Motor Cable Shield Grounding

[hlrl]

It has been our practise to ground both ends of the shields for 6.6kV motor feeders (i.e ground the shield on the controller or switchgear end as well as on the motor end). Most motor manufacturers will also recommend to ground both ends.

Does anyone know the reasons why we need to ground both ends?

In the IEEEC62.21, it was recommended to ground only motor end. Does anyone have any experience on this method of grounding the motor feeder.

 

[swn1]

Disadvantage of grounding both ends: you induce currents in the shield, which can damage it.

Advantages of grounding both ends: lower impedance to ground, expecially from the stress cone area where most failures occur.

Also, should the shield be severed by excavation, forklift, or other misadventure, both halves will still be grounded, helping to ensure timely tripping and limit overvoltages along the cableway.

If you do not ground both ends, there will be a voltage induced on the shield approximately equal to the voltage drop in the phase conductor. So the ungrounded shield strap should be taped off and must be regared as energized when the system is in operation.

I would ground both ends and evaulate the shield current magnitude at commissioning. Disconnect the load end shield grounds only if the shield current magnitude exceeds the manufacturer's recommended limits.

 

[hlrl]

Hi SWN1,
Thanks for the responds.
The recommendation in IEEE62.21 is to ground the shield on the switchgear end to reduce the surge. It was reported the surge can be reduced by 50% especially for long cable (greater than 100m).
This is also new to me... Our practice is also to ground both ends.

 

[swn1]

By surge here I assume you are talking about charging current into the cable/shield capacitance? The higher shield/ground high-frequency impedance would reduce that, but it would have to be a significant factor. For 5KV class motor leads I don't think it will be. The capacitance of the motor windings is still there, after all.

If you are using a vacuum interrupter for the starter the surge will occur on solidly-landed contact faces and won't cause much problem anyway. With an air-magnetic starter the surge will jump the gap as it closes and cause some contact face damage, but nothing compared with what happens when you open them.

At higher voltage levels the surge into the distributed capacitance will be proportionatly higher. Perhaps it is significant on long 15KV cables, I know it is on 69KV lines. But you are the first person I've heard worry about it at 5KV.

Theory says ground one end. Safety considerations come down on the side of grounding both ends, but not overwhelmingly. Common practice is to ground both ends. Experience says it almost always works without trouble.

Either way, testing the integrity of the shield should be part of the periodic maintenance program. One disadvantage of the armored triplex cable is that there is no jacket over each shield. They all short to each other and the armor, masking any defects in individual shields.