Combining control cables in Substation conduits - Best Practice

[rockman7892]

What is typically viewed as best practice for combining control cables in UG conduit in Substations? Lets assume typical control cables consists of CT wiring, PT wiring, 125VDC control, 120VAC power, 48V control.

- Is it typical to combine CT wiring with other 125V DC control wiring?
- Is it typical to combine CT & PT wiring?
- Is it typical to combine 120VAC (or any AC power) with CT wiring or DC control?
- Is it typical to combine different voltage levels for DC voltages (IE 48V and 125VDC)?

I know that the NEC and other codes likely allow for combination of the above (IE 600V class wiring) so was looking for more of a "best practice" and reliability perspective.

Thanks

 

[davidbeach]

From a practical maintenance focused approach those groups are all separate cables regardless of what might be permissible.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[rockman7892]

davidbeach - Thanks for feedback. If I'm correct there is nothing from a NEC/NESC standpoint that prohibits combination of any of these cables as long as insulation is all rated adequately for the largest voltage (typically 600V)?

That said I agree from a maintenance focused standpoint it makes sense. Are there specific reasons in your experience that support the maintenance aspect of that approach? Replacement of failed cable, easy identification of wire types in a conduit or leaving a conduit?

 

[stevenal]

Separate cables in the same conduits. Conduit runs are short, leading to or from trench channel. The trench channel is easily opened for cable replacements or additions. The future substations may just have AC and DC power cables and a bit of fiber.

 

[bacon4life]

IEEE 525-2007 has recommendation for circuit/cable separations I found somewhat confusing. I have not yet seen if IEEE 525-2016 has any clearer guidance.

I have not seen strict restrictions on which control cables can run in the same conduit. If possible, SCADA status cables are often somewhat separated. In retrofit applications, there is often very limited conduit space available.

I have not personally seen cables where CT circuits are mixed with any other type of signal. Our typical new circuit breaker installation uses 10 separate cables, which does get to be kind of pain to manage that many separate cables. During initial construction, a smaller number of larger cables would have made cable identification somewhat easier. I think one of my neighboring utilities uses control cables with more than 15+conductors, so I suspect they might be mixing several kinds of signals in one cable.

For control of a transformer load tap changer, I have seen both the raise/lower control and a single phase PT circuit share a single 9 conductor cable. Whether this qualifies as a best practice, I am not sure.

 

[davidbeach]

For a utility, "best practice" includes a healthy dose of "normal". Yes, standard practices do sometimes need to be revised and updated but that takes time and effort to get everybody up to speed on the new approach. There's a definite advantage of going into the substation knowing a lot about what your going to find just because there's been a reasonably consistent approach over the years. More evolution than revolution.

An industrial owned substation, on the other hand, will probably get by with what ever the design consultant is doing at the time of the design. They have a tendency to be much more one-off than any utility would typically put up with.

The NEC, which would apply to the industrial substation but not to the utility substation, would allow anything to be grouped together so long as the lowest insulation rating in the cable is higher than the highest voltage used. Quick, what voltage do you pick for a CT secondary? Is 600V insulation sufficient on the secondary of a C800 CT? I don't know and I doubt that there are many AHJs that could give a good answer as to why or why not. I've had to explain to the AHJ why a 15kV circuit breaker can use a 600V insulated CT or why a 600V rated window CT can be used around a 15kV insulated cable.

But I'll be glad when most of this can be put to rest in large substations where a breaker gets a couple of DC power circuits and a few fiber pairs and that's it. Probably many years yet before we do our first one that way, but I think the time will come.

I’ll see your silver lining and raise you two black clouds. - Protection Operations