OBC Detection, Lighting & Power cheat sheet

Passive infrared (PIR) sensors detect changes in infrared radiation (heat) from moving objects; they do not emit energy and are immune to radio-frequency interference but can be blinded by heat sources or sunlight.

Microwave motion sensors emit and receive radio waves (typically 2.4 GHz or 10.525 GHz); they can detect through non-metallic materials but may cause false alarms from moving water or fluorescent lights.

Dual-technology sensors require both PIR and microwave detection to trigger an alarm, reducing false alarms; the logic is AND, not OR.

Photoelectric beam detectors use an infrared beam between a transmitter and receiver; a break in the beam triggers an alarm; alignment is critical and fog or dirt can attenuate the beam.

Ultrasonic sensors use high-frequency sound waves (typically 25–40 kHz) and detect motion by Doppler shift; they are sensitive to air currents and temperature changes.

Coverage pattern for a PIR sensor is typically 90° or 110° horizontal and about 60° vertical; rated detection range is specified for a standard human target (70 kg, 1.7 m tall) walking at 1 m/s.

Detection range is reduced by high ambient temperature (approaching body temperature), by barriers, and by mounting height above the recommended maximum.

Glass-break detectors use acoustic sensors tuned to the frequency of breaking glass (typically 3–5 kHz) and may include a shock sensor; they should be mounted within 7.6 m (25 ft) of the protected glass.

Perimeter detection using active infrared beams: beam stacking (multiple beams) reduces false alarms from small animals; beam height and spacing determine the detection zone.

Incandescent lamp efficacy is about 10–17 lumens per watt; halogen is about 15–25 lm/W; compact fluorescent (CFL) is about 50–70 lm/W; LED is about 80–150 lm/W.

Luminous flux (lumens) is total light output; illuminance (lux) is lumens per square meter; 1 footcandle = 10.76 lux.

Color temperature is measured in kelvin (K): warm white ≈ 2700–3000 K, neutral ≈ 3500–4100 K, cool white ≈ 5000–6500 K.

Color rendering index (CRI) ranges from 0 to 100; a CRI of 80+ is generally acceptable for indoor lighting, 90+ for critical color tasks.

A luminaire's ingress protection (IP) rating: first digit (0–6) for solids, second digit (0–8) for liquids; IP65 is dust-tight and protected against water jets.

LED drivers may be constant-current or constant-voltage; constant-current drivers specify output current (e.g., 350 mA, 700 mA) and a voltage range.

Dimming: forward-phase (leading-edge) dimmers are for incandescent/halogen; reverse-phase (trailing-edge) are for electronic transformers and some LEDs; 0–10 V dimming uses a low-voltage control signal.

Class 2 circuits for lighting are limited to 100 VA and 30 V RMS or 60 V DC; they require separation from Class 1 circuits.

Luminaire spacing for uniform illumination: spacing-to-mounting-height ratio (S/MH) is typically 0.8–1.5 depending on the luminaire's beam distribution.

Ohm's law: V = I × R; power: P = V × I = I²R = V²/R.

A circuit breaker trips on overcurrent (overload or short circuit); a fuse melts; both are overcurrent protective devices (OCPDs).

The interrupting rating (AIC) of a circuit breaker must be equal to or greater than the available fault current at its terminals.

Time-current curves show trip time versus current; the curve's shape determines coordination with upstream devices.

Ground-fault circuit interrupter (GFCI) trips at 4–6 mA of ground-fault current within a specified time; it protects people from electric shock.

Arc-fault circuit interrupter (AFCI) detects signature of parallel or series arcing and trips; required by NEC for many branch circuits.

Voltage drop formula for single-phase: VD = 2 × K × I × L / CM, where K = 12.9 for copper, 21.2 for aluminum, I in amperes, L in feet, CM = circular mils.

Ampacity of conductors is affected by ambient temperature and number of current-carrying conductors in a raceway; apply correction and adjustment factors from NEC Table 310.15(B)(2)(a) and (B)(3)(a).

Selective coordination: the upstream OCPD should have a higher threshold and longer time delay than the downstream OCPD so that only the downstream device trips for a fault in its zone.

American Wire Gauge (AWG): smaller number = larger conductor; area doubles approximately every 3 gauge steps (e.g., 10 AWG has about twice the area of 13 AWG; 12 AWG is about twice 15 AWG).

Ampacity examples for copper conductors at 30°C ambient, 60°C insulation: 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A; at 75°C: 14 AWG = 20 A, 12 AWG = 25 A, 10 AWG = 35 A.

Stranded conductors are more flexible and resistant to vibration than solid; use stranded in areas subject to movement.

Terminations must be made with listed connectors (wire nuts, push-in connectors, terminal blocks) and tightened to the manufacturer's specified torque.

Aluminum conductors require CO/ALR or AL-CU rated devices and antioxidant compound; aluminum-to-copper connections are prone to galvanic corrosion and thermal creep.

Conductor color code (US NEC): black/red/blue = ungrounded (hot) phases; white or gray = grounded (neutral); green, green/yellow, or bare = equipment grounding conductor.

Cable tray fill, conduit fill (NEC Chapter 9 Table 1): 1 conductor = 53%, 2 = 31%, 3 or more = 40% of conduit internal cross-sectional area.

Bend radius for non-shielded conductors: at least 8 times the overall cable diameter for fixed installations; for single conductors, 8× the conductor diameter.

Soldered connections are not permitted as the sole means of termination; mechanical connectors must be used.

Grounding connects equipment to earth; bonding connects metallic parts together to ensure equal potential. Both are required to clear faults and protect people.

The equipment grounding conductor (EGC) provides a low-impedance path for fault current, allowing the overcurrent device to trip quickly.

Ground-fault current path impedance must be low enough that the OCPD trips within the time specified by the time-current curve; NEC 250.4(A)(5) requires effective ground-fault current path.

Bonding jumpers around water meters, gas piping, and structural steel ensure electrical continuity; use listed bonding clamps and conductors sized per NEC Table 250.122.

The minimum size of the EGC is based on the rating of the overcurrent device protecting the circuit, not on the conductor ampacity alone.

Ground resistance testing: fall-of-potential (three-point) method is the standard; a ground resistance of 25 ohms or less is often specified for a single electrode, otherwise a second electrode is required.

Touch potential is the voltage between a person's hand and feet; step potential is between two feet; both are reduced by equipotential bonding and low ground resistance.

Lockout/tagout (LOTO) procedures require de-energizing, testing for absence of voltage, and locking the disconnect before work; verify with a known-good tester.

Personal protective equipment (PPE) for electrical work is selected by incident energy analysis (arc flash study); arc-rated clothing and face protection are required for higher hazard levels.

Continuity testing: use a low-resistance ohmmeter; a reading near 0 Ω indicates a good connection; a high reading indicates corrosion or loose termination.

Insulation resistance testing (megohmmeter): apply 500 V DC for 600 V rated equipment; minimum acceptable value is typically 1 MΩ per kV of rated voltage plus 1 MΩ (IEEE 43).

Polarity test: verify that the ungrounded (hot) conductor is connected to the correct terminal; reversed polarity is a common fault.

Voltage drop measurement: measure voltage at the source and at the load while the load is energized; a drop greater than 3% for branch circuits or 5% overall indicates a problem.

Thermal imaging: use an infrared camera to detect hot spots at terminations, breakers, and conductors; compare similar phases under load.

Ground fault location: use a clamp-on ammeter to measure current on the grounding conductor; any significant current indicates a ground fault or leakage.

Short-circuit vs. overload: a short circuit causes immediate high current and trips magnetic element; an overload causes gradual heating and trips thermal element.

Motor circuit testing: measure winding resistance (should be balanced within 5%), insulation resistance to ground, and running current compared to nameplate FLA.

Documentation: record test results, compare to previous readings, and follow NFPA 70B recommended maintenance intervals for electrical equipment.