Got sent out for a no cool. System may or may not have been heating for first four months as it is in an auditourium with two other rtu feeding the space. Was told that terminal board in new carrier rooftop was bad and changed out board to keep customer happy as control company already told them that was the problem. All was good with the change out till duct smoke interlock was tied in. Original problem of chattering contacters returned with the addition of duct smoke shutdown wires to board no issues when jumper accross terminals. Found 16v present on wires to ground. Traced back could find no shorts. Removed from fire system voltage remained. Cut wiring at a 20' interval from closest j box and with both ends hanging in mid air still had voltage. Ran new shielded conductors measured to ground still had 6v. Inspected wiring pulled out strupped off outer casing no obvious cause found. Ground shield wire of new pull to j box voltage reading disappears but contactors still chatter. Added an isolation relay to the loop system run and duct smoke shutdown passes test.what gives.