external ground in duct bank 1

[living2learn]

Why would you run an external ground in a concrete encased medium voltage duct bank? I have found some specs that call for it and don't know why? Is it to ensure a direct connection between manholes, mitigate EMF?

Any standards - trying to educate myself and did a few hours worth of resarch with no good references.

 

[wroggent]

If this is in a facility the concrete encased conductor helps establish a low resistance ground grid. A low resistance ground grid is desired in a facility because it (1) helps maintain an equal ground potential throughout the facility, (2) helps keep the voltage on equipment lower during lightning strikes, and (3) establishes a low resistance path for ground faults (to ensure tripping of protective devices). The rebar that may be present in concrete is not sufficient for establishing a ground grid; good electrical connection between the rebar pieces is not guaranteed and rebar is largely iron whereas grounding conductors should be copper or aluminum because of their lower resistivity and they're not magnetic. A project I'm working on now makes use of two #4/0 bare copper wires for each medium voltage duct bank. You would probably find more information about this in IEEE 142 a.k.a. the "Green Book", but I don't have it so I'm not sure.

 

[rcw retired EE]

A ground wire on or in the duct bank provides an alternate path for fault currents instead of just relying on the MV cable shields. Theoretically, the one ground wire can be used as the ground return path for all cables in the bank, instead of pulling an individual ground wire with every cable group. (But we spec those also).

It also ties the vaults together as you said.

But I think the biggest reason is because that's the way the way my mentor did it.

 

[7anoter4]

I am sure it could be also in other standards however this is what I found in "my collection".
From IEEE-665 GENERATING STATION GROUNDING 5.5.3.1 Grounding with common equipment grounding conductor run to the neutral ground. Quote:
"It is recommended that a main ground bus be provided. This may be the switchgear or the motor control center ground bus to which the supply transformer neutral is connected (solidly or through impedance) and that is solidly connected to the grounding grid. It should be noted that in systems with impedance grounding, the grounding conductors should be connected to the neutral ground and not to the transformer neutral. From this main ground bus, a grounding conductor is run within the same raceway as the circuit conductors. A common equipment grounding conductor could be used for all cables in the same tray (it may also be used to ground the cable tray). It is usually a bare stranded copper conductor connected to the main ground bus and fastened to the cable tray.
Where a single cable leaves the cable tray, a branch grounding conductor is added and connected to the common grounding conductor and to the equipment enclosure or motor terminal box. The separate grounding conductor is run in the same conduit or cable duct as the outgoing power cable and is also used to ground the conduit or cable duct.
It is important that the ground conductors be run as close as possible to the circuit conductors, since this reduces the reactance of the fault current return path.
When the cable is run underground, this grounding conductor should be located in the same direct burial or duct bank run as the cable. However, in this case, grounding with individual cable ground conductors (see 5.5.3.2) should be considered to avoid installation and bonding complications."
[5.5.3.2 Grounding with individual (cable) equipment grounding conductors run to the neutral ground]"
However, since control, instrumentation and communication cables run in separate steel conduit which - if it will be well bonding could create, also, a grounding effective path for power cables in the same duct bank.