FRP cable trays
FRP cable trays
(OP)
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
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






RE: FRP cable trays
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)
RE: FRP cable trays
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?
RE: FRP cable trays
RE: FRP cable trays
"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...
RE: FRP cable trays