless it was an electrical/fuse issue....am i correct on that?

[Din]

Federal Pioneer is the make of my disconnect box.

My A/C unit is outside on the roof......cannot see dataplate of the a/c unit to see what the fuse type should be. It's snowing here in Toronto....it's April people....and it's snowing.

[Jim Fisher]

Try short cycling the ac unit which means turn the unit on and off and see if this blows the fuses. Sometimes the internal over load of the compressor goes bad and wont click out with amp draw so it will keep trying to start until the fuses blows. Most T stats have a 5 min delay to not let the ac unit come on.. Do you have a digital stat?

[Din]

I have a digital thermostat......right now the fuses are blown until my electrician comes in to replace them, so i can't do any testing.

What gets me though is i set the thermostat to 80 degrees overnight so that the a/c unit doesn't come overnight. When the power failure occured overnight, the power came back on and the thermostat wasn't on....fuses were blown when the power came back on. BUT.....the a/c unit wouldn't have come on cuz i had it set to 80. So it's not like the power came on and the a/c unit came on at the same time to overload the circuit. I dont' get it.

When the fuses blew the first time a few weeks ago.....he replaced the fuses cuz they were 1 time fuses. he said that they shouldn't have had 1 time fuses in a unit like an a/c unit esepecially of that size. He replaced them with Type D fuses for Delay. He said the a/c guys put cheap fuses in there the first time and it shouldn't happen again. If it does, he said it's an a/c problem and i'd have to get my a/c guy in here. (i'll kill my a/c if i see him.....)

Do you think the fuses he replaced them with that were Type D could have been cheap makes?

Ferraz Shamut is the make of the 3 x 30A fuses in the disconnect box.

[Jim Fisher]

There can be many problems that are making them to blow.. I would start with the wire connections at the circuit breaker box and make sure that the connections are tight and also at the disconnect box and also inside the condensing unit itself. A loose wire over heats a fuse at the disconnect box. The fuses should be slow blow fuse.. If the ac unit has a breaker down in the circuit breaker panel box the unit dos not need to be fused

[Brian Oshman]

Slow blow is time delay or type D. All fuses however are one time use. Time delay fuses allow a high inrush of current for a short period of time without blowing to start motors and compressors or other high torque devices. The fuse will then blow at its rating if the high current draw exceeds the time delay.

You need to measure current draw on the unit with an amprobe at startup and runcycle to determine if it is an overcurrent situation. Also check the motor capacitor. A bad capacitor will hinder quick starting of the motor and cause the motor to draw higher starting current longer then the time delay fuse will allow.

[mgj]

Same problem at a laundromat I own. Check fliters first, in the unit on the roof should be service regularly, I have it done once a month, cost is $35/month.

also check your 220 disconnect, if it is old or just worn out and loose, replace it. the knives should be a tight fit, loose fit will cause arcing and blow fuses.

just a thought

mgj

[nebtans]

Those pleated filters restrict airflow. The evaporator canot "unload" sufficiently. Without "unloading" you get higher "head" pressure. The result is greater wear on the compressor. Which causes harder starting.

Over time the harder starting may cause the start capacitor to begin failing. At which point the fuses blow during start up.

Most AC systems have undersized returns to begin with. If you then place a pleated air filter in place you restrict airflow by an additional 30% immediately and up to 70% when filter starts clogging.

Install MUCH more return grill area if using pleated filters. the benefits are huge. compressor life, and energy usage.