Fire alarm Detector Placement

 

Spot-type detectors are usually installed on the ceiling, not less than 4 in. (0.1 m) from the wall. If their listing permits, they are also permitted to be installed on the wall, with their highest edge no less than 4 in. (0.1 m) and no more than 12 in. (0.3 m) from  the ceiling (Figure 14.2.16).

Where subject to mechanical damage, detectors must be protected. Any mechanical guard used with a smoke or heat detector must be listed for use with the detector. Otherwise, sensor performance may be degraded. Smoke detectors are often required to be set at higher sensitivity when used with plastic or perforated metal guards due to their effect on smoke entry. One manufacturer has a unique metal guard that looks like the series of smoke detectors it protects and that is engineered not to affect detector sensitivity. 

Smoke detectors should not be installed in an air stream from an HVAC supply grill, because that will inhibit smoke from a fire in the protected space from reaching the detector. (The smoke detector would be bathed in a clean air stream when the HVAC supply fan is running.) This can also affect heat detector performance, usually to a lesser degree. Locations adjacent to a return air grill should also be avoided, because returns can affect air circulation patterns in the room so as to inhibit the detection of smoke from low-energy fires.

FIRE ALARM SYSTEM BASICS

 

The basic components of each system are:

1. A system control unit (Figure 14.1.1) 

2. A primary, or main, power supply
3. A secondary, or standby, power supply
4. One or more initiating device circuits or signaling line circuits to which manual fire alarm boxes, sprinkler waterflow alarm initiating devices, automatic fire detectors, and other fire alarm initiating devices are connected
5. One or more fire alarm notification appliance circuits to which audible and visible fire alarm notification appliances, such as bells, horns, stroboscopic lamps, and speakers, are connected
6. Many systems also have an off-premises connection to a central station, proprietary supervising station, remote supervising station, or public fire service communication center by means of an auxiliary fire alarm system

Primary and Secondary Power Supplies

 The primary power is usually supplied by a connection to utility-generated electric power. The  connection must be from a branch circuit dedicated to the fire alarm system. The circuit and connections must be mechanically protected. The circuit disconnecting means must have a red marking, be accessible only to authorized personnel, and be identified as “Fire Alarm Circuit Control.” Inside the fire alarm system control unit, a permanent legend must identify the location of the electrical panel board that contains the circuit disconnecting means.

Secondary power supply for a fire alarm system is required to automatically supply the energy to the system within 30 seconds whenever the primary power supply is not capable
of providing the minimum voltage required for proper system operation.

The size of the secondary supply usually is measured in the amount of time that the secondary supply will operate the system, followed by a prescribed time period for the system to operate in an alarm condition. Local (protected premises), central station, remote station, proprietary, and auxiliary systems must have 24 hours of standby power, followed by 5 minutes of alarm. Emergency voice/alarm communication systems must have 24 hours of standby power, followed by 2 hours of emergency operation. To allow calculation of the power required for 2-hour emergency operation, NFPA 72 specifies that the 2 hours of emergency operation are the equivalent of 15 minutes of operation under full load (i.e., with all input devices and output appliances operating).

Design of Means of Egress

 

Designing a means of egress involves more than numbers, flow rates, and densities. Safe exit from a building requires a safe path of egress from the fire environment.

The path is arranged for ready use in case of emergency and should be sufficient to permit all occupants to reach a safe place before they are endangered by fire, smoke, or heat.

Proper egress design permits everyone to leave the fire-endangered areas in the shortest possible time with efficient exit use. If a fire is discovered in its incipient stage and the occupants are alerted promptly, effective evacuation may take place.

     Maximum permitted evacuation travel distances are related to the occupant characteristics, occupant alertness, and building fire protection. The less capable people are to move, the less alert they are (such as sleeping), and the less protected a building is (such as no automatic sprinkler protection), the shorter the permissible travel distance.

Depending on the physical environment of the structure, the characteristics of the occupants, and the fire detection and alarm facilities, fire or smoke may prevent the use of one means of egress. Therefore, at least one alternative means of egress remote from the first is essential. Provision of two separate means of egress is a fundamental safeguard, except where a building or room is small and arranged so that a second exit would not provide an appreciable increase in safety.

In some proposed egress designs, all the exits discharge through a single lobby at street level, even though this procedure results in egress travel through a common space. This design philosophy presumes that the lobby may be considered a safe area for all future egress needs during the life of the building. Where two remote means of egress are required, this type of egress design is not permitted by the Life Safety Code or by most model building codes.

SOURCE :  FIRE PROTECTIONHANDBOOK 

How many feed wide required for corridors in new hospital

 

the minimum width corridor required  in hospital ?

As per NFPA 101 The width of an exit access should be at least sufficient for the number of persons it must accommodate. In some occupancies, the width of the access is governed by the character of activity in the occupancy. One example is a new hospital, where patients may be moved in beds or in gurneys. The corridors in the patient areas of the hospital must be 8 ft (2.4 m) wide to allow for a bed to be wheeled out of a room and turned 90º.