In order to save on cabling, our company uses 3 wire EMC cables to connect electric machines to their VFD's. Some folks in our company think we should really use a dedicated ground wire, many are of the opinion that using the shield as ground (at least for wire diamaters < 6 mm²) is just fine. Recently we had some problems with bearings of machines that were running for 1-2 years: fluting, and a washboard like appearance of the raceways, as described in SKF documentation.

Can this problem be related to the omission of the ground wire? Are there any other compelling reasons to use a ground wire?

We often also don't use load reactors after EMC cabling. We use Danfoss drives (type FC102), which are said to have built in filters that are able to drive about 75 meters of cable without the need for additional filtering.

Worn out bearing

  • \$\begingroup\$ What does the motor documentation say about protective earth and what does the VFD documentation say? It's not just EMC. \$\endgroup\$
    – Andy aka
    Feb 10, 2021 at 12:40
  • \$\begingroup\$ The harmonic fluting suggests harmonic conducted emission filter current may be flowing through the bearings at synchronous speeds. \$\endgroup\$ Feb 10, 2021 at 13:30
  • \$\begingroup\$ The bearings issue is a different matter, you may need something like a graphite brush on the shaft to ground the rotating components. \$\endgroup\$ Feb 10, 2021 at 13:39

3 Answers 3


The shield is not to be used as the replacement for the 4-th protective earth wire, your colleagues are wrong on that. The shield is just to prevent EMI radiation from VFD source.

Are there any other compelling reasons to use a ground wire?

Yes. Safety reasons.

If the VDF already has a filter, then it might be enough. The bearings are also consumable parts, or maybe that used motors are also very cheap and low quality, since they opted for wrong and cheaper cable VS. human lives.

enter image description here

The picture is from the Danfoss FC102 manual, without any doubt the cable has to have 4 wires. The guys who put a wrong cable are charlatans, dangerous people.

  • \$\begingroup\$ The motors are presumably not of low quality. They are used for smoke extraction and have F300 rating or higher. \$\endgroup\$
    – Bart
    Feb 10, 2021 at 12:46
  • \$\begingroup\$ @Bart Sorry, but is the first time I encounter the F300 rating. Can you describe/link what is this about? \$\endgroup\$ Feb 10, 2021 at 12:49
  • \$\begingroup\$ It is a fire rating. It means the motor has to be able to operate at 300 degrees C for at least 1 hour. The motor/ventilator combination has a test certificate for this. I already suspect that the need for a ground wire is more of a safety concern, than that it prevents bearing damage. Can you elaborate a bit on the safety aspect? Would the EMC shield not be able to carry the short circuit current, or are there other safety aspects? \$\endgroup\$
    – Bart
    Feb 10, 2021 at 12:55
  • \$\begingroup\$ @Bart Link the model of this motor, because I don't believe. Never heard about such motor that can operate at 300 C. And yes, the protective earth conductor is a safety feature, it won't prevent the bearing failure. If you think that high frequency current from VFD is responsible, then you would need a sine filter on VFD output, but clearly without any in-built filter as you already have. \$\endgroup\$ Feb 10, 2021 at 13:07
  • \$\begingroup\$ The temperature requirement is laid out in European Standard EN12101-3 for smoke extraction. See for example this motor which is rated F300-2 hours: weg.net/catalog/weg/AE/en/c/… \$\endgroup\$
    – Bart
    Feb 10, 2021 at 13:14

Horrible idea and, probably, unlawful in most of the world.


  • PE has to carry fault current so it must have at least the cross section of the phase conductor; there are exceptions but in general that's the rule;
  • PE has usually (depending on where you live) quite peculiar requirements on connectors and bonding so it can't be disconnected or left disconnected by mistake or incomplete connector mating;

It is true however that shield has to be bound to PE for EMC reasons. Where to do this is a science in itself.


I don´t see what power are the drives, but over 30 kW it is recommended to use symetrical screened cables. If the cross section of screen and (if existing) additional PE wires inside the cable is below half of active wire cross section, then you need an external cable to get necessary cross section for PE.
About bearings issues, I think it can be more explained because current circulation in rotor. The solution is to replace one of the bearings with a insulated type, normally B side is smaller and cheaper.


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