Zone 3 Tests Normal at the Panel While Two Devices Are Dead
A wing with a conventional detection circuit uses 14 devices on Zone 3: 10 manual pull stations and 4 heat detectors. The zone is supervised by an addressable monitor module in the main panel across the building. During the annual test, 13 of the 14 devices operate and annunciate correctly. Pull station P-10, the last device on the run, produces nothing at the panel, and heat detector H-4, one device upstream of P-10, also produces nothing. The panel shows Zone 3 normal with no trouble both before and after the test, and the technician concludes that the wiring is sound because supervision would have caught a break. You measure 3.8 mA on the zone at the panel, unchanged when P-10 is actuated, while testing H-3 upstream steps the current to 22 mA. The building remains occupied throughout.
1. Diagnose the most likely cause of the two dead devices and explain why the panel stays normal.
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The measurements point to an open circuit between H-3 and H-4 combined with an end-of-line resistor installed at the panel terminals rather than at the end of the field run.
With the resistor at the panel, the supervisory circuit is a loop through the panel terminals only for the portion of wiring that actually reaches the resistor. Any break beyond the last working device leaves the panel seeing the correct resistance and reporting normal, while everything downstream of the break is unsupervised and cannot signal. That explains the whole picture: 13 devices operate, H-4 and P-10 are dead, and the zone reads normal both before and during the test. The steady 3.8 mA that never changes when P-10 is actuated confirms that P-10 is electrically invisible to the panel. When H-3 is actuated, the current rises to 22 mA, showing the panel's detection method is working and that H-3 is on the live side of the break.
The technician's assumption that supervision would catch a break is correct only when the resistor sits at the end of the field circuit. Correct placement is the first thing to verify on any conventional zone that reads normal with dead devices.
2. What must be corrected, and how do you verify the correction?
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Move the end-of-line resistor to the last device on the circuit, downstream of P-10, so the entire run is supervised. Then find and repair the actual break between H-3 and H-4, which may be a loose terminal, a failed splice, a damaged conductor, or a device that was removed and not reconnected.
Verification has three parts. First, with the resistor correctly placed and the break repaired, confirm the zone reads normal and the standby current matches the expected value for the device count. Second, retest H-4 and P-10 individually and confirm annunciation at the panel. Third, prove supervision: open one conductor in the field circuit at the far end and confirm the panel reports a trouble for Zone 3. That last step is the one that demonstrates the original defect is gone; retesting only the two devices would show they work but would not prove the circuit is supervised.
Record the original and corrected resistor locations, the repaired break location, and the supervision test result in the report.
3. Two devices were out of service in an occupied building. What is the correct handling of the impairment and the documentation?
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Do not sign the zone as passing. The two devices were out of service before the inspection and the panel concealed the condition, which is more serious than a single failed device: the owner has been relying on a supervision scheme that did not supervise the last portion of the circuit.
Determine what the two devices protect. P-10 at the end of the run may be a required exit pull station, and H-4 may cover an area with no other detection. Notify the owner in writing the same day, describe the impairment in plain terms, and state the affected area. If the jurisdiction requires notification of impairment to the authority having jurisdiction, do it and give the expected duration. If the affected area loses detection that the approved design relies on, discuss interim measures with the owner; if the system as a whole must come offline for repair, an impairment program with notification and, where required, fire watch applies.
In the report, list both devices by identifier and location, describe the masking condition with the measured currents, and state that the correction must be verified, including the supervision test, before the zone is recorded as compliant.