Of all the troubles that fire alarm technicians are likely to encounter, ground-faults are one of the most difficult to locate and fix. When a ground-fault does occur in a fire alarm system, any number of things can take place.
The bottom line is the type of circuit will determine how the fire alarm control panel (FACP) will respond to a ground-fault, not only while the system is in standby, but also when a sensor goes into alarm or supervisory. Most of the time, unfortunately, the panel will simply fail to detect or sound the alarm. Once the ground-fault is cleared, however, everything will usually return to normal.
This month we’re going to discuss troubleshooting fire alarm circuits. We’ll look at some of the techniques used by top technicians and how thinking outside the box can assist you in getting the job done. Sections of code will be provided where needed.
Assuring Integrity and Survivability
Today’s fire alarm systems are designed to be as fail proof as possible. By virtue of applicable codes, redundancy and survivability are built into each and every system. For this reason, alarm panels are designed to monitor the input and output cables that carry alarm and supervisory signals. And when a ground-fault does occur, code requires that everyone knows about it, including the fire alarm company’s central station operators.
According to Section 3.3.175 of NFPA 72, 2010 Edition, pathway survivability is defined as, “The ability of any conductor, optic fiber, radio carrier, or other means for transmitting system information to remain operational during fire conditions.” This requires the FACP to closely monitor each conductor in every cable in order to assure operability.
For example, when a fault occurs on a notification appliance circuit (NAC), it’s possible that some or all NAC devices connected to that circuit will not operate. Conventional initiating device circuits (IDCs) and signaling line circuits (SLCs) are just as vulnerable to the adverse effects of a ground-fault, with the exception of SLCs that are equipped with isolation modules. These devices effectively isolate the bad portion of the circuit from the good, thus minimizing or limiting the extent of the failure as much as possible.
Ground-Fault Definition, Sources
Before we can do a thorough job of installing and servicing fire alarm systems, we need to know what a ground-fault is. According to Sci-Tech Dictionary, a ground-fault is defined as an “accidental grounding of a conductor.”
A ground-fault can come from a piece of metal that has penetrated the insulation of a conductor within a cable. This condition also can occur when a bare wire comes in contact with a metal structure that’s bonded to an earth ground. The latter can come from a displaced wire nut, a lone strand from a stranded conductor, or any number of other causes. Possible choke points include cable connectors, conduits, metal junction boxes, moisture built up inside conduits and metal boxes due to leaky gaskets on boxes, or even intense humidity. This can also be a problem when fire alarm devices must reside in a wet location.
FACPs are designed to quickly detect problems in wiring, such as short circuits, bad or intermittent connections, ground-faults and more. All of this is intended to assure the safety of those who live, work or visit a facility equipped with a code-compliant fire alarm system.
“Unless otherwise permitted or required by 10.17.1.3 through 10.17.1.14, 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 restoration to normal of a single open or a single ground-fault condition in the installation conductors or other signaling channels is automatically indicated within 200 seconds” (Section 10.17.1.2, NFPA 72, 2010 Edition).
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