Neutral-to-Ground Bond in a Detached Shop Panel
During commissioning of a detached maintenance shop, a 45 kVA, 600–208Y/120 V dry-type transformer feeds a 200 A panelboard 25 m away through a 4-wire feeder. The upstream 600 V service is solidly grounded. At the panel, the neutral and equipment grounding conductors are landed on the same bus, and the bonding screw is installed. A clamp meter reads 2.4 A on the grounding electrode conductor between transformer X0 and the building steel; a voltmeter reads 4.9 V between the neutral bus and the panel enclosure. The shop has bonded metallic water piping and structural steel. Occupants report intermittent tingling at a bench grinder and flickering lights when the air compressor starts. The transformer nameplate shows 4.5% impedance; the feeder is 4/0 AWG copper in PVC conduit. No ground-fault protection is installed on the feeder. The contractor proposes to remove the bonding screw and drive a new ground rod at the panel.
1. What is the most likely cause of the 2.4 A current and the 4.9 V neutral-to-enclosure voltage? Is this an immediate hazard?
Read the worked answer
The most likely cause is a second neutral-to-ground bond at the panelboard. With the neutral and equipment grounding conductors on the same bus, neutral return current from the 208Y/120 V loads has two paths back to transformer X0: the intended neutral and the equipment grounding conductor, grounding electrode conductor, building steel, and water piping. The 2.4 A clamp reading is diverted neutral current using that parallel ground path. The 4.9 V is the voltage drop produced by that current flowing through the grounding path.
This is objectionable current, not a normal reading. It is a shock hazard because exposed metal can rise above earth potential, and it explains the tingling at the bench grinder. It can also cause erratic equipment operation. The compressor start increases neutral current and makes the potential difference worse. The condition should be treated as an immediate hazard until the parallel path is removed and the panel is correctly separated.
2. Evaluate the proposal to remove the bonding screw and add a ground rod. What corrective actions and verification tests do you specify?
Read the worked answer
Removing the bonding screw is part of the fix, but only if it is the downstream bond. The panel must have separate neutral and equipment grounding buses, with the neutral isolated from the enclosure. The neutral-ground bond must remain at transformer X0, and the transformer enclosure and secondary grounded conductor must be bonded there. Driving a new ground rod at the panel is not a substitute and can create another parallel path.
Corrective steps: de-energize and lock out; separate neutrals and grounds; remove the downstream bonding screw or jumper; inspect receptacles, equipment, and junction boxes for other neutral-ground connections; and verify the grounding electrode conductor and bonding jumper are intact and correctly sized. Re-energize and clamp the grounding electrode conductor and bonding jumpers under load; current should be near zero. Measure neutral-to-enclosure voltage under load; it should be near zero. Perform continuity and polarity checks at receptacles, verify GFCI operation, and retest the grinder circuit. Document the findings and the retest results.