Half Cycle of Current - A half loop of current having a duration of 0.00833 seconds.

Half Loop of current - The amount of time it takes the current to go from one zero reference to another.

Harmonic - Sinusoidal component of an arc voltage that is a multiple of the fundamental wave frequency.

Harmonic Distortion - The presence of harmonics that change an ac voltage waveform from sinusoidal to complex.

Hazardous Location - Means premises, buildings, or parts thereof in which there exists the hazard of fire or explosion due to the fact that:

• Highly flammable gases, flammable volatile liquid mixtures, or other highly flammable substances are manufactured or used, or are stored in other than original containers;
• Combustible dust or flyings are likely to be present in quantities sufficient to produce an explosive or combustible mixture, or it is impracticable to prevent such dust or flyings from collecting in or upon motors or other electrical equipment in such quantities as to produce overheating through normal radiation being prevented, or from being deposited upon incandescent lamps.
• Easily ignitible fibres or materials producing combustible flyings are manufactured, handled, or used in a free open state.
• Easily ignitible fibres or materials producing combustible flyings are stored in bales or containers but are not manufactured or handled in a free open state.

Hazardous location special terminology:

Cable Gland - a device or combination of devices intended to provide a means of entry of a cable or flexible cord into an enclosure situated in a hazardous location and that also provides strain relief and shall be permitted to provide sealing characteristics where required, either by an integral means or when combined with a separate sealing fitting.

Cable Seal - a seal that is installed at a cable termination to prevent the release of an explosion from an explosion-proof enclosure and that minimizes the passage of gases or vapours at atmospheric pressure.

Conduit Seal - a seal that is installed in a conduit to prevent the passage of an explosion from one portion of the conduit system to another and that minimizes the passages of gases or vapours at atmospheric pressure.

Degree of Protection - the measures applied to the enclosures of electrical apparatus to ensure

(a) the protection of persons against contact with live or moving parts inside the enclosure and protection of apparatus against the ingress of solid foreign bodies; and
(b) the protection of apparatus against ingress of liquids.

Explosive Gas Atmosphere - a mixture with air, under atmospheric conditions, of flammable substances in the form of gas, vapour, or mist in which, after ignition, combustion spreads throughout the unconsumed mixture.

Explosive Limits - the lower and upper percentage by volume of concentration of gas in a gas-air mixture that will form an ignitable mixture.

LEL - lower explosive limit.
UEL - upper explosive limit.

Methods of Protection - defined methods to reduce the risk of ignition of explosive gas atmospheres.

Non-incendive Circuit - a circuit in which any spark or thermal effect that may occur under normal operating conditions or due to opening, shorting, or grounding of field wiring is incapable of causing an ignition of the prescribed flammable gas or vapour.

Normal Operation - the situation when the plant or equipment is operating within its design parameters.

Primary Seal - a seal that isolates process fluids from an electrical system and that has one side of the seal in contact with the process fluid.

Secondary Seal - a seal that is designed to prevent the passage of process fluids at the pressure it will be subjected to upon failure of the primary seal.

Protective Gas - the gas used to maintain pressurization or to dilute a flammable gas or vapour.

Header - A raceway for electrical conductors, associated with an underfloor raceway or cellular floor system, which provides access to predetermined raceways or cells.

Heating Systems - fixed electric space-heating systems for heating rooms and similar areas; or fixed surface heating systems for pipe heating, melting of snow or ice on roofs or concrete or asphalt surfaces, soil heating, and similar applications other than space heating.

Special terminology and components:

Central unit - any heating unit (or group of units assembled to form a complete unit) permanently installed in such a way that it can convey heat to rooms or areas using air, liquid, or vapour flowing through pipes or ducts; and including duct heaters.

Fixture - any heating unit (or group of units assembled so as to form a complete unit) permanently installed in such a manner that it can be removed or replaced without removing or damaging any part of the building structure.

Heating Device - any form of electrical heating device, including cable, fixture, panel, and strip system.

Heating Device Set - a heating device assembled with the associated parts necessary to connect it to a source of electrical supply.

Heating Panel - a rigid or non-rigid laminated plane section in which the heating element consisting of a continuously parallel resistive material, a series resistive material, or a parallel-series resistive material is embedded between or in sheets of electrical insulating material.

Heating Panel Set - a heating panel together with cold leads or a non-heating portion.

Parallel Heating Cable - a cable incorporating heating elements connected in parallel either continuously or intermittently such that the watt density along the length of the cable is not altered by changes in the cable length.

Parallel Heating Cable Set - the combination of a parallel heating cable and associated parts necessary to connect it to a source of electrical supply.

Sauna Heater - a device that is designed for heating air and that is installed permanently in a special room to produce a hot atmosphere with generally a relatively low humidity, although brief excursions to relatively high humidity may take place.

Series Heating Cable - a cable using a series resistance conductor(s).

Series Heating Cable Set - the combination of a series heating cable and a means of connecting it to a source of electrical supply where the combination is assembled by the manufacturer.

Heat Run Test - A test used to determine the temperature rise of a device under rated load.

Heavy Duty - A lightning impulse classifying current category for distribution class arresters defined by ANSI/IEEE C62.11. A heavy duty rated arrester has a 10,000 amperage impulse value crest (refer to normal duty).

Hertz (Hz) - Expression of ac frequency in cycles per second, e.g., 60 Hz.

High-voltage - see definition for Voltage.

High Voltage and Low Voltage Windings - Terms used to distinguish the winding having the greater voltage rating from that having the lesser in two-winding transformers. The terminations on the high voltage windings are identified by H₁, H₂, etc., and on the low voltage by X₁, X₂, etc.

Hoistway - Any shaftway, hatchway, well hole, or other vertical opening or space in which an elevator, escalator, or dumbwaiter operates or is intended to operate.

Hot Tubs - see definition for Pools, Tubs, and Spas

HV - High voltage, see definition for Voltage.