Intermittent Ground Fault on a Shielded Fire Alarm SLC

The wet-day reading of 8.2 kΩ is below the panel's 10 kΩ threshold, while the dry-day reading of 1.4 MΩ is normal. That pattern points to moisture ingress or a damaged jacket that becomes conductive when wet, not to a hard short or a defective panel. The fault is in the field wiring or a field device, because the measurement is taken with the loop disconnected from the panel.

Safety and test-equipment points:

• Verify zero voltage before resistance testing and use a meter with an appropriate CAT rating for the environment.
• Do not connect a 1,000 V insulation tester to a loop with devices attached. Follow the manufacturer's maximum test voltage, and many manufacturers prohibit insulation testing of connected SLC wiring entirely.
• Keep the system operational as much as possible by using an approved temporary loop or panel programming permitted by the manufacturer, and follow the impairment procedure.

Isolation strategy:

1. Disconnect both loop legs at the panel and measure each leg to ground.
2. Split the loop near its midpoint (about 575 ft) and test each half to ground.
3. The half reading near 8–9 kΩ contains the fault; continue halving.
4. Inspect junction boxes, the gym location, exterior or wet walls, roof penetrations, and any point where the cable crosses metal studs or sharp edges.
5. Check devices for water entry and the shield drain for accidental contact with a grounded enclosure.

Bonding the shield at both ends is not a fix. A shield bonded at two points creates a parallel ground path and a ground loop; circulating current and noise can cause communication faults, and it may create the very ground-fault indication the technician is chasing. The manufacturer's instructions govern shield termination, and the usual requirement is a single-point bond at the panel, with the shield continuous but isolated from ground at field boxes unless the manufacturer says otherwise. The gym bond should be removed unless the installation manual specifically requires it.

The repair sequence is:

1. Localize the wet-section fault with the half-split method.
2. Replace the damaged cable section or device, using listed splices or terminations suitable for the location; do not tape a nick and bury it.
3. Restore the single-point shield termination at the panel and verify the shield is not touching enclosures in the field.
4. With the loop isolated, measure conductor-to-conductor and conductor-to-ground resistance; a normal dry reading should be in the megohm range. Follow the manufacturer's test-voltage limits.
5. Reconnect, clear the panel trouble, and confirm all devices report normally. Check the event history, perform a functional test of affected devices, notify monitoring, and document the cause, repair, and readings.

An intermittent ground fault may or may not impair the loop, but any planned disconnect or repair that takes detection or notification out of service triggers an impairment procedure. Notify the owner, the monitoring company if applicable, and the AHJ as required by the local program; document the time out of service. NFPA 72 impairment provisions require a fire watch or other approved mitigation when a fire alarm system or portion of it is out of service and the building remains occupied.

For a school in session, schedule the work to minimize outage: pre-build and pre-test the replacement section, perform the cutover during a low-occupancy period, and keep the outage as short as possible. Use panel features that allow part of the loop to remain operational if the manufacturer supports them, but do not assume a trouble condition is equivalent to full coverage. If the affected area loses detection, post a fire watch with a documented patrol and means of notification until the system is restored and tested. Record the impairment, the mitigation, and the restoration test results in the building's fire alarm records.