Intermittent Ground Fault on a Loop With Recent Roof Work
On a five-story office building, the addressable panel has logged an intermittent ground fault on SLC loop 4 for three weeks. Loop 4 is Class B with 196 devices. The fault appears in the event log mostly between 1 and 3 p.m. and then clears. With the loop disconnected at the panel, a technician measures 0.9 megohm from the loop conductors to ground; reconnected, the reading drifts between 1 and 3 megohm. Last month a roofing contractor replaced membrane near a conduit penetration above the fourth floor. The building engineer wants ground detection disabled until the roof warranty work is finished, and the electrician recommends swapping the loop card because the card is probably bad. The panel is otherwise normal, no devices are in trouble, and the system is monitored by a central station. The building is fully sprinklered and no fire watch has been posted.
1. Evaluate the two proposals: disabling ground-fault detection and swapping the loop card.
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Both proposals treat the symptom, not the cause. A ground fault is real leakage from a conductor to ground, and the 0.9 megohm reading taken with the loop disconnected at the panel proves the leakage is in the field wiring - the card is faithfully reporting a genuine condition. Ground detection is a required supervisory function; disabling it removes the only early warning and can mask a second fault. On a Class B SLC one ground may not disable the loop, but a second ground on another conductor can short the circuit, cause devices to malfunction, or take the loop down entirely, and a ground on the grounded-conductor side can defeat supervision. Swapping the card would cost time and would not change the 0.9 megohm reading on the field wiring. The correct first step is to locate and clear the leakage, then verify the loop with ground detection enabled.
2. Describe how you would isolate the fault, including what the 1–3 p.m. pattern and the roof work suggest you check first.
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Start at the roof penetration above the fourth floor: recent membrane work, a conduit that may have been disturbed, and afternoon heat plus rooftop unit operation all fit a conductor rubbed through at the penetration or water/condensate in the conduit that expands the fault path in the afternoon. Look for a missing bushing, a pinched jacket, water in a junction box, or a loose fitting touching a conductor. Then isolate systematically: disconnect the loop at the panel, split it at a mid-loop junction box, and measure resistance to ground on each half with an insulation tester or ohmmeter. The half showing the low reading contains the fault; keep halving until you reach the device or splice. Confirm the conduit is not being used as a grounding conductor. Photograph and document each step, and do not leave any portion of the loop bypassed while you work.
3. What are your obligations regarding the monitored system and building occupants while the fault is being located?
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Because the system is monitored and the building is occupied, the impairment must be managed, not just worked on. Notify the owner's representative and the central station before taking the loop out of service, and record the time, reason, and expected duration. Restore and verify the loop as soon as the fault is cleared. If the loop must be out of service long enough that the building loses detection, follow the owner's impairment procedure - typically a fire watch with notification to the AHJ, consistent with NFPA 101 and NFPA 72 impairment practices, and no hot work during the outage. Ground-fault detection stays enabled throughout. Document the event, the tests performed, and the restoration in the system log, and leave a clean loop history when you finish.