carrier packaged unit......electrical short...HELP

Paul S_3

Pic

Paul S_3

pics

Hey fellow wallies, im having a problem with a "short" to ground , here. Fuse on 208v by 24v t-frmr keeps tripping. Problem started yesterday when store lost power briefly, i was called there for no cooling this afternoon. HVAC equipment at this store was working fine when I personally did the PM 2 weeks ago. I was in a major rush when i was there so going back in morning, the short is in the first (y1) and second (y2) stage circuits, since i was in a rush i eliminated that there was a short at stat wiring and from control board to economizer and back from economizer to control board......question is in the contactor coil for 1st stg comp and coil for 2nd stg comp and condenser fan, they both ohmed out really low......4 ohms each .....PROBLEM???? Or is that a normal reading , thanks in advance  PAUL S .. ASM Mechanical Corp BK,NY.....

nicholas bonham-carter

Mysterious outage

Is this a 3-phase unit. I recall something similar with our Lennox unit, where one of the phases was fused off due to a storm, and it took a lot of diagnostics to identify the problem (not by me).--NBC

Paul S_3

3 PHASE

It is a 3 phase unit, i checked all phases , all are fine. i ohmed out the compressor contactor and the condenser fan contactor, both read really low ohm readings...(4 ohms & 6 ohms) both 24v coils , i was in a rush and didnt have the contactor, when i removed the wires from the 24v coils the tripping stopped, but a shorted coil wouldnt show a wire that carries 24v , short to ground meaning y1 and y2 coming back from economizer using continuity on my meter??? the actual coil would have to be touching ground???thanks PAUL S ASM Mechanical Corp BK,NY

bob_46

Blown

Paul, check the contactors and make sure they can pull all the way in. If there is debris or a mechanical problem that holds one open it will draw too much current .

Techman

Short?

A couple of things. Is the line voltage also 208v ?, is the xformer a multitap ? Is the 24v really 24v? Install a 10 loop multiplier of wire on each of the xformer ,y1 and y2 contactor colis and the cond fan motor contactor coil if there is one, then start the AC and bring on 1 stage at a time and read the amps. The "multiplier" has to be just big enough to go around the amp meter jaw.

A short would be a "short to ground" or a "turn to turn " , you might have an "over amp " situation .

icesailor

Hateful Things:

Many hateful things can happen when 3/phase gets a jolt from power surges +/- and the equipment doesn't know what to do.

You really need to put on your electrical troubleshooting hat and pull it down tight. So it doesn't blow off in a foul wind. Sounds like another teachable moment coming on.

Ironman

Contractors

Techman has given you some good advice about checking out the contractors. If the tripping stops after they're disconnected, then they are obviously the culprit(s). They could be shorted or not fully engaging which would cause the coils to over-amp.



Another thing: is the 24V secondary of the transformer grounded to the chassis of the unit? If so, that could leave the door open for these issues during a power surge. You could try TEMPORARILY disconnecting it and see what happens, but that may only correct the symptom, not the cause.



What type of three phase system is supplying the unit? Wye, open or closed delta, corner grounded delta?

Paul S_3

in a rush.....

Thank you guys for all your help,I mentioned before that I was in a rush yesterday. I spent time there earlier this morning. I switched the primary side of transformer from 230v to 208v...incoming voltage is 211v.....I found a short to ground at the y1&y2 wires coming back from the economizer ,I posted a pic below unit is running now thanks again Paul S