VSD cable screen earthing - China

ASF · May 19, 2017

Hi all,

I'm currently in China to commission a job, and there are four large Danfoss FC102 VSD's (~200kW) which have been installed by the local electricians. They have only earthed the cable screens at the VSD end, not the motor end. They seem convinced that this is correct.

In Australia at least, all VSD cables are solidly earthed both ends. All the reading I've been doing seems to indicate that this is best practice. And the Danfoss manual, while it doesn't explicitly state it, does show the wiring overview with a ground symbol at both ends of the motor cable screen. So I feel like I'm correct on this, but I speak next to no Chinese and the locals don't speak a word of English, so I'm having to have this discussion via google translate.

Just thought I'd throw it out there to the drive experts of the forum - is the "earth both ends" a truly universal rule? Is there a possibility that there is some difference to the power supply/earthing system in China that would make the "earth at one end only" philosophy correct? I don't want to upset the locals, but all of the VSD's are on Ethernet comms, and I also don't want to spend the next three weeks battling strange interference issues on my drive comms.

Thanks!

rupej · May 19, 2017

I'm pretty sure you're right on this. They're probably thinking of shielding for analog signal wiring with only grounding one end. That said, it's probably not a big difference as long as at least one side is grounded.

ASF · May 19, 2017

That's my understanding too. What level of impact am I looking at if I don't enforce them earthing the other end of the cable? Is it a case of "earth neither end: 100% interference; earth one end: 50% interference; earth both ends: 0% interference" or is it more like cutting out 90% of the interference by earthing one end, and if you need the last 10% you earth the other end as well?

I know I'm massively oversimplifying it - I'm sure there's a ton more to it than what I know - but if something has intermittent faults, it's a long way back to China to fix it later. I just want to make sure I get it right.

I suspect that all the smaller VSD's have been wired the same - these are all around 2-4kW and running small pumps and fans, so I'm somewhat less concerned about those, it's mainly these big 200kW ones. Should I be concerned about the smaller ones also?

Boorabee Parker · May 19, 2017

ASF said:
That's my understanding too. What level of impact am I looking at if I don't enforce them earthing the other end of the cable? Is it a case of "earth neither end: 100% interference; earth one end: 50% interference; earth both ends: 0% interference" or is it more like cutting out 90% of the interference by earthing one end, and if you need the last 10% you earth the other end as well?

I know I'm massively oversimplifying it - I'm sure there's a ton more to it than what I know - but if something has intermittent faults, it's a long way back to China to fix it later. I just want to make sure I get it right.

I suspect that all the smaller VSD's have been wired the same - these are all around 2-4kW and running small pumps and fans, so I'm somewhat less concerned about those, it's mainly these big 200kW ones. Should I be concerned about the smaller ones also?

Not having the shield grounded at the motor end can cause issues with motor and gearbox bearings regardless of size. I did read a tech doc about this. If I can find it I will post up a link. It is a constant argument I have in Aus as well, let alone China.

From an AB doc - Wiring and Grounding Guidelines for Pulse Width
Modulated (PWM) AC Drives

"Shielded cable contains the general benefits of multi-conductor cable with the
added benefit of a copper-braided shield that can contain much of the noise
generated by a typical AC drive. Use shielded cable for installations with
sensitive equipment, such as weigh scales, capacitive proximity switches, and
other devices affected by electrical noise in the distribution system.
Applications with large numbers of drives in one location, imposed EMC
regulations, or a high degree of communication/networking, are also good
candidates for shielded cable"

"Shielded cable can also help reduce shaft voltage and induced-bearing currents
for some applications. Also, the increased size of shielded cable can help extend
the distance between the motor and the drive without the addition of motor
protective devices, such as terminator networks"

rupej · May 19, 2017

If I had to guess, I would say that the shield is 90% effective with just grounding one end. But of course I have no data to back that up.

For reference though, I see VFDs wired all the time with non-shielded wire for great distances. As long as no comms are run for any significant distance in the same wireway as the power conductors, no ill effects are ever noticed.

Boorabee Parker · May 20, 2017

rupej said:
If I had to guess, I would say that the shield is 90% effective with just grounding one end. But of course I have no data to back that up.

For reference though, I see VFDs wired all the time with non-shielded wire for great distances. As long as no comms are run for any significant distance in the same wireway as the power conductors, no ill effects are ever noticed.

Just because you see these type of installations all the time does not mean it is correct. Some ill effects that may occur are

Common mode noise
Bearing Current
Cable charging current phenomenon
Reflected wave phenomenon
Electromechanical transient interference

dmargineau · May 20, 2017

ASF said:
Hi all,

I'm currently in China to commission a job, and there are four large Danfoss FC102 VSD's (~200kW) which have been installed by the local electricians. They have only earthed the cable screens at the VSD end, not the motor end. They seem convinced that this is correct.

In Australia at least, all VSD cables are solidly earthed both ends. All the reading I've been doing seems to indicate that this is best practice. And the Danfoss manual, while it doesn't explicitly state it, does show the wiring overview with a ground symbol at both ends of the motor cable screen. So I feel like I'm correct on this, but I speak next to no Chinese and the locals don't speak a word of English, so I'm having to have this discussion via google translate.

Just thought I'd throw it out there to the drive experts of the forum - is the "earth both ends" a truly universal rule? Is there a possibility that there is some difference to the power supply/earthing system in China that would make the "earth at one end only" philosophy correct? I don't want to upset the locals, but all of the VSD's are on Ethernet comms, and I also don't want to spend the next three weeks battling strange interference issues on my drive comms.

Thanks!

Definitely both ends especially for 50 KW or or higher or when PWM above 4 kHz; most of the US large heavy industry manufacturers require load side circuitry consisting of both ends ground connected armor armored cables with both ends connected grounding conductor.

The sometimes implemented method similar to Analog signals circuitry shield grounding follows the wrong premise:

The Analog signal cable shielding intends to protect the Analog circuitry from outside induced electrical 'noise' while PWM signals are shielded in order to avoid the producing of said 'noise'.

danw · May 20, 2017

Here's what the people who make the stuff say:

1. Schneider Square D VFD-Motor Wiring Practices
http://static.schneider-electric.us/docs/Motor Control/AC Drives/8800DB1501.pdf

page 7, Rule 2 When using shielded cable between the VFD and motor, connect the cable shield at both ends (to the ground)

2. Allen Bradley Wiring and Grounding Guidelines for Pulse-width Modulated (PWM) AC Drives
http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

pg 60: diagram shows shield connection to Ground at motor end and the drive end

pg 66: The cable connector must provide 360° contact and low transfer impedance from the shield or armor of the cable to the
conduit entry plate at both the motor and the drive (or drive cabinet) for electrical bonding

Shield termination on pages 75-76

3. Eaton VFD Wiring Practices Guide App note

http://tinyurl.com/mxt5o2d

Pg 1: Shielding – For the input and output motor leads it is suggested to have the shielding terminated
on both ends to the ground location, to keep a common ground throughout the system.

4. Danfoss White Paper: Best EMC Installation Practice for Variable Speed Drives

http://www.iceweb.com.au/ElectricalWeb/Earthing/Earthing Bonding Variable Speed Drives .pdf

page 11: f. and more importantly use a screened motor cable properly terminated at both ends.

5. ABB cable glands at both ends ground the shield:

Shields at Splices are grounded:

JesperMP · May 20, 2017

IMO emc cable glands are necessary at the motor end (because there is no other way to terminate the shild at the motor end), but not in the panel. In the panel, terminating the shield at an earth bus bar is better and more convenient.

shooter · May 20, 2017

For low signalwires it is recomended to ground one side, to avoid picking up noise from outside, however for hogh voltage systems it should be grounded as the reason is to avoid noise coming out of the cable.

Apart from this the motor must be grounded with a proper cable for safety reasons, the shield can be used for this when the diameter is big enough (50% of live wires.)

DickDV · May 20, 2017

This is a common misunderstanding among VFD people. It is essential to have a ground bond wire between the motor and the VFD frame to assure that the drive's ground fault protection system can operate properly. This is a ground BOND, where both the VFD and motor are otherwise grounded to code but the two grounds are forced to the same potential by the bond wire. This bond can, as a result, carry significant currents where the two grounds are at different potentials.

VFD motor cable ground conductors are sized to carry this possible current in addition to providing the signal shielding intended to minimize RFI and EMI stray interference in the vicinity of the VFD/motor system.

The "ground only one end" principle applies for signal protection where the shielding system in the cable is not designed to carry current--other than whatever current is involved conducting the noise to ground. In those kinds of shielding applications, only one end is grounded to make foreign currents impossible. Convention has it that the signal source end is the better one to ground although I've never seen a compelling argument for why that would be true.

ASF · May 20, 2017

Thanks all. It definitely seems to be that the overwhelming consensus is to ground both ends. It's just a little tricky conveying all of this to an electrical team that probably know 30 words of english between them. Mind you, I only know about 5 words of Chinese, and I'm in their country, so I can't go pointing fingers

VSD cable screen earthing - China

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rupej

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Boorabee Parker

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