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Fire Alarm -- Maintenance

When should standby batteies be replaced in a fire alarm system? Every Year? 2, 3,4, 5 years? Only when dead?
The NFPA Code indicates when to replace the batteries, but the NFPA Code only indicates the worst case scenario.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer






When Should Fire Alarm Batteries Be Replaced?

By Douglas Krantz

The 2007 NFPA72 shows in Table 10.4.4, Item 6(d)(1), that the sealed lead-acid batteries used for battery backup in fire alarm systems need to be replaced within 5 years of manufacture. The NFPA wants the batteries replaced because the battery capacity is down to about 80% by that time.

Battery capacity, the amount of amphours in a battery, changes over time. In the first few months after manufacture, the amphour capacity of the average battery increases a few percent. For several years, this capacity doesn't change much. Near the end of the battery's useful life, the amphour capacity starts to taper off. At 5 years, it's down to about 80% of rated capacity.

If the NFPA requires replacement at 5 years, why do most fire alarm service companies replace the batteries after 3 or 4 years?

The answer is timing.

At about the same time each year, fire alarm systems are inspected. Because the battery's stamped date code is the manufactures secret, the service company has to go by the date of installation, not by the date of manufacture.
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The trouble is, after manufacture, it's usually a month or two before the batteries are installed in the first place. This month or two gets added to the NFPA's 5 years, making the total time more than 5 years from the date of installation.

Rather than figuring out the date code stamped on the battery, and then trying to time the replacement at exactly 5 years, most fire alarm service companies decide to replace the batteries after 3 or 4 years of service. That way, usually, the batteries are replaced before they go bad.







Douglas Krantz

Describing How It Works
writer@douglaskrantz.com
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612/986-4210

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Electrical Flow


On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at www.douglaskrantz.com/
ElecElectricalFlow.html
.