Thanks to all for the input. The problem appears to be RFI radiating from the conduit. After locating the radio noise along the conduit run, I used a pair of jumper cables and grounded the conduit to the lighting arrestor earth ground in the tower yard as far from the affected area as I could manage, which was about 15 feet off the ground and 10 feet from the tower and first meter. Not great surface area contact but enough to have an effect. Both meters now read approx the same and though there is still some drift it has been reduced substantially. The original meter which is the furthest down stream from the grounding clamps is now more accurate than the meter in the tower. We are having our grounds checked by our contractor on Fri. There have been no changes to the system, so it will be interesting to see what has happened. Since both drive paths are affected, and since there are two additional drives fed by the same MCC which are not affected, I’m anticipating that the problem will be located between the drives and the MCC.
