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Glossary
Glossary
Overcurrent protective coordination
Coordination of two or more overcurrent protective devices in series to ensure
overcurrent discrimination (selectivity) and/or backup protection.
Overcurrent relay or release
(trip device)
Relay or release (trip device) which causes a mechanical switching device to
open with or without time-delay when the current in the relay or release (trip
device) exceeds a predetermined value.
NOTE: This value can in some cases depend upon the rate-of-rise of current.
Overload
Operating conditions in an electrically undamaged circuit which cause an
overcurrent.
Overload current
Overcurrent occurring in an electrically undamaged circuit.
Overload relay or release (trip device)
Overcurrent relay or release (trip device) intended for protection against
overloads.
Pollution
Any condition of foreign matter, solid, liquid or gaseous (ionized gases), that
may affect dielectric strength or surface resistivity.
Pollution degree (of environmental
conditions)
Conventional number based on the amount of conductive or hygroscopic
dust, ionized gas or salt and on the relative humidity and its frequency of
occurrence, resulting in hygroscopic absorption or condensation of moisture
leading to reduction in dielectric strength and/or surface resistivity.
Prospective breaking current (for a
pole of a switching device or a fuse)
Prospective current evaluated at a time corresponding to the instant of the
initiation of the breaking process.
Prospective making current (for a pole
of a switching device)
Prospective current when initiated under specified conditions.
NOTE: The specified conditions may relate to the method of initiation, e.g.
by an ideal switching device, or to the instant of initiation, e.g., leading to the
maximum prospective peak current.
Prospective peak current
Peak value of a prospective current during the transient period following
initiation.
Prospective symmetrical current
(of an AC circuit)
Prospective current when it is initiated at such an instant that no transient
phenomenon follows the initiation.
Rated current (In)
For circuit breakers, the rated current is the rated uninterrupted current (Iu)
and is equal to the conventional free-air thermal current (Ith).
Rated insulation voltage (Ui)
The insulation voltage of an equipment is the value of voltage to which
dielectric tests and creepage distances are referred.
Rated operational voltage (Ue)
A rated operational voltage of an equipment is a value of voltage which,
combined with a rated operational current, determines the application of the
equipment and to which the relevant tests and the utilization categories are
referred.
Rated uninterrupted current (Iu)
The rated uninterrupted current of an equipment is a value of current, stated
by the manufacturer, which the equipment can carry in uninterrupted duty.
Selectivity limit current (Is)
The selectivity limit current is the current coordinate of the intersection
between the total time-current characteristic of the protective device on the
load side and the pre- arcing (for fuses), or tripping (for circuit breakers) time-
current characteristic of the other protective device.
Service short-circuit breaking capacity
(Ics)
A breaking capacity for which the prescribed conditions according to a
specified test sequence include the capability of the circuit breaker to carry
its rated current continuously.