ABB REG316 4 Series - Page 326

ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-288
(6) After a further reset delay t
Reset
plus a safety margin t
Margin
,
the BFP either detects that the fault current has been inter-
rupted and the protection function resets, or the fault cur-
rent continues to flow and the BFP initiates backup tripping.
If the BFP is only required to carry out a single breaker failure
step, timer t1 can be disabled (see ‘t1 active’). The response of
the BFP corresponds once again to Fig. 3.5.28.1, but with timer
t1 set to zero.
Timing in the case of an unconditional trip:
(1) A fault has occurred and been detected by a protective de-
vice.
(2) A signal at input ‘Ext Start’ starts the UT function.
(3) The circuit-breaker ruptures the fault current.
(4) If after the reset delay t
Reset
plus a safety margin t
Margin
,
the CB auxiliary contact “CB On” still signals to the UT that
the CB is closed, a second attempt is made by the UT
function to trip the circuit-breaker.
(5) The second attempt to trip the circuit-breaker is successful
and the fault current is interrupted.
(6) If after a further reset delay t
Reset
plus a safety margin t
Mar-
gin
the CB auxiliary contact “CB On” still signals to the UT
that the CB is closed, backup tripping is initiated by the UT
function.
ResetopenCB
tt1tDelay +³ + t
Margin
ResetopenCB
tt2tDelay +³ + t
Margin
t
CB open
CB opening time including arc extinction time
t
Reset
Reset time of the current criterion
1)
t
Margin
Allowance for variations in normal fault clearing times
2)
1)
see reset time of the current detector t
Reset
2)
see safety margin t
Margin