[IMAGE]11934[/IMAGE]Reliability and redundancy are important aspects of all fire alarm systems, whether they stand guard in commercial facilities, multiple-family dwellings or single-family residential structures. This is one key reason why battery power is taken so seriously by engineers, installers and the local authority having jurisdiction (AHJ).
It’s important to utilize the right battery capacity in a fire alarm system. This is achieved by conducting mathematical calculations prior to the field work. It’s also important how and where you install your rechargeable batteries, not only from a functionality standpoint but also to comply with code.
NFPA 72, 2010 Edition, published by the National Fire Protection Association (NFPA), provides a lot of good information on battery power as required in code-compliant, UL-Listed fire alarm systems (note that all code references in this month’s column are from the 2010 Edition unless indicated otherwise). Let’s take a look at battery power from a code standpoint as well as how it applies to real-world design, installation and maintenance practices.
Battery Capacity Review
As part of a secondary power supply in a fire alarm system, rechargeable batteries in commercial fire alarm systems are required by code to provide standby operation for a minimum period of 24 hours with a subsequent ring time of five minutes (Section 10.5.6.3.1).
Emergency voice/alarm communication systems fall under Section 10.5.6.3.1: “The secondary power supply for in-building fire emergency voice/alarm communications service shall be capable of operating the system under quiescent load for a minimum of 24 hours and then shall be capable of operating the system during a fire or other emergency condition for a period of 15 minutes at maximum connected load.”
Residential structures with a code-compliant fire alarm system are required to provide standby power for a period of 24 hours with a subsequent ring time of four minutes (Section 29.6.4[3b]).
As you would expect, the failure of either primary (public electric bus) or secondary (rechargeable batteries) must result in a trouble condition at the appropriate locations within 200 seconds (Section 10.12.1, NFPA 72). For additional information on trouble signals, please refer to Section 10.12, NFPA 72.
One of the most fundamental requirements associated with rechargeable batteries is “capacity,” which is measured in amp hours (AH). The AH rating is a required marking on all rechargeable batteries and it’s one that you cannot ignore. As a general rule of thumb, the higher the AH rating is the larger the battery and the larger the cabinet that holds it must be. In addition, the larger the AH rating, the longer the battery will last under load.
The AH rating, as it appears on a battery, relates to the length of time that you can expect normal operations while drawing 1 amp of current. For example, a 7.5AH, 12V (V = volts) battery at full capacity will provide 7.5 hours of operation under a 1A load. If the load happens to be 0.5A, then you can expect 15 hours of standby with a full charge.
Calculating load current and subsequently battery capacity is fairly straightforward. First, ask the manufacturer of the fire alarm panel if it has a calculation program (usually an Excel file) that provides all the calculations required by the local AHJ. Chances are there is one. If so, your work will be relatively easy to complete. Or secondly, if there is no pre-established program available, then you’ll have to do the calculations by hand.In brief, you’ll have to calculate both standby and alarm battery capacities. Adding them together you must add a 20-percent margin of safety. Here are initial formula and calculations to get started:
Basic formula for battery capacity
time (in hours or H) X current (in amps) = capacity (AH)
Calculating total battery capacity
Step 1 — 0.083H (standby time) X current (amps) = standby capacity (in AH)
Step 2 — alarm time (in hours) X current (amps) = alarm capacity (AH)
Step 3 — standby capacity + alarm capacity = raw capacity (in AH)
Step 4 — raw capacity (in AH) X 1.2 (20% headroom) = total capacity
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