Underground 12.5 kV Cables

[ken44]

Asking for your input according to your knowledge and understanding of 12.5 kV cables located in underground raceways/manholes. Will cables sitting below ground water for an extended period of time cause premature failure? Is it safe to try and pump out these manholes while the cables are powered up?

 

[xformer]

I would say that under ground medium voltage cables should be rated for use in wet locations.... but I would double check with an appropriate rated voltage tester just to make sure..

 

[don_resqcapt19]

The plant that I often work in has an underground 4.16kV distribution system. The number of cable failures was drastically reduced after we installed sump pumps in all of the manholes. Yes, the conductors were suitable for wet locations, but it seems they last longer if they are not always underwater.

I would think that it would be safe to pump the manholes out while the conductors are energized as long as you do not enter the manhole to do the pumping. I would expect that you could remain outside the arc flash radius and drop the suction hose in to the manhole. I would want to do some type of check to make sure the water was not energized before I did that, but I think if there was enough leakage current to energize the water, you would have already had a cable failure.

 

[ken44]

Thank you for your help. We are making it a priority to replace all the 12.5 kV cables throughout the campus, but the sump pumps are being treated as more of a secondary issue, although it seems very important to me, for the cables (particulary the connections) to stay as dry as possible.

 

[don_resqcapt19]

The plant I spoke of is approaching 50 years old and there are still some of the original cables in use and some cables that have been replaced more than once. It had been about one cable a year before we put the pumps in and after we put them in, there was only two cables over a 15 year period. I can't say for sure that it was the pumps, but that is the only thing that was changed. Most of the cables that had failed were between manholes where the normal water level was well above the elevation of the duct banks and when we would pump the manholes out the ducts would run full stream for many minutes.