6.4 Electrical Components & Equipment Identification
Key Takeaways
- Batteries in series add voltage while batteries in parallel add ampere-hour capacity: two 12 V 18 Ah units in series give 24 V at 18 Ah.
- Transformers are rated in volt-amperes, so a 40 VA 24 VAC transformer delivers roughly 1.67 amperes.
- Form C relay contacts are single-pole double-throw and drive elevator recall, HVAC shutdown, and damper control interfaces.
- A multimeter measures voltage in parallel and current in series on an energized circuit, but resistance only on a de-energized circuit.
- The fire alarm branch-circuit breaker must be marked in red, identified as FIRE ALARM CIRCUIT, and mechanically locked in the ON position.
6.4 Electrical Components & Equipment Identification
Duty B of the Oklahoma technician blueprint — Basic Electronics and Electrical Components — is 20% of the exam, roughly 14 of 70 questions, and its very first task is to "identify and describe the operation of electrical components and equipment." That is a recognition-and-function exam objective: given a component, state what it does and where it appears in a fire alarm system.
Resistors
A resistor opposes current flow and dissipates the resulting energy as heat. In fire alarm work the dominant application is the end-of-line resistor (EOLR), which lets the control unit distinguish a normal circuit from an open or a short.
- Resistance is measured in ohms (Ω); common fire alarm EOLR values are 2.2 kΩ, 3.3 kΩ, 4.7 kΩ, 10 kΩ, and 47 kΩ, dictated by the panel manufacturer, never by the installer's preference.
- Power rating matters: a resistor must be rated above the wattage it will dissipate (P = V²/R). A ½-watt EOLR on a 24 VDC circuit at 2.2 kΩ dissipates about 0.26 W, giving roughly a 2:1 safety margin.
- Color bands encode the value: the first two bands are significant digits, the third is the multiplier, and the fourth is tolerance (gold = ±5%, silver = ±10%). Red-red-red-gold reads 2-2 × 100 = 2.2 kΩ ±5%.
Transformers
A transformer transfers energy between circuits by magnetic induction and changes voltage in the process. It works on alternating current only.
- A fire alarm control unit's step-down transformer converts 120 VAC branch-circuit power to a lower AC voltage (commonly 16–24 VAC) for the panel's power supply.
- Turns ratio sets the voltage ratio: a 5:1 step-down transformer fed 120 VAC delivers 24 VAC.
- Transformers are sized in volt-amperes (VA), not watts. A 40 VA, 24 VAC transformer can deliver roughly 1.67 A (40 ÷ 24).
Batteries
Fire alarm secondary power uses sealed lead-acid (SLA) / valve-regulated (VRLA) batteries.
- A 12 V SLA battery is nominally 12 volts; two in series produce the 24 VDC that commercial panels require. Series connection adds voltage; parallel connection adds capacity (Ah) at the same voltage.
- Ampere-hour (Ah) is the capacity rating — a 18 Ah battery theoretically supplies 1 A for 18 hours.
- Panels float charge the batteries continuously and supervise them for presence, low voltage, and open charging circuit.
Relays and Contacts
A relay is an electrically operated switch: energizing a coil moves an armature that opens or closes a separate set of contacts, allowing a low-power fire alarm circuit to control a high-power load.
| Contact Designation | State When Coil Is De-Energized | Typical Fire Alarm Use |
|---|---|---|
| Normally Open (NO) / Form A | Open | Waterflow switch, most initiating devices |
| Normally Closed (NC) / Form B | Closed | Door holders, fail-safe interfaces, supervised inputs |
| Form C (SPDT) | One contact closed, one open, common pole | Elevator recall, HVAC shutdown, damper control |
Initiating devices report by contact action: a pull station or heat detector closes a normally-open contact across the circuit, shunting the EOLR and producing the alarm current the panel is watching for.
Connectors, Terminations, and Splicing Tools
- Terminal blocks are the preferred termination inside panels and back boxes — screw pressure on a stripped conductor or on a fork/ring lug.
- Crimp connectors (butt splices, lugs, ferrules) require the matched crimping tool; pliers produce a high-resistance joint that fails megger testing.
- Wire nuts / twist-on connectors and insulation-displacement B-connectors are used for field splices where permitted, always inside a listed enclosure.
- Tools of the trade: wire strippers gauged to the conductor, crimpers, cable cutters, and a punch-down tool for telecom-style blocks. Nicking a conductor while stripping creates a stress riser that snaps under vibration months later.
Fuses, Circuit Breakers, and Surge Protectors
- A fuse is a one-time overcurrent device: a metal element melts and must be replaced. Fire alarm panels use them on NAC outputs and auxiliary power taps.
- A circuit breaker is a resettable overcurrent device. The fire alarm branch circuit breaker must be marked in red, identified as "FIRE ALARM CIRCUIT," and mechanically locked in the ON position.
- A surge protective device (SPD) clamps transient overvoltage from lightning or utility switching, diverting it to ground. SPDs are rated by UL 1449 and are essential in Oklahoma, which sits in one of the most lightning-active regions in the United States.
The Multimeter
A digital multimeter (DMM) is the technician's primary instrument. Its three core functions map to Ohm's Law:
| Function | Meter Connection | Circuit State |
|---|---|---|
| Voltage (V) | Parallel across the two points | Circuit energized |
| Current (A) | Series, meter inserted into the path | Circuit energized |
| Resistance (Ω) | Across the component | Circuit de-energized |
[!WARNING] Measuring resistance on an energized circuit gives a meaningless reading and can destroy the meter; placing a meter set to amps in parallel across a source creates a dead short through the meter's shunt.
Exam Watchouts
[!IMPORTANT] Series adds volts, parallel adds amp-hours. Two 12 V 18 Ah batteries in series give 24 V at 18 Ah — not 36 Ah. The most common battery question on the exam depends on that single distinction.
[!TIP] Transformers are rated in VA, batteries in Ah, resistors in Ω and watts. Matching the component to its unit of measure answers a surprising number of Duty B items outright.
Two 12-volt, 18 Ah sealed lead-acid batteries are wired in series to supply a commercial fire alarm control unit. What is the resulting voltage and capacity?
A technician needs to measure the current drawn by an energized notification appliance circuit. How must the multimeter be connected?
Which relay contact arrangement provides one normally-open contact and one normally-closed contact sharing a common pole, making it the standard choice for elevator recall and HVAC shutdown interfaces?