D-Link - Dual element fuse link with high current capabilities to withstand transient conditions due to lightning while maintaining low current overload protection capabilities. The construction is two separate elements in series, in one fuse tube.

Damp Location - see definition for Location.

Dead When Applied to Electrical Equipment - Means that the current-carrying parts of electrical equipment are free from any electrical connection to a source of voltage and from electrical charge and does not have a voltage different from that of earth.

Dead front - Means without live parts exposed to a person on the operating side of the equipment.

Deflector - A means for directing the flow of the gas discharge from the vent of an arrester.

Delta - A standard three-phase connection with the end of each phase winding connected in series to form a closed loop with each phase separated by 120°. Sometimes referred to as a three wire.

Delta-WYE - A term indicating the primary connected in delta and the secondary in Wye when pertaining to a three-phase transformer or transformer bank.

Design Tests - Tests made on each design to establish the performance characteristics and to demonstrate compliance with the appropriate standards of the industry. Once made they need not be repeated unless the design is changed so as to modify performance.

Dielectric Strength - The voltage which insulating material can withstand before breakdown occurs' the ability of an environment to counteract an established potential over a specific distance.

Dielectric Tests - Test which consist of the application of a voltage higher than the rated voltage for a specified time for the purpose of determining the adequacy against breakdowns of insulating materials and spacing under normal conditions.

Different Systems - Those which derive their energy from different transformers or from different banks of transformers, or from different generators or other sources.

Direct Current (DC) - An essentially constant value current that flows in only on direction.

Discharge Current - The surge current that flows through an arrester.

Discharge Voltage - The voltage that appears across an arrester conducting surge current, expressed in terms of crest value of voltage that occurs during flow of surge current of a specified wave shape. The crest value of the discharge voltage for a standard 8 X 20 microsecond current wave should be less than the BIL of the protected equipment by suitable margin of protection of not less than 20%.

Disconnecting Means - A device, group of devices, or other means whereby the conductors of a circuit can be disconnected from their source of supply.

Dry Location - see definition for Location.

Dual Element Fuse Link - A fuse link whose construction consists of two separate elements in series. In general, one element clears the low current faults and the other element clears the higher current faults within the rating of the fuse (refer to D-Link.) .

Dual Sensing Fuse Link - An under-oil expulsion fuse made of an eutectic element which melts when a combined top oil temperature rise due to ambient conditions and loading plus temperature rise due to current flow of 145° is reached.

Dual Voltage Switch - A switch used to change the primary voltage of a transformer (refer to dual voltage transformer).

Dual Voltage Transformer - A transformer that consists of two separate windings which can be connected in series to handle a specified voltage and kVA, or in parallel to handle the same kVA at one half the series connected voltage. The voltage is changed by operating a dual voltage switch.

Duplex Receptacle - see definition for Receptacle.

Dust-free Room - a room, building, or other area, of a size that permits the entrance of persons for operation and maintenance purposes and constructed so that the quantity of dust that can enter will not create a hazardous condition.

Dust-tight - An enclosure constructed so that dust cannot enter it.

Duty - A requirement of service that demands the degree of regularity of the load.

Continuous Duty - A requirement of service that demands operation at a substantially constant load for an indefinitely long time.

Short-Time Duty - A requirement of service that demands operation at a substantially constant load for a short and definitely specified time.

Intermittent Duty - A requirement of service that demands operation for definitely specified alternate intervals of:
    (a) Load and no-load;
    (b) Load and rest; or
    (c) Load, no-load, and rest;

Periodic Duty - A type of intermittent duty in which the load conditions are regularly recurrent.

Varying Duty - A requirement of service that demands operation at loads and for intervals of time, both of which may be subject to wide variation.

Duty-Cycle Test - A series of 22 discharges of an 8 X 20 microsecond current wave of a specified magnitude, while an arrester is connected to a 60 Hz power source at a rated voltage. The test indicates the durability of an arrester when subjected to repeated lightning surges under actual operating conditions.

Duty-Cycle Voltage Rating - The designated maximum permissible voltage between terminals at which the arrester is designed to perform its duty cycle.

Dwelling unit - One or more rooms for the use of one or more persons as a housekeeping unit with cooking, eating, living, and sleeping facilities.

Source: Canadian Electrical Code and National Electrical Code
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