Ground Conductor - 15kV Tape Shield 1

[cslater]

I have seen a number of installs of 15kV cable with a grounded tape shield on each conductor. In all of these installs, there is not a separate ground conductor pulled through the conduit.

On a new job, the engineer is calling for a separate ground wire. Can someone point me to the relevant code section?

It's not a big deal to pull the ground wire - I'm just curious to understand the proper way to do this...

Thanks!

- Chris

 

[7anoter4]

I think if the Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment[metallic cable tray]
is more than permitted according to cable tray cross section area [see NEC Table 392.60A] a Grounding cable has to run
through the cable tray according to Table 250.122.

 

[bacon4life]

Where the cables identical? In some cases my utilities uses cables with a tape shield & separate neutral and sometimes we use cable with a concentric neutral. The tape shield is very thin and just carries the capacitive charging current. The concentric neutral cable has the same copper cross section area for both the inner conductor and the shield. Most typically we will use tape shield for balanced 3 phase applications and concentric neutral for single phase applications.

 

[220or221]

Article 250 of the NEC covers the requirements for grounding, specifically section X of Article 250 - for MV applications. In the case of your MV circuit, the use of a seperate ground conductor versus the tape shield as an equipment/system ground conductor depends on several factors: the type of raceway system and whether or not it can serve as a ground path, the system grounding method (solid or low resistance), and the short circuit capacity of the tape shield itself. Under the correct set of circumstances the tape shield will be adequate to serve as the ground conductor; however in my experience the seperate ground conductor is typically required for one or more reasons related to the items noted above.

 

[7anoter4]

Let's say the cable ran through rigid steel conduit underground and at the end will be connected to a motor terminal box.
The steel rigid conduit -according to NEC- it is a suitable mean to carry any current to Ground but the flexible does not. So you need -any way- a grounding cable from the rigid conduit to terminal box.
You could have a similar problem at the other end so you would prefer a grounding conductor along the entire length parallel
to the cable. If the cable enter the conduit from a cable tray you have to check if the cable tray can support the fault current.
IEEE Std 575 ch. 5.4.3 Parallel Ground Continuity Conductor states that
a parallel ground continuity conductor close to the shielded cable will reduce the magnetic field produce by other conductors carrying the short-circuit to Ground.
conveying the return current to Ground. This conductor may be a steel rigid conduit-of course.