Terminating shield on 5kV insulated conductor
Terminating shield on 5kV insulated conductor
(OP)
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
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





RE: Terminating shield on 5kV insulated conductor
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
RE: Terminating shield on 5kV insulated conductor
RE: Terminating shield on 5kV insulated conductor
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:
h
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://w
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].
RE: Terminating shield on 5kV insulated conductor
RE: Terminating shield on 5kV insulated conductor