OBC Detection, Lighting & Power flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

PIR sensor principle

Passive infrared sensors detect changes in infrared radiation emitted by warm bodies moving across the field of view; they do not emit energy and are immune to radio interference but can be blinded by heat sources.

Microwave motion sensor

Emits and receives radio-frequency waves (e.g., 2.4 GHz) and detects motion by Doppler shift; can penetrate non-metallic materials but may false-alarm on moving water or fluorescent lights.

Dual-technology sensor logic

Requires both passive infrared and microwave detection to trigger an alarm (AND logic), reducing false alarms compared to either technology alone.

Photoelectric beam detector

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

Glass-break detector

Acoustic sensor tuned to the frequency of breaking glass (typically 3–5 kHz) and may include a shock sensor; mount within 7.6 m (25 ft) of protected glass.

Lumens vs lux

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

Lamp efficacy

Ratio of light output to power input in lumens per watt; incandescent ≈10–17 lm/W, halogen ≈15–25, CFL ≈50–70, LED ≈80–150.

Color temperature

Measured in kelvin (K); warm white ≈2700–3000 K, neutral ≈3500–4100 K, cool white ≈5000–6500 K; indicates the light's apparent color.

Color rendering index

Scale from 0 to 100 indicating how accurately a light source renders colors compared to a reference; CRI 80+ is acceptable for indoor lighting, 90+ for critical color tasks.

IP rating

Ingress protection code: first digit (0–6) for solids, second digit (0–8) for liquids; IP65 means dust-tight and protected against water jets.

LED driver types

Constant-current drivers specify output current (e.g., 350 mA, 700 mA) and a voltage range; constant-voltage drivers provide a fixed voltage (e.g., 12 V, 24 V).

Dimming methods

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

Ohm's law and power

V = I × R; P = V × I = I²R = V²/R; these formulas relate voltage, current, resistance, and power in DC and resistive AC circuits.

Circuit breaker interrupting rating

The maximum fault current a breaker can safely interrupt; it must be equal to or greater than the available fault current at its terminals.

GFCI trip threshold

Ground-fault circuit interrupter trips at 4–6 mA of ground-fault current within a specified time, protecting people from electric shock.

Voltage drop formula

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.

Selective coordination

Upstream overcurrent protective device has a higher threshold and longer time delay than the downstream device so that only the downstream device trips for a fault in its zone.

AWG sizing rule

Smaller AWG number means larger conductor; area doubles approximately every 3 gauge steps (e.g., 10 AWG has about twice the area of 13 AWG).

Copper ampacity examples

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.

Aluminum conductor termination

Requires 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.

Conduit fill percentages

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

Grounding vs bonding

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

Equipment grounding conductor

Provides a low-impedance path for fault current, allowing the overcurrent device to trip quickly; its minimum size is based on the OCPD rating per NEC Table 250.122.

Ground resistance testing

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

Touch and step potential

Touch potential is 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

Procedure requiring de-energizing, testing for absence of voltage, and locking the disconnect before work; verify with a known-good tester.

Insulation resistance testing

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).

Voltage drop measurement

Measure voltage at source and at load while energized; a drop greater than 3% for branch circuits or 5% overall indicates a problem.

Thermal imaging inspection

Use an infrared camera to detect hot spots at terminations, breakers, and conductors; compare similar phases under load to identify anomalies.

Short circuit vs overload

A short circuit causes immediate high current and trips the magnetic element; an overload causes gradual heating and trips the thermal element.

Motor circuit testing

Measure winding resistance (should be balanced within 5%), insulation resistance to ground, and running current compared to nameplate FLA.