Terminating shield on 5kV insulated conductor 1

[eeprom]

Hello,
I am trying to find a definitive answer to a question regarding how to properly ground a shield on a medium voltage cable. I've got a 1200 HP 4160V motor, and I have shielded cable running from the breaker to the motor. The guy who did this last grounded both ends of the shield. The stress cones are in bad shape, and I believe it is because of having both ends of the shield grounded. I personally believe the cable should be grounded at one end, at the breaker only. But I cannot find any text which states one way or the other.

IEEE Red Book says that there "is much disagreement" in regards to this subject. I am looking for a source which will explain what benefit you get from the different methods of grounding. If you know of such a source, please advise.

Thanks

 

[dpc]

http://www.okonite.com/engineering/shielding.html

It's generally safer to ground the shield at both ends. If grounded at one end only, high voltage can build up on the ungrounded end.

With shield grounded at both ends, induced shield currents can occur, but these are generally not a major problem.

I would guess that the large majority of MV motor feeder cables in the US have the shield grounded at both ends.

Contact Okonite for more info on cable shield grounding.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[eeprom]

Thank you for your help. That Okonite link was very helpful.

 

[7anoter4]

I agree with dpc it's generally safer to ground the shield at both ends. If you don't have any reason not to do so, you better do it. If it is a three-phase cable then no circulation current will affect the heating so the both ends grounding is recommended.
But I think you have to check two issues connected with the both [or multiple] shield grounding.
First of all you may overcome the voltage building-up by some reasonable means. Also, if the cable is sheathed and man contact with shield is avoided, a 65 V shield voltage may be permissible.
If the main conductor cross section is 4/0 or more the shield current will heat the cable and reduce the ampacity. This is the case of a single conductor cable, mainly.
The shield losses may be calculated according to EPRI-EL5036 Power Plant Electrical ref. series VOL.4-WIRE AND CABLES –for instance.
You may find the actual ampacity of a cable including shield losses from NEMA WC50
NEMA WC 50 Ampacities Including Effect of Shield Losses for Single-Conductor Solid-Dielectric Power Cable, 15kV Through 69kV (Copper and Aluminum Conductors).See:

http://electronics.ihs.com/document/abstract/OMZSSAAAAAAAAAAA

The second, the shield capacity to withstand the short circuit has also to be checked.
As per ICEA P-45-482-1994"SHORT CIRCUIT CHARACTERISTICS OF METALLIC SHEATHS AND SHIELDS" some 15% of the shortcircuit current flowing through the main conductor could pass through the shield. Using Okonite other link:

http://www.okonite.com/engineering/shielding-currents.html

one could appreciate if the cable shield may convey 15% of the calculated short-circuit current of the system.
Let say I"k=20 kA 15%=3 kA. The single phase cable 4/0 copper conductor screen dia =0.81 inches [Okonite]. For copper tape shield of 5 mils thick shield area A=3.02*b*dm.
A=3.02*5*810=12231 CM.
I=A/sqrt(N/K) If we shall take N= 8 cycles and K=0.517 for XLPE insulation then
I=3109 A. Then according to Okonite this shield may withstand 3.1 kA for 8 cycles =1000/60*8=133 msec.
You may consider a balanced share of this current through all 3 cables of a three-phase system and then the number of cycles will be then 76 cycles [1.27 sec].

 

[eeprom]

The reason for the post in the first place is that I noticed the stress cones were significantly deteriorated, to the point that I had them replaced. This motor has only been in service for 3-4 years, and the shield is grounded at both ends. I don't know how much current is flowing through the shield, but it must be enough to cause some heating. This particular motor is a 4160V, and so one end of the cable is safely encased in the switchgear. There is another termination at a local j-box. For reasons of safety, I would solidly ground the shield at the j-box, and leave the shields floating inside the switchgear. There is no safety issue this way, and the stress cones will last a lot longer.

 

[KB3HHC]

There's another issue to be aware of. See Section 6.2 of ANSI/IEEE C62.21-2003 which states that grounding the shield at the motor end only is helpful to reduce voltage surge levels at the motor. Further explanation is given within the standard, as well as some advice on how to balance this consideration against the others that have been mentioned.