• If there is a risk of intermittent faults. If the current IED, close to the faults, picks
up and resets there is a risk of unselective trip from other protections in the system.
• Delayed resetting could give accelerated fault clearance in case of automatic
reclosing to a permanent fault.
• Overcurrent protection functions are sometimes used as release criterion for other
protection functions. It can often be valuable to have a reset delay to assure the
release function.
22.2 Principle of operation
22.2.1 Mode of operation
The function can operate in a definite time-lag mode or in a current definite inverse
time mode. For the inverse time characteristic both ANSI and IEC based standard
curves are available. Also programmable curve types are supported via the component
inputs: p, A, B, C pr, tr, and cr.
Different characteristics for reset delay can also be chosen.
If current in any phase exceeds the set pickup current value (here internal signal
pickupValue), a timer, according to the selected operating mode, is started. The
component always uses the maximum of the three phase current values as the current
level used in timing calculations.
In case of definite time-lag mode the timer will run constantly until the time is reached
or until the current drops below the reset value (pickup value minus the hysteresis) and
the reset time has elapsed.
For definite time delay curve ANSI/IEEE Definite time or IEC Definite time are chosen.
The general expression for inverse time curves is according to equation
195.
Section 22 1MRK505222-UUS C
Inverse time characteristics
1182
Technical reference manual
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