I found this last night and I am a bit confused. This picture is from down inside a 6'X 8' manhole. The three feeders you see are 12.47KV and they have an equipment grounding conductor pulled with each run as one would expect to see. One the other hand the single conductors that run with this duct bank are a bit of a mystery. They run all the way back to the 12KV substation and terminate on the equipment grounding bus. I haven't been in the MV Switches on the other end but I can assume they terminate in the same manor there. What would be the purpose of running these extra grounding conductors in a medium voltage duct bank? I have run and seen run a bare copper conductor in the encasement of similar duct banks but again I am at a lose to the theory behind it. Any one have any insight?
PS: The conductors were not continuous in this man hole. They were just coiled up on the floor. But that is a subject for another thread.
When we install a slab box for a xfrmr, we install 2 ground rods, normally at the corners of the box. It looks like these are the grounds, and although installed in an unorthodox manner, what the heck? The pull ropes are only installed to pull in the "real" pull ropes.
Thanks for the replies. I answered my own question, however. I found some of the original design drawings. The "extra" ground wires go from a lineup of 12KV switchgear, which is the service for the facility, to wall mounted ground bus' in each unit substation room. This is a typical ground bus where all the local bonds and transformer grounds are connected. I suppose the original designers meant to keep all the different buildings at the same potential. I know of two that aren't though because the original contractor never connected them! The small pull lines were left by the original contractor. The 1/4" line we installed in preparation to pull the 12KV feeder for one of the two 12KV subs we are adding here. We pulled it Friday night (all 1140 feet!). but our drawings did not call for the extra ground that the original design called for. Hmm. This brings up a question. Although this conductor is not required in the NEC, what possible benefits or problems could it bring to the facility?
The typically small cross-sectional area of tape or drain-wire shield conductors in MV90/105 cables has a problem doing what grounding is supposed to do--limit voltage rise and permit fault current to rapidly clear overcurrent conditions.