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813651A00
To change settings on a Model 555 Electronic Trip Unit
Time-current curves represent the tripping characteristics of a circuit breaker. Often referred to
as “trip curves”, they are used for design, selection, and coordination of electrical distribution
systems. Time-current curves, similar to the one shown in Figure 1, are used to show how fast a
breaker will trip at various currents.
Figure 1
Long-time Function.
The top part of the curve, which represents the overload trip function of the
circuit breaker. Long-time Pickup Setting (I
ld
) equals 105% -130% of the Current Setting (I
r
).
Short-time Function.
The short-time pickup function is adjustable with electronic trip-units only. It
determines the amount of current the breaker will carry for a short period of time before tripping, allowing
downstream devices to clear short-circuits without tripping the upstream device. Typically, this will be in
the realm of 1.5 to 10 times rated current.
Instantaneous.
The bottom part of the curve, this represents the region in which the circuit breaker will
trip without any intentional delay.
Ground Fault Protection.
A trip function that responds to current traveling outside the intended path.
L
S
I
G
G
L
I
S
