6.2 Standby and Alarm Load Logic
Key Takeaways
- Standby load and alarm load answer different operating conditions.
- Battery calculations usually combine a long standby period with a shorter alarm period.
- The candidate must know which devices draw current in each condition.
- A common mistake is using the same current value for every duration.
Standby and Alarm Load Logic
The most common fire alarm power calculation pattern is simple in concept: the system must operate during loss of primary power, then still support alarm operation. The exam challenge is assigning each load to the correct condition. Standby load is not the same as alarm load.
Standby current is the current drawn while the system monitors inputs, supervises circuits, and waits in normal condition. Alarm current is the current drawn when alarm functions operate. Some devices draw the same current in both conditions, some draw more in alarm, and some operate only during alarm or only during a specific control sequence.
| Load type | Question to ask | Typical examples |
|---|---|---|
| Standby current | What operates while the system is normal on secondary power? | FACU electronics, modules, monitoring circuits, some detectors. |
| Alarm current | What operates during the alarm condition in the scenario? | Notification appliances, relay outputs, alarm LEDs, control modules. |
| Auxiliary current | What non-alarm equipment is powered by the fire alarm supply? | Door holders, interface modules, accessories if listed. |
| Excluded current | What is not powered by this source? | Devices on a different listed supply or circuit. |
For NICET FAS scenario guidance, build two columns before using the calculator. Put each device's standby current in the standby column if it is powered in normal monitoring. Put each device's alarm current in the alarm column if it operates during the alarm condition described. Multiply current by quantity only after the device is in the correct column.
A clean battery-load workflow is:
- List all loads served by the power supply.
- Record standby current per device or module.
- Record alarm current per device or module.
- Multiply by quantity for each load group.
- Total standby current and alarm current separately.
- Multiply each total by the required duration given in the problem or reference.
- Add the amp-hour components and compare to available battery capacity.
Use consistent units. Milliamps must be converted to amps when calculating amp-hours unless the entire calculation is kept consistently in milliamps and converted at the end. A value of 80 mA is 0.080 A. A decimal shift error can make a battery look ten times larger or smaller than required.
Exam trap: adding standby and alarm current together first, then multiplying the combined number by the standby duration. That overstates alarm load across the long standby period and shows the candidate did not understand the operating sequence. Another trap is forgetting that notification appliances usually belong in the alarm column, not the standby column, unless the specific device data says otherwise.
NICET's Level II outline places power supply and loading requirements in the submittal preparation and layout domain. That means a question may ask what is missing from a shop drawing load schedule, not only what number is the final answer. The missing item may be standby current, alarm current, quantity, supply capacity, battery size, or voltage-drop information.
Practice with the built-in calculator style and write down the equation. For example, standby amp-hours equal standby amps times standby hours, while alarm amp-hours equal alarm amps times alarm hours. Use the durations supplied by the question or the governing reference you are allowed to use. Do not invent a duration from memory when the exhibit gives one.
Why should standby and alarm current be totaled separately?
What is 80 mA expressed in amps?
Which calculation approach is the common trap?