Poor planning and missing, incomplete or incorrect secondary power calculations are among the most common causes for rejection of a submittal to an engineer or to the authority having jurisdiction (AHJ). A previous article addressed the requirements for the features and performance of both primary and secondary power supplies.1 The article showed how to determine the required demand and durations for the secondary power supply. This article shows how to use the demands and durations to calculate the net required capacity for batteries that are used as part or all of a secondary power supply.
Figure 1 shows that batteries will always be a part of a signaling system power supply. The most common configuration is where batteries are incorporated to provide separate, switched secondary power to the system. In that configuration, the batteries are connected in a way that allows the control unit power supply to switch from the primary source to the secondary source when the primary is lost or disconnected.
Figure 1. Power Supply Requirements
The figure shows two arrangements for the use of batteries as a switched secondary power supply. The first is where the batteries supply the entire secondary power supply. The second is where the batteries back up a primary power supply that also includes a backup generator.
In the previous article, it was shown that the code permits a reduced duration for the operation of the batteries where the primary power supply includes a backup generator.2 NFPA 72 does not refer to the UPS option as “secondary power”. Still, the batteries of the UPS provide that function. As noted in the previous article, the batteries on the UPS require the same duration, hence capacity, as those connected directly to the control unit. Operationally, the UPS is a Type 0 (per NFPA 1113) where the batteries always provide power to the system and are recharged by the primary power supply. Thus, there is no switchover that must take place when primary power is lost.
For each of the three battery configurations permitted by NFPA 72, the code has specified the required duration (time, t) for battery operation. The load, or demand is the amount of current (I) supplied by the batteries at a particular time and is a function of the system design and configuration.
Most errors in calculating the required battery (and generator) capacity (stored energy, E) occur in determining the required load. The code specifies two types of loads (demands) and associated durations to be used for determining the required secondary supply capacity. The first is the normal, quiescent load. This is the amount of current that the system demands during its normal, non-alarm state. Depending on the type of system, the code requires that the batteries be capable of providing that amount of current for a specified period (see the first article). The code requires that at the end of the specified quiescent period, the system must be capable of supplying the alarm load for a specified period.
For general alarm systems, the demand current is based on the entire system operating in the alarm mode. This means that all notification appliances and emergency control function interfaces are operating. The demand for emergency voice alarm indication systems (EVACS) and mass notification systems (MNS) will actually vary over the required duration. Therefore, the code permits the capacity to be calculated using the full alarm load, but over a reduced duration in order to simulate the intermittent operation over a longer period. The total required capacity is determined by summing the capacity required to serve the quiescent load and the capacity required to serve the alarm load.
ETotal = ENormal + EAlarm
ET = INtN +IAtA
Where I is electrical current in amperes, t is the time in hours and E is energy in units of amp-hours.
As a minimum, the code requires that the batteries be sized to supply the actual (design) quiescent load and alarm loads for the specified durations. However, how often is the final installed quantity of devices and appliances the same as the original design? While calculations based on a design are a useful starting point, the code requires that the secondary power system be adequate for the final installed load. Therefore, engineers should do one of two things to assure compliance: 1) require recalculation after the final system configuration; or 2) require the capacity to be calculated using the full load capability of the system. If the first option is used, it is only fair that the contractor be compensated for any change orders that add load that must be accommodated for the completed installation.
The second option is the best practice, but is not required by code. For that option, if a circuit is rated for 2.0 amps by the manufacturer, the calculation would assume it is fully loaded even if only 0.75 amps of load is initially being installed. This would ensure that all future changes would not require a change in batteries. The same argument for the second option can be made for determining the required wire size.
To calculate the battery size using the minimum code approach, the quiescent and alarm loads must be tabulated and summed for all equipment, devices, and appliances connected to the power supply. If the second approach is used, the quantities of initiating devices and notification appliances is important only for determining the number of modules and circuits that will be provided to accommodate them. It is usually best to allow the installer and manufacturer to determine the number of modules and circuits because it is sometimes less expensive to install additional circuits than it would be to use fewer, but longer circuits. Also, even if the load will be calculated using the full circuit capacity, it is best for the engineer to specify that the circuits not be loaded more than a certain percentage. This will further ensure that additional devices and appliances can be added without the need to add modules and circuits.
The battery calculations should be done by the installing contractor, system distributor or equipment manufacturer and then checked by the responsible designer and by the authority having jurisdiction (AHJ). The first step is to gather the manufacturers’ published specification sheets for all devices, appliances, and equipment. It is necessary to list all components and the quantities used. For each item, the specifications will list the quiescent (normal, non-alarm) current (load) and the alarm current (load) in amps.
Table 1 is a simplified example for how the capacity of secondary batteries is calculated. For this example, the required duration is 24 hours in quiescent mode and five minutes (0.083 hours) in alarm mode. The calculation will be done for a specific system configuration with a specific quantity of devices and appliances. If the batteries are to be sized for full circuit loading (option 2), the list of initiating devices and notification appliances would be replaced with a listing of the circuits and their full load current capability.
|Standby Current, Amps||Alarm Current, Amps|
|PB Smoke Det.||2||0.0450||0.0900||0.0600||0.1200|
|15 cd strobes||15||0.000||0.0590||0.8850|
|110 cd strobes||4||0.000||0.1450||0.5800|
|Horn/15 cd strobe||10||0.000||0.0830||0.8300|
|Horn/110 cd strobe||5||0.000||0.1930||0.9650|
|Net Standby Load, Amps:||0.9409|
|Net Alarm Load, Amps:||4.6857|
|Enter Required Standby Duration:||24||Hours|
|Enter Required Alarm Duration:||5||Mins||0.083||Hours|
|Total Standby, Amp-Hours:||22.5816||Amp-hours|
|Total Alarm, Amp-Hours:||0.3905||Amp-hours|
|Total Calculated Battery Capacity:||23.0||Amp-hours|
|Required Factor of Safety:||20%|
|Code Required Battery Size/Capacity:||28||Amp-hours|
|Supplied Battery Size/Capacity:||36||Amp-hours|
|Actual Factor of Safety:||57%|
Table 1. Simplified Secondary Power Calculation Example
The required capacity is calculated by multiplying the load by the required duration for both the quiescent condition and the alarm condition. In this example, for the quiescent condition, the total standby (quiescent) load of 0.9409 amps is multiplied by 24 hours to get 22.6 amp-hours of required quiescent capacity. The total alarm load of 4.6857 is multiplied by 0.083 hours to get a required alarm capacity of 0.4 amp-hours. They add together and round to a required capacity of 23 amp-hours. New in the 2010 edition of NFPA 72 is a required 20% factor of safety, bringing the net required capacity to 27.6, or 28 amp-hours after rounding.
Most manufacturers have calculation programs to determine the battery capacity. In reality, most systems will have many more entries for panel components.
There are several entries in the above example worth discussing. The alarm current listed for the power supply is the current that the power supply uses as it supplies the other loads. The option 2 method could be modeled by simply assuming that the power supply is at full load. So, a power supply listed to provide a maximum of 4 amps would list 4 amps as the alarm load regardless of how many modules, circuits, devices, or appliances are actually connected to it.
For smoke detectors and any initiating devices that draw power, how many should be considered to be in alarm? This example has all 52 smoke detectors in alarm, but it is also common to use a number that represents the largest one or two fire areas in the building. The relays are shown to be energized in the non-alarm condition and dropping out (de-energized) upon alarm. Systems might also have relays that are normally not energized until there is an alarm.
The final step is to select a battery that has the required stored capacity and that can discharge at the required rates. In this example, a battery is needed that has a capacity of 28 amp hours or more and that can discharge at a rate of 1 amp (0.9393 rounded) for 24 hours and then be able to discharge at a rate of 4.7 amps for a duration of 5 minutes.
An analogy might help. A gravity water tank has a certain maximum stored capacity. The flow rate from the tank is a function of the outlet and distribution piping and the height of water in the tank. When full, the hydraulic head is actually greater and the system will flow at a higher rate than when the tank is near empty. As the tank empties, the flow rate decreases.
The system (tank or batteries) must be designed to provide the required discharge at all stages of use. Battery manufacturers and suppliers can provide documentation regarding a battery’s ability to discharge at certain rates at the end of the discharge cycle.
Batteries are required by NFPA 72 to be labeled with the date of manufacture. In prior editions of the code, there was a five-year replacement requirement. In the 2013 edition, replacement is required as recommended by the manufacturer or when the batteries fail during testing. The five-year requirement was removed in favor of “replacement as recommended by the manufacturer,” which may be less than five years.
With respect to occupancy hazards and risks, engineers should consider that the secondary supply is only for the fire or signaling system control unit. Any transmitters or sub-panels used for communications will have their own power supplies with the same requirements for secondary power. The public communications infrastructure is outside the jurisdiction of NFPA 72.
NFPA 72 recognizes that the Federal Communications Commission has jurisdiction over the installation requirements for parts of the communication infrastructure used to transmit signals from a protected premises to a supervising station. Traditional telephone central offices and managed facilities voice network (MFVN) facilities used by Internet service providers will typically have 24 hours or more of standby battery capacity in addition to backup generators.
However, modern communications methods, including telephone and Internet service, may not be powered entirely from the central office. Instead, they may have in-building circuits powered from a network interface device at the property that requires primary power and includes a backup battery. Those backup batteries are a part of the communications system, not the fire alarm or signaling system, and are sized for only about eight hours of standby. Both traditional telephone and Internet services provided by an MFVN will usually have field located concentrator units along the path from the protected premises to the central office or MFVN. These local concentrator units, which can frequently be seen on poles or in pedestals throughout a community, also have primary power and batteries for secondary power. So, while a fire alarm or signaling system designed in accordance with NFPA 72 might continue to operate during an extended power outage, its ability to communicate off premises might be limited to eight hours or less. This needs to be factored into emergency planning for the property.
While the actual selection of power supplies and calculations of battery capacity are not difficult, selecting the proper parameters and combinations of power supplies requires engineering consideration. The designer must consider the environmental conditions, hazards involved and the resulting risks when specifying power supply durations for fire alarm and signaling systems.
- “Power Supply Requirements for Fire Alarm and Signaling Systems,” Fire Protection Engineering, 1st Quarter 2012.
- NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2013.
- NFPA 111, Standard on Stored Electrical Energy Emergency and Standby Power Systems, National Fire Protection Association, Quincy, MA, 2010.