Understanding Secondary Power Supplies
Secondary power supplies are often misunderstood by fire technicians and security installers. There are several back-up power configurations to choose from, and knowing the appropriate one to use is essential to providing an effective, code-compliant fire alarm system.
In the fire alarm business, there are two basic sources of power for the systems we install. They are most often referred to as the primary and secondary power supplies.
Section 907.5 of the International Fire Code (IFC), 2003, published by the International Code Council (ICC), defers the issue of power to NFPA 72. “The primary and secondary power supply for the fire alarm system shall be provided in accordance with NFPA 72.”
According to Section 22.214.171.124.1 of NFPA 72, 2002 Edition, “At least two independent and reliable power supplies shall be provided, one primary and one secondary, each of which shall be of adequate capacity for the application.”
A typical primary power supply is usually nothing more than the public electric bus, or house power, that we use each and every day. Secondary power supplies, however, will vary in size, format and scope.
This month, we will discuss these choices. We’ll also talk about the fire technician’s responsibility concerning electric generators; the secondary power requirements under Section 126.96.36.199 of NFPA 72; and the use, care and physical handling of rechargeable batteries.
Secondary Power Requirements No matter what the secondary power happens to be, there are a number of criteria that must be met before a fire alarm system is considered code compliant. First and foremost, the secondary power source must be able to restore the fire alarm system within 30 seconds after primary power fails, as outlined in Section 1-5.2.6 of the 1999 Edition of NFPA 72, National Fire Alarm Code, published by the National Fire Protection Association (NFPA).
According to the 1999 edition, this changeover must occur without any loss of signal when primary voltage is less than what is required for proper operation.
The 2002 code approaches this issue from a performance-based perspective. In Section 188.8.131.52 of the 2002 edition, secondary power can come from one of three sources. Changeover as specified by the 2002 code must be accomplished in such a manner that signal loss cannot be more than 10 seconds.
The first type of secondary power, as specified by 184.108.40.206.1(1), is that of ordinary rechargeable batteries that meet with the conditions set forth in 220.127.116.11 of the same edition (see sidebar).
Although rechargeable batteries are becoming the preferred method of providing back-up power to fire alarm panels and the remote devices that connect to them, there are two other secondary power choices fire technicians might encounter. According to Section 18.104.22.168.1(2) of NFPA 72, 2002, this includes: 1) an engine-driven generator set (genset) with a 4-hour battery secondary supply for the fire alarm control panel, and 2) a multiple engine-driven generator system where one of the gensets is equipped to start automatically.
Where there’s a lone internal combustion generator providing secondary power, code requires the use of batteries for the fire alarm control panel with at least 4 hours of standby. The purpose of the 4-hour standby is to maintain the operability of the fire alarm panel while the genset starts.
The 4-hour battery capacity allows enough time for the end user to have the genset repaired/restored to working order without disrupting the system.
Generator Power Quality
Fire technicians are often put in the position where the continued operation of the fire alarm panels they install hinges on the use of an electric generator during a power outage. One potential problem technicians face in this regard is power quality.
“Power quality can be a real problem with older emergency generators that fail to meet equipment standards of today,” says Nick Markowitz of Markowitz Electric Protection in Verona, Pa. “Some of these older generators fail to provide a clean output. Hooking a modern fire alarm panel up to such a generator can cause damage to the motherboard.”
The problem is some of these older gensets fail to output a clean, pure AC sine wave.
The first consideration is proper approvals and listings. According to Article 700.3, NFPA 70, 2002, “All equipment shall be approved for use on emergency systems.”
There are also instruments available to measure the quality of a genset’s output. These devices will effectively analyze the physical characteristics of the electricity delivered to the fire alarm panel. Some models allow for documentation using a printer or an RS-232 connection to a PC with the proper corresponding software loaded thereon.
Another issue fire technicians need to be aware of is the genset’s fuel source. According to Article 701.11(B)(2), NFPA 70, 2002 Edition, “Where internal combustion engines are used as the prime mover, an on-site fuel supply shall be provided with an on-premise fuel supply sufficient for not less than 2 hours’ full-demand operation of the system.”
In addition, where the genset engine relies on a public source of natural gas, Article 701-11(B)(3) calls for a dual-fuel arrangement. “Prime movers shall not be solely dependent on a public utility gas system for their fuel supply or municipal water supply for their cooling systems. Means shall be provided for automatically transferring one fuel supply to another where dual-fuel supplies are used.” The listed exception to this is where the authority having jurisdiction (AHJ) believes there’s “a low probability of a simultaneous failure of both the off-site fuel delivery system and power from the outside electrical utility company.”
Secondary Power Supply Capacity
The required capacity of the secondary power supply must meet code in order for the AHJ to accept and sign off on the job. There are four basic fire alarm systems as defined by NFPA 72 with which fire technicians commonly work. Which of the four the fire technician happens to be working on determines the capacity of the secondary power supply. There are others per NFPA 72, but we will confine our discussion to these: 1) Protected Premise (Local), 2) Auxiliary, Remote Station, 3) Proprietary, and 4) Emergency Voice/Alarm Communication.
When working with a protected premises (local) fire alarm system, secondary power supply capacity must be at least 24 hours during standby (maximum standby load) with a subsequent ring time (full load in alarm) of 5 minutes (Section 22.214.171.124.3.1, NFPA 72, 2002 Edition).
In auxiliary and remote station systems, standby capacity is expected to be 60 hours or greater with a subsequent ring time of 5 minutes. Emergency voice/alarm communication system batteries are required to have a capacity sufficient to sustain operations for 24 hours with a subsequent period of 2 hours when operated in emergency mode per Section 126.96.36.199.3.1(A), or 15 minutes when under full load during evacuation procedures.
Emergency mode entails the use of the audio portion, while full evacuation entails the operation of all notification appliances, perhaps in conjunction with periodic audio announcements.
For the complete version of t his story, see the August issue of Security Sales & Integration magazine.
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