Secondary power supply: fire alarm term explained
The secondary power supply is the backup that keeps a fire alarm system running when normal power fails. It is usually batteries with a charger. The batteries are sized to carry the system through a standby period and then a period of full alarm, so the building stays protected during an outage.
How the backup works and how it is checked
- Primary and secondary. Normal power from the building supply runs the system and the charger. On failure of AC power, the batteries take over without a break, and the panel reports a trouble.
- Standby then alarm. The code sets how long the batteries must carry standby and then alarm, and the times differ by system type, so take them from your own copy of NFPA 72 and the project's sizing sheet. See the battery calculation method for how the numbers are used.
- Capacity from current. Add up the standby currents of everything and the alarm currents of everything. Standby amperes times hours, plus alarm amperes times the alarm time in hours, gives the amp-hours, and a margin is added before choosing a battery size.
- The charger. It refills the battery after an outage and holds it at float. A charger fault produces a trouble signal.
- Checks. The panel reports AC loss, a low battery or a missing battery. Testing includes measuring battery voltage and checking the battery's age and connections as the maintenance procedure requires.
Try one question
Question 1 of the free 10-question check on NICET FA I. No signup. Then ask Strobe, the AI study buddy, why the answer is right and the others are wrong.
Installation
A supplied module drawing labels COM, NO and NC. The door interface needs a switched contact that is open while the relay is at rest and closes when the relay operates. Which pair matches that instruction?
Press A–D or 1–4 to choose an answer while this question is on screen.
Choose an answer to begin
Worked example: standby alone
A fictional panel draws 0.5 amperes in standby, and the fictional sizing sheet calls for 24 hours of standby. The standby portion is 0.5 x 24 = 12 amp-hours before any alarm load or margin is added. The alarm load comes on top of that, and a margin and the next larger battery size follow. The battery calculator works the full sum with the same method. Notice how much of the capacity goes to standby, which is why a small standby load that is forgotten can cause a battery to be too small.
Ask Strobe before you buy AI your Alarm Ladder study buddy
See what's on the NICET FA I exam, test yourself with 5 quick questions, or find the plan that fits you.
The NICET FA I course
Available now · instant accessLearn every Level I task, then prove it on timed practice exams.
- 6modules of lessonsEvery task on the outline
- 2full-length timed exams85 questions in 110 minutes each
- 340practice questionsEach one explained
- 4worked calculationsStep by step
- 20code-navigation drillsFind it fast in NFPA 72
- 1week-by-week study planSet to your exam date
Turn a practice answer into a study step. In untimed practice, ask Strobe AI about any question you answer, follow up, then see your weakest topics and what to drill next.
Instant access. Terms and refunds · Secure checkout by Stripe
- Level I, II and I&T bundle$299$98 less than buying the 3 courses separatelyIncludes the NICET FA I, NICET FA II and NICET I&T FA courses
Terms and refunds · Secure checkout by Stripe
- Free sample lessonFree
- Question of the dayFree
Every answer comes with a full explanation.
Common mix-ups
- Secondary is not generator. Batteries are the secondary supply in the usual arrangement, and an engine generator is a different arrangement with its own rules.
- A battery that holds voltage may still be weak. Voltage alone does not prove capacity, so follow the maintenance procedure and replace batteries by schedule.
- Alarm time depends on the system. A voice system can differ from an ordinary one, so check the system type and the sizing sheet first.
- Added devices change the sum. Every new device adds current, so recalculate when the system grows.
Secondary power on the NICET Level I exam
Battery sizing is one of the calculations Level I prep includes, and power checks are part of Maintenance (40 to 50% of Level I) and Installation (44 to 54%). NFPA 72 (2022) Chapter 10 covers fundamentals including power supplies.
Practice it
Run your numbers in the fire alarm battery calculator. Read the battery calculation method for the full steps, and trouble signals for how power faults appear. The fire alarm terms glossary lists the rest.
Know your power terms? Check the rest of Level I
Take the free 10-question check. It's weighted to the Level I outline, scored by domain and needs no signup, so you see whether installation, maintenance or layout costs you the most points. The Level I course gives you full timed practice exams, an explained answer for every question and the section number to look up for each one.
Sources
Show the list
- NFPA 72 (2022), NFPA, Chapter 10, referenced by number only.
- Fire Alarm Systems Level I content outline, NICET.
Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.
Your next step
Ready to see where you stand on NICET FA I?
Free, untimed and scored by domain. No signup.
Start the free 10-question checkLevel I course. Lessons, timed practice exams and a plan around your exam date. Get Level I course · $99 See the plans or read the free sample lesson.Terms and refunds
Want a timed run? Try the free 25-question mini-mock. The first 5 need no email.