1 phase 460v cond. motor?

[jarganda83]

Today I was zapped bt the chassis of a condensing unit. I changed a cc heater on this unit because it shorted out in a puddle of water. I energized this unit after installing the new cc heater and the cond fan grill zapped me. I removed the 24v control wires, so the unit was not running. I put one meter lead on the roof flashing and the other lead on the cond motor grille, the meter showed 260v. I shut down power and removed the cond motor leads. I rechecked for chassis voltage and found none. I ohmed this motor out: c-s 74, c-r 76, s-r 82. I picked up a new motor and installed it with no chassis voltage. I ohmed the new motor before I installed it and the resistances were very similar.

How could a single phase motor ohm out all the same? The motor only uses 2 legs of power. L1 to common, L2 to run cap to run, other side of run cap to start.

???

[SBKold]

You gotta ohm it to ground.

[timebuilder]

Today I was zapped bt the chassis of a condensing unit. I changed a cc heater on this unit because it shorted out in a puddle of water. I energized this unit after installing the new cc heater and the cond fan grill zapped me. I removed the 24v control wires, so the unit was not running. I put one meter lead on the roof flashing and the other lead on the cond motor grille, the meter showed 260v. I shut down power and removed the cond motor leads. I rechecked for chassis voltage and found none. I ohmed this motor out: c-s 74, c-r 76, s-r 82. I picked up a new motor and installed it with no chassis voltage. I ohmed the new motor before I installed it and the resistances were very similar.

How could a single phase motor ohm out all the same? The motor only uses 2 legs of power. L1 to common, L2 to run cap to run, other side of run cap to start.

???

I have a more important question.

Why was the chassis not bonded to the panel? That overcurrent device should have opened on the fault.

[jarganda83]

This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.

[GT Jets]

This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.

Well there you go then....

[jarganda83]

So.... the old motor was not grounded. Right? But, it was, I replaced it and now the is no more voltage on the cabinet.

Maybe my meter is not powerful enough the detect that small of a resistance between the motor lead and the frame? But still be enough to pass voltage. Is that possible? I should also mention that the comp. has one leg with continuous power. So the comp. could be allowing some voltage to pass to the cond. fan motor when neither motor is running.

What about the motor winding resistances? I've never seen a single phase motor ohm out like that. I would think that common-start and common-run have to add up to start-run. How can the not? The new motor runs under it's rated amps.

[jarganda83]

This unit is a Carrier
M/n: 38YCC036630
S/n: 1002E30967

[GT Jets]

So.... the old motor was not grounded. Right? But, it was, I replaced it and now the is no more voltage on the cabinet.

Maybe my meter is not powerful enough the detect that small of a resistance between the motor lead and the frame? But still be enough to pass voltage. Is that possible? I should also mention that the comp. has one leg with continuous power. So the comp. could be allowing some voltage to pass to the cond. fan motor when neither motor is running.

What about the motor winding resistances? I've never seen a single phase motor ohm out like that. I would think that common-start and common-run have to add up to start-run. How can the not? The new motor runs under it's rated amps.

You misinterpreted your meter reading in your post... "Found no resistance" IS a direct short", you should have stated that the resistance was greater than what your meter was capable of reading, unfortunately for many meters, this is a mere 200ohms.

Just because your meter did not read any "CONTINUITY" does not mean the motor was good.

I am also betting that the grounding on the line side of the machine has been compromised in one way or another. Betting it is not bonded at all. (If it was, you would not have gotten "zapped". Also there needs to be a separate ground wire from the chassis of the condenser fan motor to a KNOWN good ground.

Several years ago there was a facilities guy knocked unconscious by getting shocked and falling and hitting his head on a gas line. I often use my Volt Alert during PM's to test the sheet metal before I pull them apart, especially if they have a history of tripping breakers.

GT

[jarganda83]

Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...

[GT Jets]

Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...

Now we're getting to the meat and potatoes.

A higher end meter very well may have been able to prove the motor partially grounded. What make/model meter were you using?

I agree that the ground issue needs to be investigated further. My bet is that the breaker panel the power is coming from has lost continuity with the ground stake at some point. Either that or it was never completed (both equally plausible).

For god sakes be careful....

[timebuilder]

This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.

The unit needs an electrically acceptable low impedance ground to the overcurrent device that protects the unit. ALL current should have flowed through the equipment grounding conductor, as opposed to you, wearing work boots and touching the fan grille. If you felt discomfort when touching the grille, the impedance of the ground path is too high.

[timebuilder]

Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...

If you got "zapped," then yes, the entire ground path from the service SHOULD be checked. Similarly, the connection in the unit disconnect should not rely on three sheet metal screws to ground the cabinet of the unit. A grounding conductor should run from the disconnect to a grounding lug somewhere in the unit power section.

Note to all:

when using screws for grounding or bonding, only a machine thread screw is acceptable. Typical sheet metal thread screws are not acceptable in the NEC.

A ground lug, attached with one or more machine screws, is best.

[thermojohn]

The unit needs an electrically acceptable low impedance ground to the overcurrent device that protects the unit. ALL current should have flowed through the equipment grounding conductor, as opposed to you, wearing work boots and touching the fan grille. If you felt discomfort when touching the grille, the impedance of the ground path is too high.

Great explanation.

[timebuilder]

Now we're getting to the meat and potatoes.

A higher end meter very well may have been able to prove the motor partially grounded. What make/model meter were you using?

I agree that the ground issue needs to be investigated further. My bet is that the breaker panel the power is coming from has lost continuity with the ground stake at some point. Either that or it was never completed (both equally plausible).

For god sakes be careful....

The problem would not be with the grounding stake or rod. The ground path begins at the panel with the system bonding jumper, which connects the ground bar to the cabinet and the neutral. This is the ONLY point in a system where the neutral and grounding conductors meet, so as to avoid what they call "objectionable current," that is, where current typically is flowing through the ground conductors. That current should ONLY flow during a fault. That's why they changed from three wire to four wire plugs for dryers, because we used to tie the ground (cabinet) to the neutral at the terminals in the back of the dryer. Now, only the cabinet connects to the fourth wire, and never carries current until something in the dryer shorts to the cabinet.

The grounding conductors carry fault current to the neutral at the panel, so as to assist the overcurrent device to open. The ground rod outside is there to take any lightning potential in the building wiring system to a low impedance earth ground, which only lightning uses as its sought potential. We can read voltage potential to ground because the building neutral and the utility transformer neutral (the center tap of the secondary) both tie to ground rods for lightning protection of the grid and structure.

[ararams]

Keep in mind all your electrical connections including ground is only as good as it's weakest or apparent connection. I your unit is grounded you may want to check the five or six screws on the fan guard to top cover since carrier uses screws with a rubber washer.

[timebuilder]

Keep in mind all your electrical connections including ground is only as good as it's weakest or apparent connection. I your unit is grounded you may want to check the five or six screws on the fan guard to top cover since carrier uses screws with a rubber washer.

AND, the motor should have a grounding conductor going to the unit cabinet, under a machine thread screw, such as a self-tapping 10-32 screw. Green coloring is not an NEC requirement, BTW.

[jarganda83]

Timebuilder, does this "bonding jumper" weaken when there is a large flow of current through it? The cc heater in the unit

[jarganda83]

Sorry... the wire nuts that attatch the the cc heater leads were laying in water in the botttom of the unit. So, that did trip the breaker at the roof panel and at the breaker down stairs.

[timebuilder]

Timebuilder, does this "bonding jumper" weaken when there is a large flow of current through it? The cc heater in the unit

Generally speaking, no. It is sized by the manufacturer of the panel to be sufficient for its purpose.

[jarganda83]

When the unit "zapped" me it was only a tingle.

So, if the unit was properly grounded I should have felt absolutely nothing?

The breaker only trips when there is high amperage, right? So, if the short to ground in motor was very small then maybe it wasn't over amping?