Conveyor shocking people

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alblock

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NC, TN, SC
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Industrial Electrician
A supervisor came over the radio saying a conveyor was shocking people. Its a portable metal conveyor with rubber wheels. It has a 3p 480v gear motor that's powered by a vfd. The drive gets power through a L16-20 locking plug. When the motor was running i measured 160v from the conveyor and any metal object that was grounded. The conveyor being on rubber wheels wasn't grounded. I found the ground wire going to the drive was pulled out from the terminal. I connected the ground wire and the voltage went away. I didn't find any nicked wires or lose connections. I measured the motor phase to phase resistance and all were the same. I used a fluke megger and all phases read 550 mega ohms at 500v to the motor frame. The factory installed bond between the drive and the motor was good. I don't fully understand why there was a voltage with the motor checking good. I thought an ungrounded conductor would have to be touching something to cause this. Was this because the motor was on a vfd? Would this happen to any motor that loses ground? Thanks


 
alblock said:
All phase to phase voltage was 480v
All phase to ground voltage was 277v
I didn't check the EGC with a clamp but will on Monday.
Click to expand...
If you find a significant current, look for the same amount of extra current on one line conductor.
 
190322-2420 EDT

alblock:

It does not take much current to sense a shock, possibly somewhere below 1 mA.

With the metal frame not earthed (connected to earth or the floor such as by the EGC) very little current from your power lines can easily couple to the frame machine frame. Most likely this by capacitive coupling based on your measurements of DC leakage resistance.

Obviously when you reconnected the EGC the problem went away. 1 mA at 500 V will come from an impedance of about 500,000 ohms. At 60 Hz 5000 pfd = about 530,000 ohms. This is a moderate amout of leakage capacitance but is quite dependent upon the size of components. Capacitance could be noticeable higher if there are noise filter devices.

With the EGC open it would be interesting to know the leakage current to earth, the EGC. Don't do this current measurement with a current meter, but use an appropriate resistor and measure voltage drop across the resistor. 1 mA thru a 1 k resistor is 1 V. Resistors are very inexpensive. Protective fuses in many Fluke meters are like $15.

.
 
If the output of the VFD contains an EMI filter and one capacitor has gone bad or become disconnected, that could put 160v on the frame. Maybe even if one capacitor has significantly less capacity than the other two -- I wouldn't expect them to be any more precise than ±20%, and I've seen some marked "-20%, +80%".

If the voltage is there all the time, it could be an EMI filter on the input of the VFD.

(emi: "electro-magnetic interference")
 
LarryFine said:
If you find a significant current, look for the same amount of extra current on one line conductor.
Click to expand...

gar said:
190322-2420 EDT

alblock:

It does not take much current to sense a shock, possibly somewhere below 1 mA.

With the metal frame not earthed (connected to earth or the floor such as by the EGC) very little current from your power lines can easily couple to the frame machine frame. Most likely this by capacitive coupling based on your measurements of DC leakage resistance.

Obviously when you reconnected the EGC the problem went away. 1 mA at 500 V will come from an impedance of about 500,000 ohms. At 60 Hz 5000 pfd = about 530,000 ohms. This is a moderate amout of leakage capacitance but is quite dependent upon the size of components. Capacitance could be noticeable higher if there are noise filter devices.

With the EGC open it would be interesting to know the leakage current to earth, the EGC. Don't do this current measurement with a current meter, but use an appropriate resistor and measure voltage drop across the resistor. 1 mA thru a 1 k resistor is 1 V. Resistors are very inexpensive. Protective fuses in many Fluke meters are like $15.

.
Click to expand...

I will take the measurements on Monday and post results.

drcampbell said:
If the output of the VFD contains an EMI filter and one capacitor has gone bad or become disconnected, that could put 160v on the frame. Maybe even if one capacitor has significantly less capacity than the other two -- I wouldn't expect them to be any more precise than ±20%, and I've seen some marked "-20%, +80%".

If the voltage is there all the time, it could be an EMI filter on the input of the VFD.

(emi: "electro-magnetic interference")
Click to expand...

The voltage was only there when the motor was running.
 
Aside from EMI/RFI, all drives create Common Mode Noise, basically a flow of electricity teferenced to ground due to the way the PWM output functions. That CM noise needs a solid path to ground and if the ground wire was missing, it looks for the next best path.
 
I just now got time to do this.

LarryFine said:
If you find a significant current, look for the same amount of extra current on one line conductor.
Click to expand...

There was no current on the EGC.

gar said:
190322-2420 EDT

alblock:

It does not take much current to sense a shock, possibly somewhere below 1 mA.

With the metal frame not earthed (connected to earth or the floor such as by the EGC) very little current from your power lines can easily couple to the frame machine frame. Most likely this by capacitive coupling based on your measurements of DC leakage resistance.

Obviously when you reconnected the EGC the problem went away. 1 mA at 500 V will come from an impedance of about 500,000 ohms. At 60 Hz 5000 pfd = about 530,000 ohms. This is a moderate amout of leakage capacitance but is quite dependent upon the size of components. Capacitance could be noticeable higher if there are noise filter devices.

With the EGC open it would be interesting to know the leakage current to earth, the EGC. Don't do this current measurement with a current meter, but use an appropriate resistor and measure voltage drop across the resistor. 1 mA thru a 1 k resistor is 1 V. Resistors are very inexpensive. Protective fuses in many Fluke meters are like $15.

.
Click to expand...

169.6 VAC
168.2 VAC across 1000 ohm resistor

Low impedance setting
25.3 VAC
24.7 VAC across 1000 ohm resistor
 
alblock said:
I just now got time to do this.



There was no current on the EGC.
Click to expand...

Less than the minimum resolution of a clamp meter, but more than you'd want to carry through your body!

169.6 VAC
168.2 VAC across 1000 ohm resistor

Low impedance setting
25.3 VAC
24.7 VAC across 1000 ohm resistor
Click to expand...

168V across 1000 ohms means there was 168mA flowing through the grounding path, even with a 1Kohm impedance in the way. That's more than enough to kill somebody, and much more than would be expected for capacitive leakage from the motor windings to the frame or through an EMI filter. There's an actual ground fault in there somewhere, with an impedance of around 650 ohms {(277/0.168)-1000=648}. Since the megger confirmed the motor doesn't have a fault this big, you can bet it's the VFD or a faulted EMI filter or inductor. If the EGC current goes away when the motor's not running, then you know the problem is in or after the VFD, and not on the line side of it.
 
ptonsparky said:
Using a hi impedance meter we get 168 ma. Low, gives us 24. Either one more than ample to feel and on up to kill. So which reading should be used?
Click to expand...
To me, this means the voltage source is of a relatively high impedance, because the low-impedance reading lowers the voltage enough to 1/7 of the high-impedance reading.
 
ptonsparky said:
Using a hi impedance meter we get 168 ma. Low, gives us 24. Either one more than ample to feel and on up to kill. So which reading should be used?
Click to expand...

The high-impedance reading is the relevant one.

Neither is truly a "high impedance" voltage measurement, since you had a 1000 ohm resistor across the meter. Your actual equivalent circuit was this:

(277V PHASE)------>(fault impedance)-------->(1000 ohm resistor)------->(EGC GROUND)

Your volt meter was across the 1000 ohm resistor. You can see that if your meter has a high impedance (which is usually many megaohms for a digital multimeter), then the voltage you measure is equal to the current passing through the ground fault in milliamps (since your 1k resistor will drop 1V per milliamp). But if you put your meter in low-impedance mode, then it has a much lower internal resistance (but we don't know exactly what that impedance is). This is the same as putting another resistor in parallel with your 1k resistor. Now your voltage measurement is much less meaningful because you don't know the total impedance and can't do the quick 1V-per-milliamp math.
 
I will be replacing that drive. I may have some more questions after I do some research and note taking. We also have some used drives I'm gonna play around with. Thanks to everyone.
 
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