Definite Time Non-Directional Overcurrent Protection (Toc); Application; Functionality - ABB REL 501-C1 2.5 Technical Reference Manual

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Definite time non-directional overcurrent
protection (TOC)
Definite time non-directional overcurrent
protection (TOC)

Application

The time delayed overcurrent protection, TOC, operates at different system conditions for cur-
rents exceeding the preset value and which remains high for longer than the delay time set on
the corresponding timer. The function can also be used for supervision and fault detector for
some other protection functions, to increase the security of a complete protection system. It can
serve as a back-up function for the line distance protection, if activated under fuse failure con-
ditions which has disabled the operation of the line distance protection.
The time delayed residual overcurrent protection is intended to be used in solidly and low resis-
tance earthed systems. The time delayed residual overcurrent protection is suitable as back-up
protection for phase to earth faults, normally tripped by operation of the distance protection. The
protection function can also serve as protection for high resistive phase to earth faults or as a
fault detection for some other protection functions.

Functionality

The current measuring element continuously measures the current in all three phases and com-
pares it to the set operate value IP>. A filter ensures immunity to disturbances and dc compo-
nents and minimizes the transient overreach. If the current in any of the three phases is above
the set value IP>, a common start signal STP and a start signal for the actual phase(s) are acti-
vated. The timer tP is activated and the phase overcurrent trip signal TRP is activated after set
time. The general trip signal TRIP is activated as well.
The input signal BLOCK blocks the function. The input signal BLKTR blocks both trip signals
TRP and TRIP.
The residual current measuring element continuously measures the residual current and com-
pares it with the set operate value IN>. A filter ensures immunity to disturbances and dc com-
ponents and minimizes the transient overreach. If the measured current is above the set value
IN>, a start signal STN is activated. The timer tN is activated and the residual overcurrent trip
signal TRN is activated after set time. The general trip signal TRIP is activated as well. The input
signal BLOCK blocks the function. The input signal BLKTR blocks both trip signals TRN and
TRIP.
Chapter 5
Current