The calculations used to design power supplies for fire alarm and signaling systems are not rocket science and rarely even rise to be considered "engineering." However, knowing the conditions under which a system must perform and specifying the parameters necessary for system power supplies. A future article will address the calculation of a net required energy capacity based on the required demands and durations.

Power requirements for fire alarm and signaling systems are specified in the National Fire Alarm and Signaling Code.3 The code requires a system to have either two sources of power (primary and secondary) or a single Uninterruptible Power Supply (UPS). Where primary and secondary power supplies are used, the secondary supply can consist of batteries or batteries plus a standby generator. Figure 1 summarizes the power supply options and lists the applicable code sections.

Most systems use commercial light and power sources for the primary power supply. A key requirement is that primary power must be supplied by a dedicated branch circuit. The intent is that no other system or equipment can be powered from the same circuit. So, for example, it would be improper to install an electrical receptacle outlet on the circuit for technicians to use when servicing the system.

This requirement does not limit that one dedicated branch circuit to serving only one power supply within a system. Thus, the dedicated branch circuit could supply several power supplies within a control unit or within multiple interconnected control units that serve the signaling system.

The circuit must be properly sized and protected in accordance with NEC4 requirements. The dedicated circuit can be supplied from any properly installed electrical panel board - even sub-panels.

A common misconception is that circuit breakers are required to have locks that permit them to operate properly, but that prevents inadvertently turning off the breaker. NFPA 72 only requires that the breaker be properly labeled and that it be accessible only to authorized personnel. Although neither NFPA 72 nor NFPA 70 defines "accessible only to authorized personnel," NEC 110.26(F) does say that "Electrical equipment rooms or enclosures housing electrical apparatus that are controlled by a lock(s) shall be considered accessible to qualified persons."4

The circuit disconnecting means needs to be accessible for technicians' use and for safety purposes and its location must be listed at the point of connection to the control equipment. However, the intent is that it be protected against accidental operation. Therefore, breakers located in locked panel boards (or located in panel boards that are in locked electrical rooms) meet the requirements of the code as well as those secured with breaker locks.

The UPS option requires that the UPS be a Type 0, Class 24, Level 1 system per NFPA 111.5 Type 0 means that there is no switchover time when power is transferred from the primary power source to the UPS batteries. Essentially, the load is always on batteries that are being charged by the primary supply.

This type of UPS is a valuable tool for critical systems, such as those controlling large suppression systems as it can prevent computer chip malfunctions that sometimes occur when a system switches from primary to secondary power and when the switchover causes a short duration dip in system voltage.

The Class 24 requirement addresses the duration that the supply must operate. A Level 1 system is one where "failure of the equipment to perform could result in loss of human life or serious injuries."

An important consideration for design engineers is protection of the circuit between the UPS and the signaling system. That circuit is responsible for carrying both primary and secondary power and is, therefore, subject to common mode failures. A system with separate primary and secondary supplies has two different circuits for power delivery and is not subject to common mode failures (except at the control unit itself).

The code does not require any special protection for the UPS circuit and does not limit its length, hence exposure. Both should be considered by the design engineer in the context of the risks involved. (Note that this same consideration should be given to the circuit between a transfer switch and the load where a backup generator is used as part of a secondary supply.) Also, although the UPS must be supplied by a dedicated branch circuit, the code does not say that the UPS must only serve the fire alarm or signaling system. However, from risk management and failure mode standpoints, sharing the UPS with other loads should not be done unless care is taken to ensure that it can serve the entire connected load for the required duration or shed load as needed to meet its mission.

Figure 1 makes it evident that there will always be batteries on a fire alarm or signaling system. Batteries must be properly sized to provide adequate capacity and the ability to discharge at a rate demanded by the system load for a specified duration.

It is a failure when a secondary (standby) power system does not have either the stored capacity or the ability to discharge at a rate that will meet a signaling system's demand over a set time period. The energy capacity to be stored depends on the demand (load, discharge or rate of consumption) and on the duration over which that demand must be met.

CAPACITY = DEMAND x DURATION

As shown in Figure 1, there are three different configurations using batteries: 1) batteries only as secondary power; 2) batteries with a backup generator as secondary power; and 3) batteries as integral part of a UPS. NFPA 72 has specific requirements for each.

For a basic fire alarm system that uses primary power with batteries only as secondary power, the battery capacity must be sufficient "to operate the system under quiescent load (system operating in a non alarm condition) for a minimum of 24 hours" and then still be able to operate "all alarm notification appliances" and all other connected loads for a period of five minutes. The code specifies that the net capacity be based on two different demand rates (quiescent and alarm) for two different durations (24 hours and 5 minutes).

Emergency communications systems (ECSs) used for mass notification or for in-building fire emergency voice/alarm communications service have the same 24-hour quiescent load requirement but require 15 minutes of full-load alarm capacity. This is because these systems are usually operated for longer periods during an emergency. They sometimes may be used for 30 - 60 minutes, but only under partial load as announcements are made to certain floors. They might then be called upon to operate under an increased or even full load for some period. The code requirement for 15 minutes of full load should be evaluated by the system designer in conjunction with a risk analysis to determine if a larger capacity should be provided.

Where a backup generator is part of the secondary supply, NFPA 72 addresses the required capacity for the batteries and the generator. The battery capacity must be based on four hours of demand load, but the code does not indicate if the quiescent demand or the alarm demand should be used to calculate the resulting net battery capacity.

The generator must have a Class 24 rating per NFPA 110,6 which means it must be capable of operating for 24 hours at its rated load. Since the generator must be sized to handle the greatest load imposed upon it, this requirement indicates that the generator must have sufficient fuel capacity to operate for 24 hours under full-alarm mode. (Note that if the generator supplies other loads in addition to the signaling system, they must be included in the fuel capacity calculation.) The required fuel capacity for a generator operating at full load will usually be calculated by the generator manufacturer or supplier.

Where the UPS option is used to condition primary power and provide secondary power, the UPS must be Class 24, which means that 24 hours of capacity must be provided; however, the code does not indicate if the quiescent demand or the alarm demand should be used to calculate the net battery capacity for that duration. It would seem logical to follow the requirements for secondary batteries and provide 24 hours of capacity at the quiescent load and five minutes of capacity at the alarm load.

A future article will address calculating the net required energy capacity using the established loads and durations.

 

  1. "Engineering Failure or Failure to Engineer?" Fire Protection Engineering, Winter 2003.
  2. "Professional Qualifications for Designers of Fire Alarm and Signaling Systems," Fire Protection Engineering, 3rd Quarter 2011.
  3. NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2010.
  4. NFPA 70, National Electrical Code, National Fire Protection Association, Quincy, MA, 2011.
  5. NFPA 111, Standard on Stored Electrical Energy Emergency and Standby Power Systems, National Fire Protection Association, Quincy, MA, 2010.
  6. NFPA 110, Standard for Emergency and Standby Power Systems, National Fire Protection Association, Quincy, MA, 2010.