Integration: Power and fire/life safety systems

11/13/2013


Life safety loads: Fire detection and alarm systems

NFPA 72: National Fire Alarm and Signaling Code, contains requirements for emergency power to fire alarm and detection systems. NFPA 72 specifies the need for two independent power supplies with adequate capacity to serve the connected loads.

The primary source may be either a commercial utility source or an engine generator. The secondary source maybe either a storage battery or an engine generator.

A dedicated branch circuit for the primary power supply must be provided. It is not acceptable for the fire detection/alarm system to share power with any other load. The branch circuit connections must be mechanically protected against physical damage, have suitable overcurrent protection capable of interrupting the maximum short-circuit current they may be subjected to, and be clearly marked as a “FIRE ALARM CIRCUIT.”

Typical operating voltage for fire detection and alarm systems in the United States is 120 Vac supplied to the primary side of the system power supply, which is then rectified and stepped down to 24 Vdc (system operating voltage). The most typical configuration for the primary power is a dedicated branch circuit fed from a commercial utility source.

The most typical configuration of the secondary power supply is battery backup, usually two 12 Vdc batteries connected in series. The battery amp/hour rating (size) is calculated by the system designer based on the maximum load conditions of the system and the time period the system will need to receive the secondary power upon loss of the system primary power. NFPA 72 also requires that the battery calculations include a 20% safety margin added to the calculated amp-hour rating.

The intent of providing secondary power to the fire detection/alarm system is to allow the system to operate in a “standby” or normal mode for a period of time (typically 24 hours) in a scenario where the primary power has been entirely or partially lost, and also to provide sufficient operating power to the system at the end of this supervisory period to perform evacuation functions (alarm mode) for a sufficient period of time (typically 5 minutes) to allow occupants of the protected premise to evacuate safely. 

Therefore, the battery calculation method is as follows:

Required standby time (hours) x total system standby current (amps) = Required standby capacity (amp/hours) + required alarm time (hours) x required alarm current (amps) x 120% (20% safety factor) = adjusted battery capacity (amp/hours)

NFPA 72 also requires that the secondary power supply shall automatically provide power to the protected premises’ fire detection/alarm system within 10 seconds whenever the primary power supply fails to provide the minimum voltage required for proper operation.

Further, any required signals shall not be lost, interrupted, or delayed by more than 10 seconds as a result of the primary power failure.

It is important to understand that there are several types of protective signaling systems that are also addressed in NFPA 72 that may have differing requirements related to secondary power supply capacity. Table 1 summarizes some of these systems and the associated capacity requirements for secondary power.

Life safety loads: Fire pumps

The major electrical power requirements for fire pumps are found in NEC Article 695. The intent of the requirements is to provide uninterrupted power to the fire pump and to protect all power equipment and wiring from fire. Other major requirements for the installation of fire pumps are found in NFPA 20.

The following four items are major electrical power requirements for fire pumps per NEC 695:

  1. Electric motor-driven fire pumps shall have a “reliable” source of power. Typically the “reliable” power source is an electrical utility service connection. The service connection is located to minimize damage from a fire. Many times the power connection is found remotely located from the building or major fire load area. An electrical tap used for the fire pump power connection can be located ahead of the service disconnecting means when installed in accordance with NEC 230. Alternate feeders can also supply power to the fire pump if those feeders are supplied from separate utility service connections.
  2. Circuits that supply electric motor-driven fire pumps shall be supervised from inadvertent disconnection. One can minimize the opportunity for disconnection by directly connecting the supply conductors to the power source of the listed fire pump controller, or listed combination fire pump controller/power transfer switch. The disconnecting means is typically supervised by a central monitoring station so that the operation of the disconnect is reported to a constantly attended location. A local supervisory alarm signal may also be installed to alert local service personnel.
  3. All power supplies shall be located and arranged to protect damage against fire from within the premises and exposing hazards, and multiple power sources shall be arranged so that a fire at one source does not cause an interruption at the other source. Physical location(s) of fire pump power sources in relation to the specific fire hazards at those location(s) must be carefully considered during the electrical power design phase of a project to accommodate the requirements of Article 695.
  4. Power circuits and wiring methods shall comply with the requirements of Article 695. The power conductors feeding the fire pump must be physically protected from the fire hazard, structural failure, or operational accident. This is typically accomplished by physically routing the conductors outside of the building, encasing the conductors/raceways in a minimum of 2 in. of concrete, or installing them in a minimum 2-hour fire-rated assembly.

Life safety loads: Special facilities

Specific requirements for emergency power and life safety loads will vary based on building occupancy type, facility use, and critical function. Various codes such as the International Building Code (IBC), NFPA 5000: Building Construction and Safety Code, NFPA 99: Health Care Facilities Code, and others will have specific requirements. Facilities with special requirements include hospitals, high-rises, large places of assembly, and hospitals.

One interesting example of a special building type is semiconductor manufacturing facilities (H4/H5 occupancies). Continuous gas detection and emergency alarm systems are commonly used in these facilities. In this type of facility, emergency power is provided following NFPA 110 requirements for both continuous gas detection and emergency alarm systems.

Lastly, recent versions of the NEC have added Article 708: Critical Operations Power Systems (COPS). These are systems, operations, or facilities designated by local, state, or federal government as “mission critical”; examples can include police or fire stations or other facilities for reasons of public safety, national security, or business continuity. This new section (introduced in 2008) has some notable requirements for things like commissioning, which has long been practiced in data centers and other previously unclassified mission critical facilities.


Brian Rener is electrical engineering discipline platform leader and quality assurance manager at M+W U.S. He has more than 20 years of experience in management and engineering for new and existing facilities, and is a member of the Consulting-Specifying Engineer editorial advisory board. Josh McConnell is the life safety systems discipline platform leader at M+W U.S. He has 27 years of experience in engineering and construction of life safety systems.


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