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Glossary
Glossary
Conventional free-air thermal current
(Ith)
The conventional free-air thermal current is the maximum value of test current
to be used for temperature-rise tests of unenclosed equipment in free-air. The
value of the conventional free-air thermal current shall be at least equal to the
maximum value of the rated operational current of the unenclosed equipment
in eight-hour duty.
Conventional non-tripping current
(of an overcurrent relay or release
(trip device))
Specified value of current which the relay or release (trip device) can carry for
a specified time (conventional time) without operating.
Conventional tripping current
(of an overcurrent relay or release
(trip device))
Specified value of current which causes the relay or release (trip device) to
operate within a specified time (conventional time).
Creepage distance
Shortest distance along the surface of an insulating material between two
conductive parts.
NOTE: A joint between two pieces of insulating material is considered part of
the surface.
Critical load current
Value of breaking current, within the range of service conditions, at which the
arcing time is significantly extended.
Critical short-circuit current
Value of breaking current, less than the rated short-circuit breaking capacity,
at which the arc energy is significantly higher than at the rated short-circuit
breaking capacity.
Current-limiting circuit breaker
A circuit breaker with a break-time short enough to prevent the short-circuit
current reaching its otherwise attainable peak value.
Cut-off current (Let-through current)
Maximum instantaneous value of current attained during the breaking
operation of a switching device or a fuse.
NOTE: This concept is of particular importance when the switching device or
the fuse operates in such a manner that the prospective peak current of the
circuit is not reached.
Disconnector
Mechanical switching device which, in the open position, complies with the
requirements specified for the isolating function.
Exposed conductive part
Conductive part which can readily be touched and which is not normally
alive, but which may become alive under fault conditions.
NOTE: Typical exposed conductive parts are walls of enclosures, operating
handles, etc.
Frame size
A term designating a group of circuit breakers, the external physical
dimensions of which are common to a range of current ratings. Frame size
is expressed in amperes corresponding to the highest current rating of the
group. Within a frame size, the width may vary according to the number of
poles.
Fuse
Device that, by the fusing of one or more of its specifically designed and
proportioned components, opens the circuit in which it is inserted by breaking
the current when this exceeds a given value for a sufficient time. The fuse
comprises all the parts that form the complete device.
Fuse-element
Part of the fuse-link designed to melt under the action of current exceeding
some definite value for a definite period of time.
Fuse-link
Part of a fuse (including the fuse-element(s)) intended to be replaced after
the fuse has operated.
Impulse withstand voltage (U
imp
)
Highest peak value of an impulse voltage, of prescribed form and polarity,
which does not cause breakdown under specified conditions of test.
Instantaneous relay or release
(trip device)
Relay or release (trip device) which operates without any intentional time-
delay.