Electrician Talk banner

1 - 20 of 23 Posts

·
Registered
Industrial electromechanial maintenance technician
Joined
·
7 Posts
Discussion Starter · #1 ·
This morning I experienced something that I do not quite understand, and I am hoping that someone can help me to better understand this very unsafe condition. Today was my first day on this job.

Two operating personnel started up a line. The line segments that were involved in this fault were a motorized feed-roll machine and a motorized cutter. These two mahines are mechanically and electrically isolated from each other, and both have their own ground conductor going back to the service panel. When either of the two operators touched both of these machines at the same time they received a shock. I immediately stopped the operation and checked both machine chassis continuity-to-ground with a multimeter. I read a very low resistance on each ground, so I felt ok with the integrity of their individual grounds. I disconnected power from the motorized feedroll machine and I turned-on the cutter. I recorded 35 vac when I placed a probe on the motorized cutter chassis and placed the other probe on the motorized feedroll chassis. I turned off the motorized cutter and, using the multimeter, checked for continuity between the two machine chassis. I read infinite resistance. Then I set-up a test. With the 35 vac present I placed a bonding conductor across the two machines and the 35 vac dropped to zero. I then left this bonding condutor/jumper in place.

My questions;

1. Since both machines are grounded to a common earth ground, shouldn't I have been able to read at least some low resistance when I tested continuity between the two machine chassis, or is the battery in the multimeter too weak for that kind of test? Should I have used a megger?

2. In reference to the 35 vac "leakage" that I read from the motorized cutter, is that evidence of an electrical breakdown that is not yet at the level to trip the motor or control circuit's OL device?

Thank You for any help you may offer!

Michael
 

·
Registered
industrial E,I&C
Joined
·
3,339 Posts
1/ if they do not ohm low then they are not connected to a common ground point. One may not be connected to any ground point.

2/ There are a lot of things that can explain the 35v potential (its lower than we would expect for a machine thats not grounded)
 

·
Just trying to get home
Site Engineer
Joined
·
2,411 Posts
1) "When either of the two operators touched both of these machines at the same time they received a shock. I immediately stopped the operation"
-- You weren't "immediate" enough for their own good. It's an individual choice, but maybe you can try to emphasize to your workers that having one person verify that another received a shock by trying it for themselves is not a valid method of troubleshooting.

2) If they are derived from separate power sources, the power sources themselves may not be bonded or grounded correctly. If the machines can both be touched simultaneously, which is obvious from your description, they have to be at the same potential.

3) Are there any neutrals involved here?

4) What do you consider "a very low resistance on each ground"?

5) What do you mean by "disconnected" power from the motorized feedroll machine?

6) What do you mean by "turned off" the motorized cutter?

re: 5) and 6) disconnects/knife switches, pulled fuses, removed wiring
 

·
Registered
Scada Supervisor
Joined
·
3,136 Posts
You have a machine that is ungrounded and has a fault.
Does this machine use conveyors with permanent magnet DC Motors.
This is common if a machine becomes ungrounded. I have seen it many time with plug in conveyors, ground pin breaks off and carbon dust conducts. Did you check for any DC to ground?
 

·
Just trying to get home
Site Engineer
Joined
·
2,411 Posts
I think the op died'd
 

·
Registered
Industrial electromechanial maintenance technician
Joined
·
7 Posts
Discussion Starter · #8 ·
1/ if they do not ohm low then they are not connected to a common ground point. One may not be connected to any ground point.

2/ There are a lot of things that can explain the 35v potential (its lower than we would expect for a machine thats not grounded)
I confirmed that both machines individually are grounded to the ground bus in the service panel that supplies power to both of the machines, and also that the service panel is grounded. I went so far as to test resistance between the service panel ground bus and the service panel grounding electrode (<150 ohms).
 

·
Registered
Industrial electromechanial maintenance technician
Joined
·
7 Posts
Discussion Starter · #9 ·
1) "When either of the two operators touched both of these machines at the same time they received a shock. I immediately stopped the operation"
-- You weren't "immediate" enough for their own good. It's an individual choice, but maybe you can try to emphasize to your workers that having one person verify that another received a shock by trying it for themselves is not a valid method of troubleshooting.

2) If they are derived from separate power sources, the power sources themselves may not be bonded or grounded correctly. If the machines can both be touched simultaneously, which is obvious from your description, they have to be at the same potential.

3) Are there any neutrals involved here?

4) What do you consider "a very low resistance on each ground"?

5) What do you mean by "disconnected" power from the motorized feedroll machine?

6) What do you mean by "turned off" the motorized cutter?

re: 5) and 6) disconnects/knife switches, pulled fuses, removed wiring
No neutrals involved. Both 480/3 Phase.
"Very low resistance" - less than 15 ohms.
"Disconnected" - unplugged from the male/female twist-loc coupling, completely opening the power ciruit.
"Turned off" - I pressed the stop PB and opened the control circuit, thus removing control power from the motor starter and stopping the motor..
 

·
Registered
Industrial electromechanial maintenance technician
Joined
·
7 Posts
Discussion Starter · #10 ·
You have a machine that is ungrounded and has a fault.
Does this machine use conveyors with permanent magnet DC Motors.
This is common if a machine becomes ungrounded. I have seen it many time with plug in conveyors, ground pin breaks off and carbon dust conducts. Did you check for any DC to ground?
There is no DC present and both machine chassis test <20 ohms to ground.
 

·
Registered
Scada Supervisor
Joined
·
3,136 Posts
both machine chassis test <20 ohms to ground.
turned off the motorized cutter and, using the multimeter, checked for continuity between the two machine chassis. I read infinite resistance.
This tells me you missed something, you can't have it both ways.
As for the meter not being able to read it, no it will read it. To proof this measure a roll of wire from end to end that will be reading 2500 foot your ground circuit is not longer than that.
You have two problems to find.
1. Why is the voltage there.
2. Why is the ground not common.

You answered about the DC voltage but not the DC motors, does it have DC motors? If so does the voltage you read change with machine speed? I have also seen this same thing with heavy dust, but it only shows up after losing a ground.
You said you ran a bonding jumper from machine to machine and it went away, what happens when you run a bonding jumper from the panel to one machine, then move it to the other machine?

Keep at it
 

·
Registered
Industrial electromechanial maintenance technician
Joined
·
7 Posts
Discussion Starter · #12 ·
turned off the motorized cutter and, using the multimeter, checked for continuity between the two machine chassis. I read infinite resistance.
This tells me you missed something, you can't have it both ways.
As for the meter not being able to read it, no it will read it. To proof this measure a roll of wire from end to end that will be reading 2500 foot your ground circuit is not longer than that.
You have two problems to find.
1. Why is the voltage there.
2. Why is the ground not common.

You answered about the DC voltage but not the DC motors, does it have DC motors? If so does the voltage you read change with machine speed? I have also seen this same thing with heavy dust, but it only shows up after losing a ground.
You said you ran a bonding jumper from machine to machine and it went away, what happens when you run a bonding jumper from the panel to one machine, then move it to the other machine?

Keep at it
t
No DC motors either.

Tomorrow I will recheck ground continuity between each machine and the panel, the panel and the system grounding conductor, and I am also going to meg the motor. This condition feels like the running machine (cutter) is poorly grounded, has an energized frame/chassis, and the other machine (feed roll) is well grounded.
 

·
Registered
Scada Supervisor
Joined
·
3,136 Posts
t
No DC motors either.

Tomorrow I will recheck ground continuity between each machine and the panel, the panel and the system grounding conductor, and I am also going to meg the motor. This condition feels like the running machine (cutter) is poorly grounded, has an energized frame/chassis, and the other machine (feed roll) is well grounded.
That makes sense, if it is only when it runs.
Cowboy
 

·
Just trying to get home
Site Engineer
Joined
·
2,411 Posts
I'd measure the current on that the bond wire between machines while running as another data point, as well has normal configuration current on EGCs (with and without the bond wire). In the future, the experience of checking current with a clamp on meter would be second nature. You can connect a scope to that as well to get what I call a fingerprint for VFDs and other known equipment.
 

·
Registered
industrial E,I&C
Joined
·
3,339 Posts
Generally when i wire any machine thats classified as a ground island (not welded to another machine) then i always ohm the machine casing to something that is grounded like the building.
Now its possible that some panels on a machine are mounted on rubber bushings but even then there should be a grounding jumper in the machine.

The voltage could be coming from a step down transformer, surge suppressor or a hundred other places which is why you need to make sure its grounded. Grounding will either bleed it off or short it out. P.s anything more than 2 ohms on copper/metal is high resistant
 

·
Just trying to get home
Site Engineer
Joined
·
2,411 Posts
Grounding will either bleed it off or short it out. P.s anything more than 2 ohms on copper/metal is high resistant
I like that, but where do you reference that for your use? Is that per manufacturer specs or your agency/corporation?

For my installs I am required to achieve 1 ohm. That's according to contractor installation guidance and some MIL-STD, and MIL-HDBK references.
 

·
Registered
industrial E,I&C
Joined
·
3,339 Posts
I like that, but where do you reference that for your use? Is that per manufacturer specs or your agency/corporation?

For my installs I am required to achieve 1 ohm. That's according to contractor installation guidance and some MIL-STD, and MIL-HDBK references.
Its more of a reply to his statement earlier that he was reading ""Very low resistance" - less than 15 ohms. "

Using a t5 which is generally about 1 ohms probe to probe 2 and under is exceptable unless its in the contract then im breaking out the quality meter for testing.
 

·
Registered
Joined
·
156 Posts
I agree: item one would be why is the voltage there? Item two would be grounding. You might find one while looking for the other. On ships, it wasn't continuous if it was more than 1 ohm. Anything over half ohm was suspect. My mental image of people touching both machines to see if there's actually a shock is of a current going right over their hearts. Jeenkys...
My next question would be what is being processed on these machines- dusty, caustic, hot, etc.?
The thing about 35 volts is a teaser- my first thought was ghost voltage. It almost looks like a shared neutral but no. I wonder if someone incorrectly branched somewhere off one of the legs for an outlet or something.
 

·
Registered
Industrial electromechanial maintenance technician
Joined
·
7 Posts
Discussion Starter · #19 ·
I agree: item one would be why is the voltage there? Item two would be grounding. You might find one while looking for the other. On ships, it wasn't continuous if it was more than 1 ohm. Anything over half ohm was suspect. My mental image of people touching both machines to see if there's actually a shock is of a current going right over their hearts. Jeenkys...
My next question would be what is being processed on these machines- dusty, caustic, hot, etc.?
The thing about 35 volts is a teaser- my first thought was ghost voltage. It almost looks like a shared neutral but no. I wonder if someone incorrectly branched somewhere off one of the legs for an outlet or something.
This is 480 3-phase. There would be no possibility of an outlet and zero branch circuits. The operators were subjected to the shock by accident. They did not intentionally touch both machines just to see if the voltage was present. The operation is very dusty. MAybe the extreme dust is conducting?
 

·
Registered
industrial E,I&C
Joined
·
3,339 Posts
This is 480 3-phase. There would be no possibility of an outlet and zero branch circuits. The operators were subjected to the shock by accident. They did not intentionally touch both machines just to see if the voltage was present. The operation is very dusty. MAybe the extreme dust is conducting?
Controls are probably 120v which are more exposed and more likely to get damaged then the main phases.
 
1 - 20 of 23 Posts
Top