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Section 6: Glossary of Terms and Formulas

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The following terms and formulas appear in this manual and in discussions of highway lighting. For electrical terms not listed here, consult the National Electrical Code (NEC).

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American Association of State Highway Transportation Officials

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Alternating Current (AC)

Current and voltage alternates from maximum positive to maximum negative in a sinusoidal pattern.

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Ampere (A)

The unit of electric current.

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A device that includes a transformer that modifies incoming voltage and current to provide the circuit conditions necessary to operate electric discharge lamps.

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Blanket Agreement

An agreement covering installation, operation, and maintenance responsibilities between a municipality and the Texas Department of Transportation (TxDOT) for all safety or continuous lighting within the municipality.

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Breakaway Support

A light pole support designed to shear easily under vehicular impact. The breakaway feature can be an aluminum transformer base, a frangible insert between pole base foundation, a slip base, or other device. The breakaway support must meet current AASHTO and FHWA requirements.

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Candela or Candlepower (cd)

The unit of luminous intensity emitted by a light source in a given direction.

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Capacitance (C)

Ability to store energy in an electrostatic field. Measured in farads or microfarads.

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Clear Zone

The area provided along highways to allow vehicles veering off the travel lane an opportunity for safe recovery or stopping. The clear zone width is always measured from the edge of the travel lane and depends on several roadway factors. The Roadway Design Manual contains a full discussion of the clear zone ("Horizontal Clearance to Obstructions") and provides the minimum and desirable widths for various roadways.

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Complete Interchange Lighting

The lighting, within the limits of the interchange, of the main lanes, direct connections, ramp terminals, and frontage road-crossroad intersections. (See partial interchange lighting.)

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A flexible wire that may be stranded or solid, insulated or bare, and that can carry electrical current.

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Continuous Lighting

Roadway lighting providing uniform illumination on all main lanes and direct connections and complete lighting for all interchanges.

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Conventional Lighting

A highway lighting system in which the luminaires are typically mounted no higher than 50 feet. (See also high mast lighting.)

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A luminaire light distribution is designated as cutoff when the candlepower per 1,000 lamp lumens does not numerically exceed 25 (2.5 percent) at an angle of 90 degrees above nadir (horizontal), and 100 (10 percent) at a vertical angle of 80 degrees above nadir. Cutoff type luminaires usually have flat glass lenses. Cutoff ratings have been replaced with BUG (Backlight, Uplight, Glare) ratings by the IES.

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Direct Current (DC)

Flow of electricity in a single direction.

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Electrical Details (ED)

TxDOT's standard sheets showing specifications for electrical specifications and standard construction practices for the installation of conduit, conductors, ground boxes, electrical services, and other electrical equipment.

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Electrical Service

Point of receiving power from utility company. Typical service voltages used on highway electrical systems are 120/240 VAC and, 240/480 VAC.

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Federal Highway Administration.

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An aimable luminaire generally employed for spot or wide-angle lighting.

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Footcandle (FC)

English unit of measurement for the illumination (E) on a surface. (See "lux" for metric.) One footcandle is the illumination on a surface that is one foot from and perpendicular to a uniform point source of one candela. Combining the inverse square law and the cosine law, the formula for footcandles (FC) is:

where CD is the candlepower, A is the angle of incidence of the light beam (see diagram under "lux,") and D is the distance of the surface from the light source.

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The property of a material designed to be readily or easily broken.

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High Mast Illumination Details (HMID)

TxDOT's standard sheets showing specifications for high mast illumination rings, lowering devices, and related mechanical and electrical systems.

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High Mast Illumination Pole Foundations (HMIF)

TxDOT standard sheets showing specifications for high mast illumination foundations and drill shafts.

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High Mast Illumination Poles (HMIP)

TxDOT standard sheets showing specifications for high mast illumination poles.

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Illuminating Engineering Society of North America (IESNA, also IES)

A non-profit society of lighting engineers and professionals. The IESNA writes many of the industry standard specifications for lighting fixtures.

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High Mast Lighting

Lighting units mounted at heights of 100 feet or more.

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Inductance (L)

Ability to store energy in electromagnetic field. Measured in henrys or millihenrys.

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Kilowatt (KW)

A measure of real power (generators, lamps, and heating elements are rated in watts or kilowatts).

(1KW = 1.34 horse power)

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Kilovolt-amp (KVA)

A measure of apparent power. Equipment is rated in KVA when heat dissipation is a concern (transformers are rated in KVA)

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A replaceable light source in a glass enclosure. A lamp may be high pressure sodium, metal halide, fluorescent, incandescent, or induction fluorescent. Also called a "light bulb."

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Light Source

The device that converts electric energy to visible light.

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Lumen (lm)

The unit of luminous flux.

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A device that directs, controls, and modifies the light produced by a light source. It consists of a light source, reflector, refractor, housing, and such support as may be integral with the housing

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Lux (lx)

Metric unit of measurement for the illumination (E) on a surface. (See footcandle for English unit.) One lux is the illumination on a surface one meter from and perpendicular to a uniform point source of one candela. The formula for lux is:

where CD is the candlepower, A is the angle of incidence of the light beam, and D is the distance of the surface from the light source.

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Mast Arm

An attachment to a light pole on the end of which a luminaire is mounted.

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Mounting Height

The vertical distance between the base of the pole and the luminaire.

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National Electric Safety Code (NESC)

A standard for the safe installation, operation, and maintenance of electric supply and communication lines and equipment, for use by electric utilities.

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National Electrical Manufacturer's Association (NEMA)

An organization of electrical manufacturers that develops specifications and industry standards.

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National Fire Protection Association (NFPA)

An organization devoted to eliminating death, injury, property, and economic loss due to fire, electrical, and related hazards. The NFPA develops and maintains safety standards such as NFPA 70: National Electrical Code, and NFPA 70E: Standard for Electrical Safety in the Workplace.

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NFPA 70: National Electric Code (NEC)

An NFPA standard for the safe installation of electrical wiring and equipment in the United States. The NEC is considered the minimum acceptable standard for a safe installation. Its purpose is the practical safeguarding of persons and property from hazards arising from the use of electricity. An electrical installation that complies with the NEC will be essentially free from electrical hazard.

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NFPA 70E: Standard for Electrical Safety in the Workplace

An NFPA standard for safe work practices to protect personnel by reducing exposure to major electrical hazards such as electrical shock, electrocution, arc flash, and arc blast.

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Ohm (O)

The unit of electrical resistance.

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Partial Interchange Lighting

The lighting of acceleration and deceleration lanes, ramp terminals, crossroads at frontage road or ramp intersections, and other areas of nighttime hazard. (See "complete interchange lighting.")

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A galvanized steel or aluminum shaft to support the lighting unit (also called "lighting standard").

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Power (P)

Measured in watts. Formula as follows:

For power loss due to resistance in lighting circuits, the power factor can be considered equal to one. This power can also be calculated: .

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Power Factor (pf)

Time relationship between current wave and voltage wave in an A.C. system.

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Polyvinyl chloride, a material used for underground non-metallic conduit.

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Polished aluminum device used to reflect light.

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Prismatic glass element used to refract light.

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Regulated Output Ballast

A form of electrical transformer that maintains the wattage of the lamp at a nearly constant value, though the line voltage may fluctuate as much as ±10 percent. Such ballasts or transformers may be integrally mounted within the luminaire or separately mounted in a ballast enclosure.

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Rigid metal conduit.

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Roadway Illumination Assembly

The luminaire and supporting members (pole, mast arm, etc.) with other related lighting equipment attached.

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Roadway Illumination Details (RID)

TxDOT's standard sheets showing specifications for roadway illumination to be used with TxDOT standard specification Item 610.

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Roadway Illumination Poles (RIP)

TxDOT standard sheets showing specifications for roadway illumination poles to be used with TxDOT standard specification Item 610.

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Safety Lighting

Roadway lighting installed at interchanges, highway intersections, and other points of nighttime hazard to the extent necessary to provide for the safe and orderly movement of traffic.

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Starter or Starting Aid (also called igniter)

A device producing a high voltage pulse to begin arcing in a lamp.

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An electrical device that changes one AC voltage to another.

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Transformer Base

A breakaway device for light poles, also called a T-base. It is a hollow cast aluminum base, the bottom of which is bolted to a concrete foundation and to the top of which the bottom flange of the pole is bolted.

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The ratio of the average level of illuminance to the minimum level of illuminance on the roadway.

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Volt (V or E)

The unit of electromotive force, electrical pressure, or difference of potential. Analogous to water pressure. One volt will cause one ampere of current to flow through a resistance of one ohm.

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Voltage Drop

A result of current flowing through a resistance.

Example: A current of 30 amperes flowing through 300 feet of No.8 conductor whose resistance loop is 0.3924 ohms will result in a voltage drop of 11.77 volts.

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Warrants are applied to determine whether or not the lighting system is justifiable at a particular location on an eligible highway.

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