EMC Cable shield used as PE wire, great idea or not?

Question

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.

Answer 1

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.

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.

Answer 2

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

Reasons:

• 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.

Answer 3

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.