Ground Fault and an Overloaded NAC in an Occupied Assisted Living Facility
A four-story, 78-bed assisted living facility runs a 12-year-old addressable fire alarm panel. After a weekend of heavy rain, the panel reports a ground fault and trouble on SLC loop 2, plus a NAC 3 open-circuit trouble. Maintenance admits the roof drain above the second-floor corridor ceiling has leaked for months. With the loop isolated from the panel, the technician reads 6.8 kilohms from the loop conductors to ground. On NAC 3, a clamp meter shows 2.05 A against a panel nameplate rating of 2.0 A per circuit. The circuit is 14 AWG with a one-way length of 240 ft; appliances are rated 24 VDC and listed to operate down to 16 V. The administrator wants the system silenced and left in service until the roofers finish in three weeks, and asks for a signed statement that the system is fully functional. The technician has four hours on site, 34 devices on loop 2, and 22 appliances on NAC 3.
1. How would you isolate the ground fault on loop 2, and what would you tell the administrator about leaving the system in service as-is?
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Isolation method
- Disconnect loop 2 from the panel before any resistance testing; never megger a loop with devices attached.
- Note that 6.8 kilohms to ground is a partial, likely wet, ground rather than a dead short, so it will drift with humidity and temperature and can look intermittent.
- Half-split the loop: open it at a mid-point junction box or at a loop isolator, then read each half to ground and keep halving the half that shows the low reading. A 34-device loop should resolve in roughly three or four splits.
- The reported leak above the second-floor corridor is the prime suspect; inspect those junction boxes and splices first.
- Repair or replace wet conductors and splices, dry the boxes, then verify insulation resistance against the manufacturer's minimum (commonly 1 megohm) before reconnecting.
What to tell the administrator I would not sign a statement that the system is fully functional. A ground fault defeats the panel's ability to supervise the loop, and a second fault can take out devices or the whole loop. Instead: document the impairment, notify the facility and the authority having jurisdiction, and put a fire watch or interim coverage in place for the affected area until the repair is verified. Offer to return daily to re-check while the roofers work.
2. Evaluate NAC 3. Is the connected load and voltage drop acceptable, and what would you do about it?
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Load The panel is rated 2.0 A per NAC and the measured draw is 2.05 A. That is over the rating, not at it. Even at exactly 2.0 A there would be no headroom, and good design practice is to keep notification appliance circuits well under the panel rating so that appliance additions, inrush, and aging do not push the circuit past its listing.
Voltage drop 14 AWG is about 3.07 ohms per 1,000 ft, so a 240 ft one-way run is 480 ft of conductor, or about 1.47 ohms. Drop = 2.05 A x 1.47 ohms = about 3.0 V. Starting at 24 V, the far end sees roughly 21 V, and even at a 20.4 V end-of-discharge battery the far end would be about 17.4 V, above the 16 V listed minimum. Voltage drop is therefore acceptable, but only marginally if appliances are added.
Action Find and repair the open circuit first; a bad splice at the same wet corridor location is a strong candidate. Then rebalance the 22 appliances across a spare NAC so each circuit runs with real margin, re-measure current, and update the as-built drawings and the record of completion.
3. What notifications and records does this work require before you leave the site?
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- Impairment notice: the facility administrator and the AHJ are told what is out of service, for how long, and what compensating measures apply. In a health care occupancy this usually means a fire watch or staff patrol of the affected corridor on a documented schedule.
- Tag and log entry: a dated tag at the panel and an entry in the system's inspection/maintenance record stating the trouble found, the tests run, and the parts repaired.
- No false sign-off: the technician signs only what was verified. A statement that the system is fully functional when a ground fault and an over-current NAC remain would be false and would put the technician and the company at risk.
- Follow-up schedule: a written return date, plus instructions to the facility to call if the ground fault returns before then.
- Final close-out: after repair, a retest of the affected loop and NAC, updated as-builts if wiring changed, and a revised record of completion if device counts or circuit assignments changed.