8 Steps to Unearth Ground-Fault Locations
Circuit integrity is an important aspect of fire detection. For example, a ground-fault condition in an initiating device circuit (IDC) could potentially cause a fire alarm control panel (FACP) to not see a fire condition detected by a detector on the same circuit. Or, a ground-fault condition on a notification appliance circuit (NAC) could potentially result in no sound when the panel attempts to warn the occupants of a detected fire.
Ground Faults Must Be Reported
Although today’s panels are designed to function despite ground faults, such a condition must be detectable and reported locally within a given period of detection, as outlined in the National Fire Alarm Code, NFPA 72.
All means of interconnecting equipment, devices, and appliances and wiring connections shall be monitored for the integrity of the interconnecting conductors or equivalent path so that the occurrence of a single open or a single ground-fault condition in the installation conductors or other signaling channels and their restoration to normal shall be automatically indicated within 200 seconds (Section 1-5.8.1, NFPA 72, 1999 Edition).
OK, so let us say that a fire alarm panel has done its job by alerting the occupants of a building that a ground-fault condition has been detected on a NAC. Also, the length of the circuit and the means of installation are such that a physical inspection for points of damage is impractical or impossible. Under these conditions, what is the best course of action for the fire technician?
Techniques for Troubleshooting
Bob Ruyle, a long-time fire professional from Lincoln, Neb., once shared with me his secret for successfully troubleshooting ground-fault conditions. According to Ruyle, the following eight steps will aid the technician in finding his/her problem in record time:
1. Disconnect the NAC from the main alarm control panel.
2. Locate and remove a notification appliance device that is approximately halfway between the panel and the end-of-line (EOL) resistor. Use this point to do resistance tests with a volt-ohm meter (VOM).
3. Remove all the other NAC devices on the circuit and install jumper wires at each location so that the circuit continuity is preserved, all the way from the head end panel to the halfway point, as well from the EOL to the same physical location.
4. At the halfway mark, use the VOM to measure resistance between each conductor and a known earth ground. To do this, clip one of the VOM leads to ground while applying the second probe to the other side of the circuit. Thus, when an indication of continuity (resistance) takes place, you know which side of the NAC you must focus your efforts on.
5. Start at the opposite side of the circuit. Begin by removing each jumper one at a time, starting at the farthest part of the circuit. You are very close to the problem when the VOM indicates that continuity between earth ground and the faulted conductor has ceased to occur, or the reading on the meter has significantly changed.
6. In this case, the portion of the NAC to check is the length of cable between the point where you are now and the previous point where you disconnected the last jumper.
7. If the NAC has open wire conductors, a visual inspection will usually reveal the problem. Replacement of the offending length of wire may also be an option if a visual inspection of the conductors is impossible or if they lie within metallic conduit.
8. If the NAC is installed within metallic conduit, the task of finding the exact point of the ground fault may not be cut and dry. One means of detection is to reinstall your VOM and use a screwdriver to bang on the conduit. When a change in reading or tone occurs, you are physically close to the fault point.
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