Alaska Low Voltage Practice flashcards

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

Ohm's Law

Voltage equals current multiplied by resistance (V = I × R). Used to compute expected current, voltage drop, or resistance in any low-voltage circuit.

Power formula

Electrical power equals voltage times current (P = V × I), also expressed as I²R or V²/R. Determines conductor sizing and heat generation.

Series circuit rule

Total resistance is the sum of individual resistances, and the same current flows through every component. Voltage divides proportionally across each element.

Parallel circuit rule

Reciprocal of total resistance equals the sum of reciprocals of branch resistances. Voltage is identical across all branches and current divides.

Decibel definition

A logarithmic ratio: 20 log10(V2/V1) for voltage or 10 log10(P2/P1) for power. A 6 dB voltage increase doubles signal level.

TIA-568 channel length

Copper horizontal channel maximum is 100 meters, comprising 90 meters of permanent link plus up to 10 meters of patch cords at each end.

Category 6A distance

Category 6A balanced twisted-pair supports 10GBASE-T to the full 100-meter channel. Category 6 supports the same rate only to 55 meters.

T568A versus T568B

Two pin/pair assignments for eight-position modular jacks. They differ in which pairs occupy pins 1-2 and 3-6; both ends must match.

PoE 802.3at

Power over Ethernet standard delivering up to 30 watts at the power sourcing equipment and 25.5 watts at the powered device over two pairs.

Fiber APC connector

Angled physical contact connector with an eight-degree polished ferrule. Lower return loss than UPC but not intermateable with flat-polish connectors.

NEC Article 725

Governs Class 1, Class 2, and Class 3 remote-control and signaling circuits. Defines power-limited thresholds and permitted cable types.

NEC Article 760

Covers fire alarm systems wiring, distinguishing non-power-limited (NPLFA) from power-limited (PLFA) circuits and requiring listed fire alarm cable.

Class A circuit

Fire alarm signaling circuit that continues to operate with a single open or ground fault. Class B circuits lose function on a single open.

Smoke detector spacing

Nominal 30-foot spacing on smooth ceilings with 15 feet maximum to any wall. Always confirm with the adopted code edition and local amendments.

NAC voltage drop

Notification appliance circuits must deliver rated voltage to the last device. Calculate using conductor resistance and total appliance current at worst case.

Fail-safe lock

Access-controlled lock that unlocks when power is removed. Required where life safety egress depends on the door releasing automatically.

OSDP protocol

Open Supervised Device Protocol: bidirectional, encrypted reader-to-controller communication that supersedes Wiegand for secure access control installations.

Wiegand interface

Legacy unidirectional reader-to-controller signaling using data-zero and data-one lines. Commonly 26-bit format; not encrypted and easily compromised.

Request-to-exit device

Sensor or pushbutton that signals the controller to release the lock for egress. Required by life safety codes where free egress is mandated.

RS-485 termination

A 120-ohm resistor at each end of the bus prevents signal reflections. Mid-bus termination or missing terminators cause intermittent communication faults.

BACnet MS/TP

Building automation protocol running over RS-485 twisted pair. Uses a token-passing master-slave scheme for device communication on a segment.

Modbus RTU

Serial register-based protocol over RS-485 using binary encoding. Master polls slaves by address; no built-in authentication or encryption.

HDBaseT extension

Technology extending HDMI, Ethernet, and control up to 100 meters over Category 6A cable. Used for displays beyond passive HDMI cable reach.

70V distributed audio

Constant-voltage speaker system using step-down transformers at each speaker. Higher voltage lowers line current and reduces conductor loss over long runs.

Lockout tagout

OSHA 1910.147 requirement to isolate, lock, tag, and verify de-energization before servicing equipment. Each worker applies a personal lock.

Arc flash boundary

Distance at which incident energy equals 1.2 calories per square centimeter. Inside this boundary, arc-rated PPE is required per NFPA 70E.

Assured grounding program

OSHA alternative to GFCI on construction sites: tools are tested, logged, and inspected on a documented schedule with visual checks before each shift.

GFCI on construction

Temporary receptacles on construction sites require ground-fault protection unless an assured equipment grounding conductor program is implemented and documented.

TDR testing

Time-domain reflectometer sends a pulse and measures reflections to locate opens, shorts, and cable length. Common on copper cabling commissioning.

OTDR testing

Optical time-domain reflectometer characterizes fiber by measuring backscatter and reflections. Locates breaks, splices, and connector loss along the link.

Insulation resistance test

Megohmmeter applies DC voltage to measure leakage between conductors or to ground. Low readings indicate moisture, damage, or contamination in the cable.

Ground loop symptom

Hum in audio, rolling bars in video, or intermittent data caused by multiple ground references. Resolve with isolation or single-point grounding.