This article originally appeared in the July 2012 issue of SD&I magazine
Summer heat waves and the over-use of air conditioners can wreak havoc on an electrical grid. Just reading that sentence probably made you recall the Northeast blackout in August 2003. Fire alarm panels have to be designed in such a way that they will still operate (albeit not forever) in situations where there is no power, or minimal power, available through the panel’s primary power source. Calculating sufficient stand-by battery power and taking into consideration serious voltage drops is a must.
When calculating the voltage drop for your Notification Appliance Circuits (NAC), you should be planning for the worse-case scenario. NFPA 72 states the minimum conditions under which the system must still function properly. This originates at rule 10.14.1 “Voltage, Temperature, and Humidity Variation. Equipment shall be designed so that it is capable of performing its intended functions under the following conditions: (1)*At 85 percent and at 110 percent of the nameplate primary (main,) and secondary (standby) input voltage(s).”
Consider the following two scenarios:
In the first scenario, a 24VDC fire alarm panel has lost 15 percent of its primary AC input voltage during a brown-out. Anytime less than 85 percent of the normal voltage is available, the panel is permitted to shut down in order to protect itself from permanent damage, as described in 10.4.1(1) above. However, the batteries are there to help supply the additional power needed in order to keep the system functioning.
In scenario two, a black-out has caused the panel to rely solely on its battery power. As time passes, the batteries begin to deplete. This depletion rate will assuredly be faster for older, weaker batteries. At some point, the secondary power will be depleted to the point that the panel is hovering right at its 85 percent cutoff point, which means the supply voltage is now 20.4 volts (85 percent of 24 volts).
At this point, the panel is still fully functional and providing the available voltage to the NACs, and may continue to do so until the secondary input voltage falls below 20.4 volts. The fire alarm system panel is still functional but is operating on 85 percent of its 24-volt battery backup voltage, or 20.4 volts.
Voltage drop calculations
When performing your voltage drop calculations, you must ensure that the system, especially the notification appliances, have sufficient operating voltage, even during times where the worst conditions exist. This means we are to use an initial voltage of 20.4 volts when performing our calculations, not 24. Although NFPA 72 and UL standards do not prevent manufacturer’s from designing their equipment to “transfer to secondary supply at less than 85 percent of nominal primary voltage,” you should prepare your calculations to compensate for the lowest primary voltage.
All 24-volt notification appliances will function properly all the way down to16 volts.
This is because their UL 1971 listing requires that the alarm notification appliances function properly (i.e. loudness, brightness) throughout their listed voltage range of 16 to 33 volts. However, not every device from every manufacturer has the highest current draw at 24 volts. Because of this, UL will list the maximum current draw the device drew when UL tested it throughout the required 16 to 33 volt range. This worst-case current draw is reflected in the UL Maximum Current Draw tables provided by each manufacturer.
The standby battery time is based on the worst-case load of the panel, which includes the same of the current draw of the notification appliances. At the end of your calculations, you simply calculate the amp hours needed for this black-out event, and add an extra 20 percent capacity to allow for battery degradation due to aging. The inclusion of this 20 percent safety factor is now mandated by NFPA 72, 2010, in order to build-in another layer of reliability. (The requirement for supplying backup power at all is the first layer.) Should the batteries fail to recharge within 48 hours after providing back-up power to operate the fire alarm equipment for 24-hours, followed by the system’s minimum alarm functionality (five minutes for general evacuation or 15 minutes for EVAC systems providing partial evacuation), they must be replaced.