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Problem background: Medium Voltage (MV) underground cables have failed the insulation resistance test after heavy rains have flooded the area. Some of the cables have been submerged in the flood water. Even though these cables were fitted with end caps to prevent moisture and water penetrating the cable inner section through the end terminations, the heavy rains and probably the end caps were not tightly placed allowed the water to enter the cables.

Solution to the problem: The site contractor has proposed that nitrogen purging be done to get rid of the moisture and water content inside the cables.

Related video of nitrogen purging process (not the same project mentioned above)

Cable nitrogen purging process: One end of the cable is open while the other end is inserted to a pump where nitrogen is forced into the cable. The standard pressure is 15 psi to pump the nitrogen into the cables. Nitrogen is pumped into the cable continuously while being observed. If no water is seen coming out of the cable for an 8-hour continuous duration, then the cable is assumed to be free of moisture. If at any time, water is seen coming out of the cables, the timer is reset for a new 8-hour duration countdown. The purging process takes 2 to 5 days to complete.

Warranty: One thing to consider when conducting a procedure such as this is the manufacturer warranty. The buyer needs to make sure that the manufacturer would retain the warranty. Most of the time, this is not an issue. Manufacturers would retain the warranty as long as the company and personnel conducting the purging procedure is certified and followed the correct procedure.