Stray Current on a Rooftop Conduit After a Second Neutral Bond
A 1,200 A, 480Y/277 V service has its main bonding jumper installed in the service disconnect, and the neutral is bonded to the enclosure there. A 225 A panelboard about 300 ft away feeds line-to-neutral loads; its neutral bar is mounted on insulated standoffs. After an electrician 'stabilized' several receptacle ground readings by driving a green bonding screw through the neutral bar into the panel enclosure, the building water-pipe bonding conductor reads 9 A, the 4-inch EMT between the service and panel is warm near a coupling, and a clamp meter shows 6 A on the feeder equipment grounding conductor. The electrician argues that the bond cannot be the cause because the panel neutral bar was already isolated and the feeder has a copper EGC. Production cannot shut down for a full outage, but the area can be de-energized for a weekend.
1. Explain the current path and why the electrician's change produced these symptoms. Is this objectionable current?
Read the worked answer
The green screw creates a second neutral-to-ground connection at the 225 A panel. Neutral return current from line-to-neutral loads now has two paths back to the service: the intended neutral conductor and the equipment grounding conductor, EMT, and enclosure. Those paths are in parallel, so part of the neutral current flows on grounding and bonding metal. The 6 A on the EGC and the 9 A on the water-pipe bond are diverted neutral current. This is objectionable current. The panel neutral bar was isolated before the screw was added; the screw defeated that isolation. The symptoms fit: the water-pipe bond carries diverted current, the EMT warms at a coupling because current is flowing through a connection that was not intended to carry load current, and the EGC current confirms a parallel return path. Objectionable current can cause shock, overheating, arcing at joints, erratic electronics, and corrosion. The electrician's 'fix' masked a real EGC or receptacle-grounding problem rather than correcting it.
2. What is the correct bonding arrangement at the service and at the 225 A panel? What corrective actions would you take during the weekend outage?
Read the worked answer
At the service, the main bonding jumper is correct and must remain the only neutral-to-ground connection on the premises wiring system. At the 225 A panel, remove the green screw and restore the neutral bar to an isolated state. The panel enclosure and EGC bus must be bonded together, but the neutral must not be bonded to the enclosure. During the weekend outage, inspect every downstream panel and disconnect for a second bonding screw, a neutral lug contacting the enclosure, or a neutral-ground connection at a receptacle or plug-connected appliance. Confirm continuity and low resistance of the feeder EGC and branch-circuit EGCs. The unstable receptacle ground readings that prompted the screw should be corrected by repairing the EGC path, not by creating a neutral bond. Check terminations for looseness and test the EGC if readings are intermittent. After the neutral bond is removed, re-energize and measure current on the neutral, EGC, conduit, and water-pipe bond under balanced and unbalanced load.
3. What test sequence would confirm the fix, and what would you inspect if current remains on the grounding metal?
Read the worked answer
Use a clamp meter to measure neutral, EGC, conduit, and water-pipe bond current at several load levels. With a correctly isolated neutral, EGC and conduit current should be essentially zero except for leakage or capacitive charging; a significant 60 Hz current that tracks neutral load indicates another neutral-ground connection. Open branch breakers one at a time and watch which circuit changes the EGC current. Isolate the feeder and test the EGC to ground to detect a second path. Inspect receptacles for ground-to-neutral jumpers, check plug-connected equipment for internal bonding, and verify that any separately derived system has its bonding jumper only at the source. If current remains, divide the system at disconnects and test each section. Do not rely on the water-pipe reading alone; metallic piping and building steel can provide parallel paths. Document readings before and after correction.