FRP cable trays

[cmelguet]

Hi, I am involve in a project where we are running several 3 phase circuits of 33 kV using single phase conductors arranged in a trefoil formation. The cables are installed on a FRP cable ladder which is no good as a grounding conductor. The circuits lengths is around 1000 m, and all of the circuits goes from the main substation GIS switchgear to transformers 33/4.16 kV unitsubstations located around the plant.

Question: NEC requires to run a grounding conductor with every circuit. Normally the cable ladder is good enough for this. Since in this case i have an FRP cable ladder what are my alternatives?. Should I run an aditional grounding conductor for each circuit?. I understand the grounding grid is not good enough as a grounding conductor, even if all the grounding grids of the plant are joined together.

Regards,
Cristian

 

[7anoter4]

According to NEC 250.4 General Requirements for Grounding and Bonding. (B) Ungrounded Systems. (1) Grounding Electrical Equipment. you have to connect the equipment non-carrying current metallic parts to the local Grounding.
But according to (4) Path for Fault Current.:
"shall be installed in a manner that creates a low impedance
circuit from any point on the wiring system to
the electrical supply source to facilitate the operation of
overcurrent devices should a second ground fault from a
different phase occur on the wiring system. The earth shall
not be considered as an effective fault-current path."
you have to install a grounding conductor along 33 kV cable on the FRP cable tray.
The grounding conductor has to withstand the Line-to-Ground-to-Line short-circuit current.
I am not sure the metallic cable tray-even aluminum made-could withstand a medium voltage system short-circuit current. Aluminum cable tray is good only up to 2000 A.-a low voltage short-circuit, may be. See NEC Table 392.60(A)

 

[cmelguet]

Thanks 7anoter4,

The 33 kV system is resistance grounded (600 A).
Another idea, the 33 kV cables installed in the cable ladder have a screen. Can i use the screen as a grounding conductor?. In this way i dont need to run a grounding cable along the FRP ladder.

Regards?

 

[davidbeach]

Generally the higher the voltage, the lower the fault current. 40kA is real easy to achieve at 480V, but rather rare in the 15kV range. Getting above 40kA at 115kV requires lots of sources.

 

[7anoter4]

In my opinion, it is permitted according to NEC art.250.190 Grounding of Equipment.(C) Equipment Grounding Conductor. (2) Shielded Cables.:
"If the cable assembly is suitably rated for the ground fault current and is of the concentric neutral type, the shield conductors shall be permitted as the equipment grounding conductor."
NEC Table 250.122 for Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, Not Exceeding 600 A recommends 1 awg [83.69 kcmil] copper conductor cross section.
See[for instance ] some concentric neutral cables:

http://www.prysmianusa.com/energy/product-families/medium_voltage.html

 

[7anoter4]

I have to mention since the shield is grounded at both ends the losses of circulating current induced in the shield requires cable derating. However, you can reduce this current by cross bonding and transposition.