MV CABLE SHIELD GROUNDING 1

[duki]

At our facility we have an unwritten rule that mandates that all shields of MV cable be grounded at both ends in order to ensure the the voltage to ground on the shield is zero. I have asked my superintendents and engineers is there an IEEE, NEMA or any standard that backs this practice. No doubt it is good practice, but I would like to know if there is some standard that recommends this.
Thanks

 

[rbulsara]

It is no different than having a metal conduit, which is grounded at both ends (and many other places in between , at supports etc.)

 

[dpc]

Grounding of shields at both ends of a circuit is permitted, but not required by the NEC. In fact, the ampacity tables for MV cable given in Article 310 are based on grounding at one end only. If shields are grounded at both ends, the cable must be de-rated because the circulating current in the shield reduces the thermal capacity of the cable.

Grounding at both ends is a common safety practice to avoid high voltages on un-terminated shield.

Okonite used to have a section in their Cable Installation Manual covering the de-rating required due to multiple grounding points on shield. Sometimes the shields are transposed on long runs to minimize circulating current.

 

[john0109]

All ampacity tables given in Article 310 are not based on single point grounding. The ampacity tables for shielded power cable are based on single point grounding when individual phase conductors are seperated (eg one per duct; ducts on 7.5" centers).
If all three phases are in the same duct or grouped together (as in air), the tables give ampacities for multipoint grounded shields. Therefore, for all phases grouped together no derating to the table values are needed since they were calculated based on multipoint grounding.
A derating would be needed if multipoint grounding is used on a circuit where the phases are seperated (eg Table 310.81). For additional info see:

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

john0109

 

[cuky2000]

1- At our facility we have an unwritten rule that mandates that all shields of MV cable be grounded at both ends in order to ensure the voltage to ground on the shield is zero.

COMMENT 1: The shield potential may not necessarily be equal to zero even with both ends. Both ends grounded or multiple grounding points may minimize the shield potential to a safe level.

2- I have asked my superintendents and engineers is there an IEEE, NEMA or any standard that backs this practice.

COMMENT 2: Check the following standards:

IEEE standard 525: “IEEE Guide for the Design and Installation of Cable Systems in Substations”.

NEMA WC 50: “Ampacities, Including Effect of Shield Losses for Single-Conductor Solid Dielectric Power Cable 15 kV through 69 kV (ICEA P-53-426)”.

3- No doubt it is good practice, but I would like to know if there is some standard that recommends this.

COMMENT 3: The IEEE Std 422 and 525 recommend limiting the shield potential to 25 volts. The enclose graph shows the induced shield potential in function of typical lengths for cables with shields grounded at one point to limit shield voltage to as recommended in the IEEE Std 525.

_{NOTE: The first 3 points in the graph correspond to cable size 1/0, 2/0 and 4/0 AWG respectively.}