FI840F ABB distributed control system industrial DCS module card

Description

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Operation, protection functions and self-test
Basic Protection functions
The PR122 trip unit offers the
following protection functions
(according to the version):
• overload (L)
• selective short-circuit (S)
• instantaneous short-circuit (I)
• earth fault (G)(2)
• phase unbalance (U)
• self-protection against overtemperature (OT)
• thermal memory for functions L and S
• zone selectivity for functions
S and G
• residual current (Rc) with
external toroid
• source ground return with
external toroid
Setting the neutral
In PR122/P, and PR123/P as
well, the neutral protection is
50% of the value set for phase
protection in the standard version. The neutral protection
can be excluded or set to
100% for E1, E2, E3, E4/f and
E6/f. In installations where very
high harmonics occur, the resulting current at the neutral
can be higher than that of the
phases. Therefore it is possible
to set the neutral protection at
150% or 200% of the value set
for the phases. In this case it
is necessary to reduce the setting of protection L accordingly(1).
The table below lists the neutral settings for the various possible combinations between
type of circuit-breaker and the
threshold I1 setting.
Start-up function
The start-up function allows
protections S, I and G to operate with higher trip thresholds
during the start-up phase. This
avoids untimely tripping
caused by the high inrush currents of certain loads (motors,
transformers, lamps).
The start-up phase lasts from 100
ms to 1.5 s, in steps of 0.05 s. It
is automatically recognized by
the PR122 trip unit as follows:
– when the circuit-breaker
closes with the trip unit selfsupplied;
– when the peak value of the
maximum current exceeds
0.1 x In. A new start-up becomes possible after the
current has fallen below the
threshold of 0.1 x In, if the
trip unit is supplied from an
external source.