Delaware 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 times resistance (V = I × R). Power equals voltage times current (P = V × I). These two relationships underlie every low-voltage circuit calculation, including power supply sizing and voltage drop.

Voltage Drop Formula

For a two-conductor DC circuit, V_drop = 2 × L × I × R, where L is one-way length in feet and R is resistance per foot. Doubling accounts for the return conductor. Keep drop under 5% for most low-voltage loads.

Characteristic Impedance

The transmission-line impedance of a cable, such as 100 Ω for CAT6 or 120 Ω for RS-485. It depends on conductor geometry and insulation, not length. Mismatches cause signal reflections and increased return loss.

Twisted Pair Noise Rejection

Equal and opposite currents in a twisted pair create canceling magnetic fields, rejecting common-mode noise. The twist rate determines which frequencies are rejected. This is why pair integrity must be maintained at terminations.

Class 2 vs Class 3

Both are power-limited circuits under NEC Article 725, each limited to 100 VA per circuit. Class 2 is lower voltage and power; Class 3 allows higher voltage. Class 1 uses Chapter 3 wiring methods with no power limit.

Plenum Rated Cable

Cable listed CMP for installation in air-handling spaces. It uses low-smoke, low-flame-spread insulation. Riser cable (CMR) is for vertical shafts; general-purpose CM is for horizontal runs outside plenums.

T568A vs T568B

Two standard RJ-45 pinouts that differ only in the assignment of pairs 2 and 3. Either works if used consistently on both ends. T568B is more common in US commercial installations.

Cable Bend Radius

The minimum curve a cable can tolerate without damage. Unshielded twisted pair typically requires 4× the cable diameter during installation; fiber requires 10× or the manufacturer's rating, whichever is greater.

Article 725 Scope

NEC Article 725 governs remote-control, signaling, and power-limited circuits that are not integral to equipment. It defines Class 1, Class 2, and Class 3 circuits and their wiring, separation, and listing requirements.

Intersystem Bonding Termination

A listed device required by NEC 250.94 that provides a single accessible point for bonding communications, CATV, and satellite system grounds to the building grounding electrode system. Usually mounted outside.

Environmental Air Space Rule

NEC 300.22(C) permits only wiring methods specifically listed for plenum use inside environmental air spaces. This is why CMP cable, not general-purpose CM, must be used above suspended ceilings used as return air.

Box Accessibility

NEC 314.29 requires junction boxes and conduit bodies to remain accessible without removing building finish. Low-voltage splice points must not be buried in walls or ceilings, even though box fill rules may not apply.

Category Cable Ratings

CAT5e is rated to 100 MHz, CAT6 to 250 MHz, and CAT6A to 500 MHz. Higher categories support higher data rates over longer distances, with CAT6A reaching 100 m at 10GBASE-T.

Permanent Link Limit

The horizontal cable run may not exceed 90 m (295 ft). Adding patch cords at each end gives a 100 m channel. Exceeding the limit causes insertion loss and crosstalk failures during certification.

NEXT Test

Near-end crosstalk measures signal coupling from one pair to another at the transmitter end. High NEXT indicates poor termination, untwisted conductors, or a split pair and is a common cause of certification failure.

Single-Mode Fiber

Fiber with a small core (about 9 µm) that carries one propagation mode, using 1310 or 1550 nm lasers. It supports kilometers of distance and is used for campus and building backbone links.

Class A vs Class B Circuit

NFPA 72 circuit classifications. Class A keeps the circuit operational after a single open or ground fault by providing a redundant path. Class B supervises for opens but loses the circuit on a single fault.

Smoke Detector Spacing

On a smooth ceiling, detectors are spaced 30 ft apart nominally, with the first within 15 ft of a wall, covering up to 900 sq ft. Spacing is reduced for ceiling heights above 10 ft.

NAC Voltage Drop

Notification appliance circuits must deliver adequate voltage to the last device. Calculate V_drop = 2 × L × I × R and verify the final appliance stays within its listed operating range, often 16–33 V.

Pull Station Height

Manual fire alarm pull stations are mounted 42–48 in. above finished floor and within 5 ft of an exit door. They must be unobstructed and visible along the normal path of egress.

Wiegand Interface

A legacy access control interface using clock and data lines at 5 V, carrying a 26-bit format with facility code and card number. It is unidirectional and unencrypted, limited to about 500 ft.

Fail-Safe vs Fail-Secure

Fail-safe locks unlock when power is lost and are used on egress doors. Fail-secure locks stay locked on power loss and require a mechanical release for life safety. Selection is driven by code.

OSDP Protocol

Open Supervised Device Protocol is a bidirectional, encrypted access control standard that replaces Wiegand. It supports secure channel encryption, tamper reporting, and firmware updates over RS-485.

RS-485 Termination

A differential bus requiring 120 Ω termination at each physical end, daisy-chain topology, and up to 32 unit loads. Stubs and star wiring cause reflections and communication errors.

70.7 Volt Distributed Audio

A constant-voltage speaker system where each speaker has a wattage tap. Total tap wattage must not exceed amplifier output. Higher line voltage reduces loss over long cable runs compared with 8 Ω systems.

HDBaseT

A standard that extends HDMI video, audio, Ethernet, control, and power over a single twisted-pair cable up to 100 m. It uses a proprietary chipset at both ends rather than standard IP networking.

DMX512

A lighting control protocol using RS-485 at 250 kbps, carrying 512 channels per universe. It is unidirectional, uses 5-pin XLR connectors, and requires termination at the end of the daisy chain.

Speaker Impedance Loading

On 8 Ω systems, parallel speakers halve the load: two 8 Ω speakers present 4 Ω. Never go below the amplifier's minimum rated impedance, or the amplifier will overheat or shut down.

Grounding Electrode System

NEC 250.50 requires all available electrodes, including metal water pipe, building steel, concrete-encased electrode, ground ring, and rods, to be bonded together into one system.

Single-Point Bonding

Bonding a cable shield or drain wire at only one end prevents ground loops caused by potential differences between locations. The far end is left floating or capacitively terminated to drain RF.

Surge Protective Device Ratings

SPDs are specified by MCOV, clamping voltage, surge current capacity in kA per mode, and response time. Type 1 installs at the service, Type 2 at distribution panels, and Type 3 at point of use.

Delaware Licensing Authority

The Delaware Board of Electrical Examiners, within the Division of Professional Regulation, licenses electricians and electrical contractors. Low-voltage work falls under the state electrical statutes and local permit authorities.