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Old 07-06-2016, 01:10 PM   #1
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Default Ground/Shield VFD cable termination

Hi.
I try to connect a VFD to a motor. It is VFD drive to 4C Flexible VFD Cable to Junction box to 3C Arm VFD Cable to 3-phase motor. The Ground and shield are terminated in both VFD Drive and Motor side. The Junction box is panel grounded to closest earth ground rod. The 4C Flexible VFD cable is for 3-phase power + Ground + shield. The 3C Arm VFD cable is for 3-phase power + Ground + Aluminum Sheath.

I have question about Cable Grounding/Shield inside the Junction Box:
Inside the Junction box, does anyone know if I should connect VFD Flexible cable Ground to isolated terminal block to Arm VFD Ground? (Cable Ground is isolated to Junction box Grounding) Also, Should I use isolated Grounding Bushing to isolate Arm VFD Cable Aluminum sheath to Junction box Panel Grounding? and Connect Aluminum sheath to Flexiable VFD Shield? Can I connect Flex Cable Grounding conductor, Flex cable shield, Arm VFD Ground conductor, Aluminum Sheath together inside the Junction Box and Isolate those Grounding to Junction Box Grounding?

In another word, how to connect Grounding/shield for VFD cable inside the Junction Box?
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Old 07-06-2016, 01:53 PM   #2
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Power cable shields should be connected on both ends to ground.
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Old 07-06-2016, 02:13 PM   #3
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The VFD drive to Motor is about 200 Meter apart. its about 100meter for flex VFD cable and 100 meter for Arm VFD Cable.
you mean, connect Ground/shield cable to Earth Ground on Motor, Junction Box, and VFD drive? There will be grounding current (shield current) issue for this power cable if we connect cable Ground conductor to the earth ground at the Junction Box location.
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Old 07-06-2016, 03:03 PM   #4
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It's a power cable not an instrument cable. The shield has to be connected at both ends to provide a path for common mode current back to the drive.

Your cable manufacturer with have a spec on how to prepare the shield and drain depending on the exact type of shield you are using.
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Old 07-06-2016, 04:21 PM   #5
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It's a power cable not an instrument cable. The shield has to be connected at both ends to provide a path for common mode current back to the drive.

Your cable manufacturer with have a spec on how to prepare the shield and drain depending on the exact type of shield you are using.
Yes. you are correct that VFD and Motor need to grounded. My concern is Junction box that is between VFD drive and Motor. Should I connect Flex VFD Cable Ground Conductor and Shield, and Arm VFD Cable Ground Conductor and Aluminum Sheath and Connect all to the Earth Ground Lug inside the Junction Box? By saying this way, the Flex VFD Cable is grounded at the VFD Drive and at Junction Box. The Arm VFD is Grounded at the Junction Box and at the Motor side. Is this correct way to do it?

Many engineer are say to connect Ground and shield to the isolated Terminal inside the Junction Box to aviod current leakage that many flow to the Junction Box Panel Grounding. so all shield current are grounded at the motor and VFD drive only not at the Junction Box. Is this way more correct?

https://www.belden.com/resourcecente...tion_Guide.pdf
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Old 07-06-2016, 05:18 PM   #6
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Yes. you are correct that VFD and Motor need to grounded. My concern is Junction box that is between VFD drive and Motor. Should I connect Flex VFD Cable Ground Conductor and Shield, and Arm VFD Cable Ground Conductor and Aluminum Sheath and Connect all to the Earth Ground Lug inside the Junction Box? By saying this way, the Flex VFD Cable is grounded at the VFD Drive and at Junction Box. The Arm VFD is Grounded at the Junction Box and at the Motor side. Is this correct way to do it?

Many engineer are say to connect Ground and shield to the isolated Terminal inside the Junction Box to aviod current leakage that many flow to the Junction Box Panel Grounding. so all shield current are grounded at the motor and VFD drive only not at the Junction Box. Is this way more correct?

https://www.belden.com/resourcecente...tion_Guide.pdf
That is technically correct, isolate the Gnd in the J-Box. But in reality, not a big deal if there is nothing else inside of that J-box, and it is steel (not plastic) because of radiated EMI/RFI. If you do ground it inside of the box, you add a risk of creating an unintended Common Mode noise path back to it's source (usually the VFD itself) along the ground plane. CM noise always wants to go back to where it originated and you want to make that as easy as possible to avoid it causing other havoc on the way. At 200' total, assuming more than 100' from the VFD to the J-box, the CM noise path back to the VFD would have a lot of resistance to overcome (presumably) compared to the shield drain. But still, the safer path is to not ground the shield in the j-box.
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Old 07-06-2016, 10:45 PM   #7
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You mean you have an intermediate junction box between motor and drive? Then you want to isolate.
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Old 07-07-2016, 08:19 AM   #8
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At 200' total, assuming more than 100' from the VFD to the J-box, the CM noise path back to the VFD would have a lot of resistance to overcome (presumably) compared to the shield drain. But still, the safer path is to not ground the shield in the j-box.
The OP said about 200 and 100 meters... Not sure if that would make a big difference....
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Old 07-07-2016, 12:58 PM   #9
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You mean you have an intermediate junction box between motor and drive? Then you want to isolate.
Yea, His question was about the J-Box in between the drive and the motor.
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