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Glossary
Glossary
Inverse time-delay overcurrent relay or
release (trip device)
An overcurrent relay or release (trip device) which operates after a time-delay
inversely dependent upon the value of the overcurrent.
NOTE: Such a relay or release (trip device) may be designed so that the time-
delay approaches a definite minimum value for high values of overcurrent.
Isolating distance (of a pole of a
mechanical switching device)
Clearance between open contacts meeting the safety requirements specified
for disconnectors.
I
2
t characteristic of a circuit breaker
Information (usually a curve) giving the maximum values of I
2
t related to
break time as a function of prospective current (r.m.s. symmetrical for AC) up
to the maximum prospective current corresponding to the rated short-current
breaking capacity and associated voltage.
Joule integral (I
2
t)
Integral of the square of the current over a given time interval:
I
2
t =
³
i
2
dt
Let-through current ( = Cut-off current)
See "Cut-off current."
Live part
Conductor or conductive part intended to be energized in normal use,
including a neutral conductor but, by convention, not a PEN conductor.
NOTE: This term does not necessarily imply a risk of electric shock.
Magnetic overload relay or release
(trip device)
Overload relay or release (trip device) depending for its operation on the force
exerted by the current in the main circuit exciting the coil of an electromagnet.
NOTE: Such a relay or release (trip device) usually has an inverse time-delay/
current characteristic.
Main circuit (of a switching device)
All the conductive parts of a switching device included in the circuit which it is
designed to close or open.
Main contact
Contact included in the main circuit of a mechanical switching device,
intended to carry, in the closed position, the current of the main circuit.
Make time
Interval of time between the initiation of the closing operation and the instant
when the current begins to flow in the main circuit.
Maximum prospective peak current
(of an AC circuit)
Prospective peak current when initiation of the current takes place at the
instant which leads to the highest possible value.
NOTE: For a multipole device in a polyphase circuit, the maximum
prospective peak current refers to one pole only.
Molded case circuit breaker
A circuit breaker having a supporting housing of molded insulating material
forming an integral part of the circuit breaker.
Neutral conductor (symbol N)
Conductor connected to the neutral point of a system and capable of
contributing to the transmission of electrical energy.
NOTE: In some cases, the functions of the neutral conductor and the
protective conductor may be combined under specified conditions in one and
the same conductor referred to as the PEN conductor [Symbol PEN].
Opening time (of a mechanical
switching device)
Interval of time between the specified instant of initiation of the opening
operation and the instant when the arcing contacts have separated in all
poles.
NOTE: The instant of initiation of the opening operation, i.e. the application of
the opening command (e.g. energizing the release), is given in the relevant
product standard.
Overcurrent
Current exceeding the rated current.