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