The current setting should be chosen in accordance to the setting of the sensitive ground
fault protection. The setting can be given within the range 2 – 200 % of IBase.
t1: Time delay of the re-trip. The setting can be given within the range 0 – 60 s in steps of
0.001 s. Typical setting is 0 – 50 ms.
t2: Time delay of the back-up trip. The choice of this setting is made as short as possible
at the same time as unwanted operation must be avoided. Typical setting is 90 – 200 ms
(also dependent of re-trip timer).
The minimum time delay for the re-trip can be estimated as:
_
2 1³ + + +
cbopen BFP reset margin
t t t t t
EQUATION1430 V1 EN (Equation 141)
where:
t
cbopen
is the maximum opening time for the circuit breaker
t
BFP_reset
is the maximum time for breaker failure protection to detect correct breaker
function (the current criteria reset)
t
margin
is the safety margin
It is often required that the total fault clearance time shall be less than a given critical time.
This time is often dependent of the ability to maintain transient stability in case of a fault
close to a power plant.
Section 7 1MRK 506 334-UUS A
Current protection
236
Application manual
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