Connecting to Optimal Smoke Control
The control of fire safety in a burning building is an extremely important part of the fire detection mission. Last month, we talked about ancillary relays, smoke detectors and door release as a function of smoke control. This month, we will discuss a variety of other fire safety functions to which a fire alarm system must interconnect.
Activating Subsystems
When smoke is detected by a spot-type or duct-type smoke detector in a high-rise building, it is common for the alarm panel to respond by signaling a variety of other subsystems in the building at the same time or a short time afterward.
Some of the more common functions include the activation of audio/visual devices at the system control station and the floor where detection took place. Door holders and other hold-open devices should also be released on the same floor to limit the movement of smoke from one fire compartment to another.
The air handlers on the fire-involved floor should also be shut down and the mechanical smoke removal system activated. This will prevent the transmission of smoke throughout the entire structure and remove smoke from the affected area(s) to preserve the lives of those who may be trapped on those floors.
Stairwells are also a concern when trying to limit the spread of smoke, while making it readily possible for the occupants to escape. For this reason, when detection takes place, the fire alarm control panel must activate any available stairwell pressurization equipment on the affected floor(s). In addition, the stairwell doors must be unlocked so occupants can escape onto a clear floor above or below the fire-involved floor.
In some cases, the system may be required to signal a supervising station or remote central station facility.
Assuring Ancillary Relay Capacity
Almost any fire detection or system device that is used to control the fire safety functions of a building will involve a relay, whether it is incorporated within the device itself or as an add-on ancillary component. For this reason, there are several areas of concern fire technicians must contend with to assure proper operation of the systems they install.
The first concern is optimum performance. To assure this is the case, fire technicians must make sure the relay possesses the current capacity to perform the required function. In addition, the voltage rating of the relay must match the operating device’s output voltage.
For example, one smoke alarm manufacturer specifies the relay they make for this application as not to exceed 120VAC @ 15A or 30VDC @ 15A. With this kind of AC ampacity, you should be able to power a 120VAC light that is fused at 15A and uses a minimum of 14 AWG electrical wire.
Additionally, the low-voltage specification allows use of any bell or siren device that uses up to 30VDC at a current rating not in excess of 15A.
This is a hefty relay, but be aware that not all relays are capable of safely handling 15A of current. For example, another smoke alarm manufacturer specifies its ancillary relay for 120VAC @ 5A or 28VDC @ 5A. In this case, you will most assuredly need to use a second relay when switching to larger loads (see diagram in August issue).
For most applications, a 5A relay will do fine, but it is always wise to double-check the numbers to make sure. For example, if you wish to activate a 150W floodlight, the way to check is:
Power (P) = Volts (E) X Current (I)
150W = 120VAC X I
150W = I
120VAC
1.25A = I
In this case, a 5A relay should do the job fine. If you intend to activate two 150W floodlights, then double the current to 2.5A and you should still be all right.
With all of the above in mind, NFPA 72, Section 11.7.6.4 and 11.7.6.5, 2002 Edition, specifies that an ancillary relay cannot in any way interfere with the proper operation of the fire alarm system itself.
Placing Smoke Detectors
Open-area, spot-type smoke detectors, as well as those used to control a specific door or set of doors at a single location can be used for door release as a function of smoke spread control.
According to Section 5.14.6.1, NFPA 72, 2002 Edition, “Smoke detectors that are part of an open-area protection system covering the room, corridor or enclosed space on each side of the smoke door and that are located and spaced as required by 5.7.3 shall be permitted to accomplish smoke door release service.”
Also, Section 5.14.6.3 says, “When smoke door release is accomplished directly from the smoke detector(s), the detector(s) shall be listed for releasing service.”
Placement at the fire door is specifically addressed in Section 5.14.6.5, entitled “Number of Detectors Required.” For example, if the height of the wall above the door is 24 inches or less from frame to ceiling, only one ceiling-mounted smoke detector on one side of he door is required. However, if the wall height is more than 24 inches, two ceiling-mounted smoke detectors are required, one on each side of the door. In some applications, you may be called upon to install a door frame-mounted smoke detector door holder/closer unit, in which case only one is required on one side of the door. However, Section 5.14.6.5.1.4, NFPA 72, 2002, specifically instructs that this device must be installed as directed by the manufacturer.
If the purpose of the smoke sensing device is to detect smoke on one side of the door only, the detector must be installed on the side where the risk of smoke exists. In addition, where there are multiple doorways between spaces, code may require the use of additional smoke detectors.
Third-Party Listings Defined
Third-party listing, in accordance with NFPA 72, Section 3.2.5, 2002 Edition, involves the testing of fire detection devices to verify operability and compatibility with a variety of fire control systems and fire safety devices.
FM Approvals, for example, conducts rigorous tests on the automatic smoke detectors that it lists. According to FM Global Technologies LLC of Norwood, Mass., smoke detectors are exposed to a temperature of 32° F for a minimum of 16 hours as well as 125° F for another 16 hours.
These devices are also subjected to a vibration test for four hours at a frequency of 10 to 30 cycles per second and 0.02 inches of total displacement. In addition, they are exposed for another 24 hours to an ambient temperature of 100° F at a relative humidity of 90 percent.
FM also cycles one or more proposed smoke detectors through 500 operational cycles where they reset and check for operation each time. Here, FM checks for changes in the detector’s operability, including sensitivity. There must not be any appreciable change in either area for a detector to pass.
FM conducts other tests as well. According to Section 4.2.3 of Approval Standard for Smoke Actuated Detectors for Automatic Fire Alarm Signaling, Class 3230, 3250: “One or more detectors, adjusted to maximum sensitivity, will be energized for normal standby operation in a clean air (working-office type) atmosphere for a period of at least 30 days. There shall be no false signal nor evidence of instability.”
FM also tests duct-type detectors in a variety of ways. One of these tests is designed to assure the detector can handle air velocities of 250 and 1,500 feet per minute without changing its ability to operate as designed.
For more information on FM Approval listings, go to www.fmglobal.com or phone (401) 275-3000.
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