Understanding Basic Fire Alarm Systems

The modern fire alarm system is capable of detecting smoke and heat from a small flame, water flow in a sprinkler system or an activated pull station, and reporting this information to on-site personnel via dedicated phone line to any location in the world. Although a seemingly straightforward device from an installation standpoint, fire alarm work can be quite complex, especially when you consider the enormous moral and legal responsibilities involved. There have also been some recent updates to the technology over the last few years worth noting.

Recent advances. The latest major development in the fire alarm system arena has been the introduction of the addressable head. Before these updates, in the event of an alarm, the alphanumeric display at the control panel indicated which zone was affected — something like “Fire Alarm — Zone 6, East Wing Third Floor.” With an addressable head system; however, the exact location is pinpointed. Moreover, the addressable head system has enhanced diagnostic capabilities. This is a great advantage because when a system goes down, time is of the essence in restoring fire protection to the building.

To upgrade to addressable heads, it's not usually necessary to do a complete system replacement. Typically, installers must put in new heads, pull some extra wire, and insert new printed circuit cards into the existing control panel. Each new head possesses an address, which conveys its exact location. You may be asking yourself if this means a spare head has to be kept in inventory for each location. No, each initiating device has on its back a set of DIP switches by means of which you enter a binary number that comprises the address prior to installation. If replacement is necessary, use a small screwdriver to set the DIP switches on the new device.

The option to upgrade with addressable heads or to completely replace a legacy system has to be carefully considered by building owners with the input of in-house electricians and outside consultants. For a large set of buildings, the expense to upgrade can be formidable.

For example, besides addressable and non-addressable heads, there are high- and low-impedance initiating devices, 2- and 4-wire circuitry, and various operating protocols. These are reflected in the different states a control panel can be in as reported by the alphanumeric display. A system may also be power limited, or, less commonly, non-power limited.

In addition to familiarizing yourself with the most recent technology trends as outlined above, it's also important for electrical contractors to realize how sensitive these devices are to certain design, installation, and operational issues — all of which can result in lost revenue, unplanned downtime, and unhappy customers. Here's a good example. Say an expensive commercial building is all but finished; however, the fire alarm doesn't pass inspection, meaning the facility cannot legally be used. As a few rattled electricians work feverishly to get the bugs out of the system, the owners lose thousands of dollars every day. Another potentially problematic scenario might involve slightly creased conductors coming out of a conduit connector at the detector head base. Although this situation would pose no problem in ordinary power or telephone circuits, it could throw one of these systems into false alarm.

Realizing that these types of unforeseen circumstances can throw a wrench into even the best conceived plans, it makes sense for contractors to review fundamental design, installation, and operational considerations for fire alarm systems to keep their skills sharp.

Design considerations. Typically, a fire alarm system is made up of the following components:

• Initiating devices, capable of placing the system in the alarm state. These can be photoelectric smoke and heat detectors, ionization smoke detectors, heat detectors, in-duct smoke detectors, manually operated pull stations and sprinkler waterflow sensors.

• Indicating appliances, whose purpose is to announce builidng occupants or at a remote location when the system enters the alarm state, such as horns, strobe lights, chimes, bells, or combination units. They are also available in weatherproof and hazardous location versions.

• A control panel, containing programming and operating electronics and user interface, is fed by standard branch-circuit wiring and contains replaceable circuit cards — one for each zone. This includes an alphanumeric display, showing the state of the system and providing troubleshooting information, and a touchpad so that onsite personnel can silence an alarm or trouble signal, reset the system following an event, and reprogram if necessary (Photo on page C10).

• Sealed batteries similar to emergency light batteries, but listed for fire alarm systems. These are usually 6V batteries wired in series to make up 24VDC for a power-limited system. The batteries can be contained in the control panel or in a separate enclosure. When AC power fails, the batteries take over with no interruption in fire protection. Of course, there is also a charger.

• Auxiliary devices, including remote annunciators with LEDs showing the state of the system, an alarm silence switch, and visual LED indication of the zone from which a fire alarm is initiated. Electromagnetic door holders (floor- or wall-mounted) are available. In case of alarm, the magnet is de-energized, allowing the door to swing shut. Later, it is reopened manually.

Initiating devices are connected to the control panel by a 2- or 4-wire initiating device circuit. In the case of a power-limited system, 24VDC is applied to two wires going to a string of initiating devices, which are wired in parallel. Neither wire is grounded, and they are isolated from EMT or other raceways, which are grounded through the connector at the control panel. Polarity is also critical. This voltage is used to power the solid-state circuitry within each detector. It's also used by the control panel to monitor the state (alarm or no alarm) of the initiating devices and zone wiring.

A typical fire alarm system has numerous initiating devices divided among separate zones — each connected via an initiating device circuit to a central control panel. The control panel performs supervisory functions over the initiating devices, indicating appliances, all associated field wiring, telephone ties, and its own internal wiring and circuit cards.

Installation tips. During initial setup, all zone wiring, initiating device, and indicating appliance installation should be completed before the telephone tie is hooked up, typically by means of a ribbon connector. This is so that the monitoring agency won't receive false alarms.

The control panel should be located where it can be responded to as necessary either around the clock or during operating hours. This can be at building security headquarters, adjacent to a telephone switchboard or in a maintenance office — whichever location offers maximum coverage. It should also be positioned in a fairly central location because if the system goes into alarm, a person needs to be able to race to the location and verify fire status before the alarm is silenced.

Operational issues. A fire alarm system operates in one of three (or more) states: normal, alarm, and trouble. The state is reported at all times on the alphanumeric display. If the system goes into alarm, the indicating appliances throughout the building go off. These could be very loud horns for some occupancies, or softer chimes in others, such as a nursing home.

The control panel monitors the initiating device circuits at all times for shorts and open wiring by means of the applied DC voltage. The initiating devices are normally open. In the event of a fire they become conductive at close to zero ohms. How, then, is it possible for the control panel to differentiate between a non-alarm state and an open wiring fault? This is accomplished by means of an end-of-line resistor.

A 4.7 kilohm (typically) resistor is placed across the line after the final device. When this resistance is seen by the control panel, normal status is maintained. If the resistance increases, it means that an open has developed, and the panel goes into the trouble state. A buzzer sounds to alert maintenance personnel but the much louder horns throughout the building do not go off. The alphanumeric display will read something like “Open Circuit in Zone Three.” The trouble alert can be silenced by pressing a touchpad location under the trouble alert LED.

The control panel also monitors the functionality of its own wiring and zone cards, and trouble is reported in the display.

A low-level voltage is applied to the indicating appliance circuits when the system is normal. This voltage is not sufficient to set off the horns, but it is monitored as part of the control panel's supervisory function. If current ceases to flow, the trouble alert buzzer sounds, and the display indicates the presence of an open circuit.

Several troubleshooting techniques are appropriate when the system enters the trouble state. Initially, you can unhook a zone in the control panel (after disabling the system) and place an end-of-line resistor across the output terminals. This will simulate a zone in place and the actual field wiring (including devices) can be worked on while the rest of the system is operational. Another approach is to break the zone at the middle of the run and insert an end-of-line resistor. Using the “half-splitting” troubleshooting method, as discussed in “Maintenance Facts” on page 16 of the November issue, you can easily pinpoint a fault — either short or open.

Another capability of the fire alarm system is to call out in case of alarm. Two dedicated phone lines are connected, and the system performs test calls periodically in accordance with programmed instructions. If either phone line won't connect, the system goes into the trouble state, so repairs can be made.

The essence of a fire alarm system, as opposed to individual smoke detectors, even if they are wired to indicate in concert, is that it is supervised from a central location. The whole notion of supervision is critical. It does not mean that a person sits at the console and watches it at all times. What it means is that a supervisory voltage is applied to all circuitry, and current flow is monitored electronically to verify that equipment and wiring are intact.

If the system goes into alarm and won't silence due to touchpad malfunction, for example, it can be disarmed after the zone is checked for fire by cutting off the power. First, unhook one side of the battery array, then unhook the black-white-green incoming power connector. If a fire alarm system is disabled, maintenance and security personnel should initiate fire patrols throughout the building. The telephone monitoring agency should be informed, and the insurance company contacted to verify that coverage is not voided.

Herres is a New Hampshire licensed master electrician in Stewartstown, N.H.

Sidebar: Regulatory Mandates At a Glance

The following regulatory documents apply to the fire alarm system as opposed to individual smoke alarms of the residential type, even when they are AC powered and used for group operation.

NFPA 101 Life Safety Code — Denotes which occupancies are required to have fire alarm systems.

NFPA 72 National Fire Alarm Code — Lays out overall system design parameters, such as location and spacing of heads and pull stations, testing and maintenance procedures, minimum performance requirements and operational protocols.

NFPA 70 National Electric Code — Article 760 covers the equipment and wiring of the fire alarm system, both power to the control console and zone wiring to initiating devices and to annunciators, as well as any phone lines for automatic calling. Also included are other fire alarm functions, such as guard's tour, sprinkler waterflow, sprinkler supervisory equipment, elevator capture and shutdown, door release, smoke doors and damper control, fire doors and fan shutdown — only where these functions are actually controlled by the fire alarm system. Article 725, Class 1, Class 2 and Class 3 Remote Control, Signaling and Power-Limited Circuits, covers wiring emanating from the control panel. Where these circuits are power-limited, alternative requirements take effect for minimum wire sizes, derating factors, overcurrent protection, insulation requirements, and wiring methods and materials.

Underwriters Laboratory or other inspecting agencies — List all components such as control panel, smoke detecting heads, horns, pull stations, and any other equipment.