Calculating the Battery Backup for your Fire Alarm System

All fire alarm systems must be fitted with a standby supply and this is achieved by using Sealed Lead Acid (SLA) batteries.

The standards state that in the event of a mains supply failure, the backup batteries should be able to run the system for a minimum of 24hrs, followed by 30minuites in full alarm condition (all sounders running). This applies to category M & L.

If the system is category P then the above applies if the building is continuously manned. If however the building is not continuously manned, then the standby time may need to be 72hrs.

Standby Battery Calculation

In order to calculate the standby battery size required, the following formula can be used;

Battery Size (Standby time in Amp Hours) = 1.25 x [(T_ALM x I_ALM) + (T_SBY x (I_QP + I_QZ))]

Where;

T_ALM = Maximum time in hours required for the alarm [½ hour is most common time]
I_ALM = Total Alarm Current in amps for all alarm devices connected to the alarm circuits
T_SBY = Standby time in hours for the system after mains failure [normally 24, 48 or 72 hr]
I_QP = Quiescent current in amps of control panel in fault condition [because of mains failure]
I_AP = Alarm current in amps of control panel
I_QL = Quiescent current in amps of all loop devices, for instance an Ionisation Detector = 0.00005 Amp (50 μA) , Optical Detector = 0.0001 Amp (100 μA)

Typical Example

A system comprises of a 1 loop Premier Quatro panel, with 80 Fyreye Extra Addressable Optical Smoke Detectors, 15 Zeta Addressable Manual Call Points, 20 Zeta Addressable Securetone 2 Sounders and the required standby is 24 hours. It will need to operate in alarm for ½ hour.

Calculate the battery size required;

T_ALM = 0.5 Hr
I_ALM–_SND = 20 x 0.004 =0.08A
T_SBY = 24 Hr
I_QP = 0.20A
I_AP = 0.19A
I_QL = 80 x 0.00044 + 15 x 0.001 + 20 x 0.0005 = 0.0602A

I_ALM = I_ALM-SND + I_AP + I_QL = 0.08 + 0.19 + 0.0602 = 0.3302

Therefore using the equation;

Battery Size (Standby time in Amp Hours) = 1.25 x [(T_ALM x I_ALM) + (T_SBY x (I_QP + I_QL))]
Battery Size (Standby time in Amp Hours) = 1.25 x [(0.5 x 0.33) + (24 x (0.20 + 0.06))]
Battery Size (Standby time in Amp Hours) = 1.25 x [(0.5 x 0.33) + (24 x 0.26)]
Battery Size (Standby time in Amp Hours) = 1.25 x [0.165 + 6.24]
Battery Size (Standby time in Amp Hours) = 1.25 x 6.405
Battery Size (Standby time in Amp Hours) = 8.01 Amp Hours

For a system like this, 2 x 12V sealed lead acid batteries must be used each with a capacity greater than 8.01 Ah, for example, 12Ah.

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