Fire alarm circuits

The change in resistivity with temperature is used in thermistors, which can be used as thermometers and in fire alarm circuits. 

Have fire-alarm circuit integrity cable in accordance with NFPA 70. 

Fire alarm circuits are wired as either normally open or normally closed. In a normally open circuit the alarm call points are connected in parallel with each other so that when any alarm point is initiated the circuit is completed and the sounder gives a warning of fire. In a normally closed circuit the alarm call points are connected in series to normally closed contacts. When the alarm is initiated, or if a break occurs in the wiring, the alarm is activated. 

A simple normally open fire alarm circuit. 

Fire alarm circuits are Band I circuits and consequently cables forming part of a fire alarm installation must be physically segregated from all Band 11 circuits unless they are insulated for the highest voltage (lEE Regulations 528.1 and 560.7.1). 

If the occupants of a building have impaired hearing it may be appropriate to provide visual alarm systems in the areas occupied by those people. This should include any toilets and welfare facilities. If people with impaired hearing sleep in the building it may be appropriate to provide tactile devices linked to visual alarm devices. These devices may, for example, be placed under pillows and wired into the fire alarm circuits or be triggered by radio signals. 

Fire alarm circuits are wired as either normaiiy open or normally closed. 

The detection and alarm circuits of fire and gas detection systems should be continuously supervised to determine if the system is operable. Normal mechanisms provide for a limited current flow through the circuits for normal operation. During alarm conditions current flow is increased while during failure modes the current level is nonexistence. By measuring levels at a control point the health of the circuit or monitoring devices can be continuously determined. End-of-line-resistors (EOLR) are commonly provided in each circuit to provide supervisory signal levels to the control location. 

Chapter 7 is the chapter dealing with Special Conditions and it addresses most of the cables with highly improved fire performance. Thus, Articles 725 (Class 1, Class 2, and Class 3 Remote-Control, Signaling, and Power-Limited Circuits), 760 (Fire Alarm Systems), and 770 (Optical Fiber Cables and Raceways) all use the same two schemes for fire performance of cables, as shown in Figures 21.4 and 21.5. The figures show that the best is NFPA 262,65 a cable fire test for flame spread and smoke, conducted in a modified Steiner tunnel (86 kW or 294,000 BTU/h), for which the requirements in the NEC are that the maximum peak optical density should not exceed 0.5, the maximum average optical density should not exceed 0.15, and the maximum allowable flame travel distance should not exceed 1.52m (5 ft). The next test, in the order of decreasing severity is UL 1666,64 known... 

Photoelectric fire sensors that detect changes in infrared energy radiated by smoke or by the smoke particles obscuring the photoelectric beam. A relay closes to complete the alarm circuit when smoke interferes with the intensity of the photoelectric beam. 

In a manual fire alarm system, the device used to activate the alarm in most cases is a pull box or pull station. The mechanism is very simple—puUing the switch either makes or breaks an electrical circuit which in turn causes an alarm to sound. As wiU be discussed more fiiUy in the next section, the alarm may sound only in the individual building, or the alarm could initiate a signal at a remote location as well. 

Where an installation comprises a mixture of low-voltage and very low-voltage circuits such as mains lighting and power, fire alarm and telecommunication circuits, they must be separated or segregated to prevent electrical contact (lET Regulation 528.1). 

Band I telephone, radio, bell, call and intruder alarm circuits, emergency circuits for fire alarm and emergency lighting. 

Gas Sensors These detectors sense the presence of certain gases produced by combustion in most fires. Electrical circuits coupled to sensing devices trigger alarms. 

Fusion detectors The simplest application consists of an electrical circuit which contains a switch that is held in either the open or closed condition by a piece of low melting point alloy. When this melts in the heat of a fire, the switch is released and the circuit condition changes, i.e. it is either made or broken Figure 25.7). A variation of this type of detector functions by the melting alloy running into a small cup to complete the alarm circuit. 

The rate-of-rise of temperature detector employs two bimetallic strips, one exposed and one partially insulated. For slow rates of rise, such as increases in ambient temperature, both strips warm up and expand at the same rate. However, if the rate of rise is high, as in the event of a fire, the exposed strip expands faster than the insulated one and trips the alarm circuit (Figure 25.9). 

Ceramizable (ceramifiable) silicone rubber-based composites are fire resistant materials developed especially for cable covers application. In case of fire, electrical installations are endangered of short circuit effect which can deactivate lots of important devices, like fire sprinklers, elevators, fire alarms or lamps indicating route to emergency exits. Ceramizable composites are able to sustain functioning of electric circuit on fire and high temperature up to 120 min by producing ceramic, porous layer protecting copper wire inside a cable. 

An alloy consisting of 24% indium and 76% gallium is liquid at room temperature. Low-melting tin-bismuth alloys contain indium. They are used for safety fuses, in fire-alarm boxes and in sprinkler systems. Also indium alloys are used as solders for printed circuit boards. Indium alloys are easily composed for a specified melting range. Alloys of this type are used to give a signal dear for the Christmas ham or turkey to be taken out from the oven. 

PTC devices are available in a range of specifications to meet defined requirements in low voltage electronic circuits where they may, for example, protect automotive electronic circuits, cellular phones, laptop computers, loud speakers, power transformers, rechargeable battery packs, security and fire alarm systems and other products. 

Auxiliary Systems. This type connects devices in the protected plant with the municipal fire-alarm system. Alarms are received at fire-alarm headquarters on the same equipment and by the same alerting methods as alarms transmitted from municipal street boxes. Signals are recorded at a municipal fire department connecting facilities between the protected property and the fire department are part of the municipal fire-alarm system. Devices in the protected plant are customarily owned and maintained by the property owner. Equipment that connects the devices to the city s circuits is owned and maintained by the municipality, or leased by it, as part of the municipal alarm system and limited to alarm service only. 

Where cables pass through fire barriers, check that the opening is sealed. Ensure that segregation of the telecommunication, fire alarm, emergency lighting and extra-low voltage circuits from the rest of the installation circuits has been effected. 

Any of the above reasons tnay result in noise and an increase in temperature and must be corrected. Critical installations such as a refinery, a petrochemical plant, a chemical plant or a petroleum pipeline may require special precautions and control to avert any excessive heating of the bearings, which may become fire hazards. For these installations, bearing temperature detectors with a relay and alarm facility may also be installed in the control circuit of the switching device to give warning or trip the motor if the temperature of the bearing exceeds the preset safe value. 

When dealing with an entire fire detection system that utilizes more than one type of detector, a Detonator Module greatly expands the flexibility and capability of the system. An individual Detonator Module can accept multiple inputs from UV and IR controllers, other Detonator Modules, manual alarm stations, heat sensors, smoke detectors or any contact closure device. In the event of a fire, any of these devices will cause the internal fire circuitry of the module to activate the detonator circuit, sound alarms, and identify the zone that detected the fire. When properly used, a Detonator Module will add only one millisecond to the total system response time. See Figure 8 for an illustration of a fire detection system with a Detonator Module. 

Alarm systems can be integrated with fire detection systems, intrusion detection systems (IDSs), access control systems, or closed circuit television (CCTV) systems, so that these systems automatically respond when the alarm is triggered. For example, a smoke detector alarm can be set up to automatically notify the fire department when smoke is detected, or an intrusion alarm can automatically trigger cameras to turn on in a remote location so that personnel can monitor that location. 

Remote Alarm System An alarm signaling system with a direct, privately owned circuit that goes to a fire department into privately owned receiving equipment. 

This alarm is given due to short-circuit arcing inside the p-enclosure, or fire, or malfunction of the pressure relief valve)... 

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