Lost Ground

[PSM007]

I recently came across a problem where the ground became disconnected on a 120 VAC outlet and the machine chassis that was connected to it became live with 60 VAC. Why would this occur and how do I fix it from happening again?

As a side note , we ground bond and HI-POT all are equipment before it is brought into production.

 

[petersonra]

I recently came across a problem where the ground became disconnected on a 120 VAC outlet and the machine chassis that was connected to it became live with 60 VAC. Why would this occur and how do I fix it from happening again?

As a side note , we ground bond and HI-POT all are equipment before it is brought into production.

what do you mean by "lost ground"? or "ground became disconnected"?

what do you mean by "live with 60 VAC"? how was this measured? what was it referenced to?

 

[GoldDigger]

I recently came across a problem where the ground became disconnected on a 120 VAC outlet and the machine chassis that was connected to it became live with 60 VAC. Why would this occur and how do I fix it from happening again?

As a side note , we ground bond and HI-POT all are equipment before it is brought into production.

A machine which passes its Hi-Pot testing can still have capacitive leakage to the metal shell of the machine. If this happens in a motor winding or resistive heating element, the equal capacitance to the neutral and the 120V ends will cause the shell to go to 60V AC. But you cannot pull much current at that voltage, again because of the series capacitance.
If you use a low impedance meter, you should not read 60V anymore. There may still be enough current from the shell through a user to ground to be felt, although it will probably not be dangerous.
To prevent if from happening again, do a better job of providing the EGC.

 

[PSM007]

OPEN GROUND DETECTION

OPEN GROUND DETECTION

Gentlemen,

My apologies, I should have put more information in my original post. At my plant we have mobile equipment that we designed and built that can be plugged into our main production machines throughout our plant.
The issue we had last week involved an extension cord plugged into our main machine and one of our moble units plugged into the extension cord. We had a open ground in the extension cord of which caused an
shock when the operator touched the mobile equipment frame and the frame of the main machine. There was a potential of 60VAC between the frame of each machine without the ground connection.

We are in the process of removing of all extension cords and placed in our PM plan that all outlets will be checked on a regular basis.I am researching possibly adding components to our machines to detect an open ground condition. I have found a loop monitor on the market that measures that resistance of the PE condutor connections. Is there any other ideas that I can look into to prevent this from happening in the future?

Thanks.

 

[charlie b]

A properly connected Equipment Grounding Conductor can prevent a shock. But lack of a proper EGC connection can never cause a shock. You have something else going on. It could be a breakdown in the insulation of an ungrounded conductor or motor winding that is in contact with an equipment casing. It could be capacitive coupling, as has been suggested before. It will not be another open ground, so I would not spend time looking for that ghost.

 

[jxofaltrds]

Sounds like you are bonded in the machine that you connect. Correct?

The cord lost it's grounded conductor. Correct?

The EGC is acting as a grounded conductor AND the operator when he touches both machines.

You engineers can you explain it better?

 

[GoldDigger]

Sounds to me a lot more like the grounding conductor (EGC) that broke.
Interrupting the neutral would not put 60V on the EGC and at the same time allow the machine to operate normally.

Sent from my XT1080 using Tapatalk

 

[petersonra]

Sounds to me a lot more like the grounding conductor (EGC) that broke.
Interrupting the neutral would not put 60V on the EGC and at the same time allow the machine to operate normally.

Sent from my XT1080 using Tapatalk

How would an ipen EGC put 60V on the EGC?

 

[GoldDigger]

As stated earlier, equal capacitive coupling to the machine shell from hot and neutral would cause a voltage divider effect. 120/2= 60.
One possible such source would be a motor winding. Another would be resistive wire around a grounded core. Yet another would be a linear fluorescent tube close to a grounded reflector.
Any such mechanism could leave a 60V difference between the ungrounded machine shell and grounded machinery.

Sent from my XT1080 using Tapatalk

 

[dereckbc]

Something else might be going on. The clue is as you stated a machine operator was shocked. If it were capacitive coupling, it is not likely the operator would have felt anything as there is just would not be enough current to cause a sensation. I am shooting in the dark here but if I were to bet, my money is says the machine in question has the neutral and EGC bonded together in some shape, form, or fashion. Could be a fail component like a filter capacitor or MOV between the neutral and EGC, or it could be intentional bond a ignorant electrician did some time ago.

 

[GoldDigger]

A surge protector or EMI filter combining capacitors and MOVs from both hot and neutral to the EGC would have the same voltage divider effect.

Sent from my XT1080 using Tapatalk

 

[cuba_pete]

Capactive coupling

Capactive coupling

A machine which passes its Hi-Pot testing can still have capacitive leakage to the metal shell of the machine. If this happens in a motor winding or resistive heating element, the equal capacitance to the neutral and the 120V ends will cause the shell to go to 60V AC. But you cannot pull much current at that voltage, again because of the series capacitance.
If you use a low impedance meter, you should not read 60V anymore. There may still be enough current from the shell through a user to ground to be felt, although it will probably not be dangerous.
To prevent if from happening again, do a better job of providing the EGC.

Years ago as a young ET, I and a couple others decided to test our strength by holding a large disk drive motor (out of the chassis) while it was spun up. We only considered the torque, and not the shock that would result. Since the motor was no longer bolted into the chassis the EGC for the motor was lost. What resulted was involuntary clamping of one's hands to the case. Unable to let go, the torque/strength test was passed...involuntarily.

It was only a year or so later when an upgrade to the hard drives was made by the manufacturer to all of our drives' EGC's, requiring a larger dedicated bonding strap for static charge dissipation. Gee, we thought...no kidding? We could have told you that!

Young and stupid, and maybe lucky to be alive.