Intermittent Ground Fault on a Class B SLC
A 12-story office building has a Class B addressable signaling line circuit with 48 devices. The panel logs an intermittent ground fault that appears during rain and clears when the SLC is lifted from the panel terminals. Lifting individual devices does not clear the fault. The SLC is in EMT that runs through an underground parking garage, a sprinkler riser room, and a tenant space. A new roof and plumbing work were completed six months ago. The technician measures 0.8 VDC between the SLC negative and building ground with the circuit connected; the voltage disappears when the SLC is disconnected at the panel. Surge protection is installed at the panel. The building remains occupied, and the panel is not indicating a trouble on any specific device.
1. Explain what the readings indicate and list the isolation steps you would take.
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The readings point to a field-side ground fault on the SLC, not a panel fault and not a short. The fault disappears when the SLC is lifted at the panel because that removes the field wiring from the panel's ground-reference circuit. Lifting individual devices does not clear it because the fault is likely in the cable or conduit between devices, or in a device base that remains connected through the daisy chain. A 0.8 VDC offset between SLC negative and ground is consistent with a resistive ground path. Isolation should proceed by splitting the circuit at accessible junction boxes, using a ground-fault locator or clamp if available, and disconnecting sections until the fault moves. Do not megger the SLC with devices connected. Inspect the garage, riser room, and tenant space for water, pinched conductors, damaged jacket, and shared conduit.
2. Given the rain correlation and recent roof and plumbing work, what are the likely causes?
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The rain correlation strongly suggests water intrusion or condensation. Likely points are conduit entries in the underground garage, the sprinkler riser room where humidity and leakage are common, and any new roof or plumbing penetration. Water in EMT can bridge a conductor to the conduit; a damaged jacket at a coupling or box connector can do the same. Check for missing bushings, improper drip loops, and conduit runs that slope toward a box. Also check the surge protector's ground connection and any shared ground with other systems. The recent roof and plumbing work makes a new leak path plausible. If the fault appears only during rain, trace the raceway with the building envelope and plumbing chases in mind.
3. What temporary measures and documentation are required if the fault cannot be corrected immediately?
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A ground fault may not disable a Class B SLC, but it is a trouble condition that must be corrected and documented. If the fault cannot be found immediately, notify the building owner and the AHJ, record the trouble in the system log, and increase monitoring until repair. If the fault is accompanied by a loss of communication or a device trouble, the affected portion must be considered impaired; provide a fire watch or other approved mitigation per NFPA 72 and the Oregon Fire Code. Do not simply reset the panel and leave. After repair, verify that the fault is cleared, test the affected devices, and record the cause, location, and repair on the inspection report. If the building is occupied, coordination with the owner is essential.