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• If there is a risk of intermittent faults. If the current IED, close to the faults,
starts 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.
20.2 Principle of operation
IP15777-1 v2
20.2.1 Mode of operation
M16687-3 v7
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 start current value (here internal signal
startValue), 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 (start 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 61 .
[ ]
= + ×
-
>
æ ö
ç ÷
ç ÷
ç ÷
æ ö
ç ÷
ç ÷
è ø
è ø
p
A
t s B k
i
C
in
EQUATION1189 V1 EN-US
(Equation 61)
where:
p, A, B, C are constants defined for each curve type,
in> is the set start current for step n,
k is set time multiplier for step n and
i is the measured current.
Section 20
1MRK 505 384-UEN A
Inverse time characteristics
736 Breaker protection REQ650 2.2 IEC
Technical manual