The goal, when measuring the voltage on the Notification Appliance Circuit (NAC), is to make sure that there is enough voltage to turn on all the horns and strobes when there is a fire. The power may be off to the building during a power blackout, but the voltage still has to be there for the horns and strobes to warn people of fire.
Life Safety Systems
All fire alarm systems are life safety systems, though. They are designed to keep operating when things go wrong, like when the power has been blacked out for 24 hours in the building, and the batteries are near the end of their life.
The rules we have to follow are based on Life-Safety considerations. When people write the rules that we have to follow, they're thinking about life safety, so we have to think about life safety.
Measure During Alarm
All measurements that you make have to be made while the panel is in alarm, and the horns and strobes are turned on. This is the only time that the measurements actually mean anything.
Battery Voltage - 24 Volts Nominal is not 24 Volts Actual
The word "Nominal" means "In Name Only". The name stamped on the side of the battery only represents the range of normal voltage for the battery.
The battery voltage will range from a high of 26 to 27.5 volts when the battery is fully charged to a low of 20 to 22 volts when there isn't any life left in the battery. For you, this voltage range goes from a high, while the power is on in the building, to a low, after 24 hours of the power turned off (blacked out) out in the building.
The power supply voltage on the fire alarm panel is the same as the battery voltage.
Because this is a life-safety system, you have to assume the worst. The best you can assume for the power supply is the 20 volts of when the power is off. You can't use the 24 volts, which really isn't what's measured on the power supply at any time.
Installation Sheets for Horns and Strobes
To know what this voltage is supposed to be at the end of line resistor, however, you will have to look up the information of the minimum requirements shown on the Installation Sheet that comes in the package for every horn, strobe, or horn/strobe. This is the minimum voltage that you need to measure.
However, you have to know when to measure this voltage because it will be different when the circuit is off and when the circuit is on and warning people to get out of the building. This voltage is also affected by how long the power has been off in the building.
Measurement Comparison - Rule of Thumb
The rule-of-Thumb is an educated guess.
The rule-of-thumb is not a guarantee that the system will work; the rule-of-thumb is only a good guess that the system will probably work. Any voltage measurement you get outside of this rule-of-thumb means that more precise measurements need to be made, and possibly redesigning of the building wiring needs to be done.
You can only do your measurements when the horns and strobes are turned on, warning people of fire.
For any fire alarm system, the voltage is always changing. The best you can do is compare different voltages.
You compare the voltage you measure at the start of the NAC (Notification Appliance Circuit) which is the screw terminals on the control panel, and compare that to the voltage at the end of line resistor. Many factors are involved, but according to rule-of-thumb logic, the voltage at the end of line resistor
should be no more than 3 volts
less than the voltage at the start of the NAC circuit
The four volts is the voltage lost in the wiring of the building when the horns and strobes are turned on. The four volts is all the voltage you are allowed to lose when the fire alarm system is warning people to get out of the building.
However, also keep in mind that the voltage at the end of line resistor still cannot ever be below what the voltage shown on the installation sheet for the horns and strobes. If the measured voltage is less than that, the horns and strobes may not work during a power blackout. If the installation sheet is missing or already thrown away, contact the technical support for the manufacturer. Technical support will be happy to tell you this voltage.
Measurement Time - Power Out and Horns & Strobes On
The proper time to make measurements is when the power to the building has been turned off for 24 hours, and after that the horns and strobes have been turned on and warning people for 5 minutes.
The installation sheet for the horns and strobes shows what voltage is needed to operate any horn or strobe, and that is the voltage that has to reach the end of the line during the testing.
When the circuit is installed, and going to be inspected soon, turning off the power to the building is not going to be done. You will have to use a rule-of-thumb described earlier.