Why does Current Go Up as Voltage Goes Down in a Fire Alarm Strobe?
In a Fire Alarm System, a Strobe gets people's attention. In case of fire, it has to work even the fire alarm panel is on batteries during a power outage. When on batteries, the voltage of the panel decreases.
By Douglas Krantz
In a fire alarm system
, strobes are notification appliances; the flashing strobes get people's attention. They're also constant power devices.
Constant Power Used by the Strobes
The strobe flashes at a constant rate, and the brightness of the flashes doesn't get dimmer as the strobe's input voltage decreases. In spite of differences in voltage on its input terminals, when flashing, a strobe is always putting out the same amount of light power.
To keep a constant light output, the input power has to stay the same.
I hate to use formulas in the explanations, but Power = Voltage X Current
. The gist of the formula, though, is if power is going to remain constant, when voltage goes down, current, to compensate, has to go up.
Voltage varies quite a bit in a fire alarm system. This is important. The question is "why?"
24 Volts Nominal is not 24 Volts Absolute
Commonly, many people think the power supply of a fire alarm panel
is 24 volts. It is, sort of.
Whereas the literature about a fire alarm panel shows it's using 24 volts, in most cases, the 24 volts shown in the specifications is a nominal, or approximate, voltage: anywhere between 20 volts and 28 volts.
Varying Power Supply Voltage
While AC power is applied to the panel, the panel is charging the backup batteries
. As the batteries are charging, the panel power supply voltage is somewhere between 26 and 28 volts.
When the AC power is lost, the batteries take over. As they're used, the batteries will slowly lose voltage. From the original 26 to 28 volts, the battery voltage drops down to about 20 volts. Without AC power, this is the voltage of the panel. This is also about all the life one can expect from the batteries.
Even with these variations in power supply voltage, the strobes still require enough power to produce a constant flash rate.
Voltage is Lost in the Field Wiring
Added to the variation in panel power supply voltage, voltage is lost in the wire to the strobe; as current travels in the wire from the panel to the strobe, some voltage is lost to heat.
This means, the voltage at the strobe is always going to be less than the voltage at the panel.
But even during an extended AC power blackout, as the batteries lose their voltage, and with voltage loss in the wiring, the voltage at the strobe still should never go below 16 volts.
Current Rating and Life Safety
Common input voltage specifications on strobes range from 16 volts to 33 volts. The current use of the strobe is lowest at 33 volts, and highest at 16 volts. Because strobes are life safety devices, the current ratings are given at the highest current level, which is 16 volts.
Power Stays the Same
To summarize this:
The number of flashes and the brightness of each flash has to remain the same -- The power used by the strobe for light output cannot change even as its input voltage changes
The fire alarm panel is 24 volts nominal -- It's voltage at the output terminals can vary from about 28 volts to about 20 volts
The wire to the strobe loses voltage -- With the fire alarm panel varying its voltage, and the voltage loss of the building wiring, the strobe has to be able to operate properly
Strobes are Life Safety Devices -- They are constant power devices and are rated to work no matter what the voltage
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms
. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.
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