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Time delayed phase and residual
overcurrent protection (TOC1)
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The definite time-delayed protection TOC1 is intended for single and two-phase system
with a rated frequency of 16 2/3, 50 or 60 Hz.
The time delayed phase overcurrent protection is non-directional and can be used as in-
dependent overcurrent protection, particularly for radially fed systems, or as back-up
function to the main distance protection. In the first case the protected zone of the time
delayed overcurrent protection reaches up to the next overcurrent protection and works
in its zone as back-up protection. The programmable time delay (definite time) of the
function allows the time selectivity through an appropriate time grading among the
overcurrent relays protecting the system.
Where the function acts as back-up for the main line protection, the trip from the over-
current protection can be activated when the main protection function is blocked (e.g.
by the fuse failure protection) or it can be active all the time.
In some cases, where it could be difficult to achieve a selective trip, the function can be
used as a helpful overcurrent signalization for the post-fault analysis.
The time delayed residual current function, IN> (earth-fault protection), serves as a lo-
cal back-up function to the main protection function. In most cases, it is used as a back-
up for the earth-fault measuring in distance protection.
The IN> function is intended to be used in solidly earthed two-phase systems.
The time delay makes it possible to set the relay to detect high resistance faults and still
perform selective trip.
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The current measuring element in one of the integrated signal processors continuously
measures the phase currents (IL1 and IL2) and the residual current (IN) in two-phase
systems. In single-phase systems it measures the line current IL1. The measured cur-
rents are compared with the set operate values I> and IN>. A recursive Fourier filter
filters the current signals, and a separate trip counter prevents high overreaching of the
measuring elements. The logical value of the current signal on the output of the digital
processor is equal to 1 if the measured current exceeds the set value.
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