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ISO 9001:2008
4.5. 51N Function. Neutral inverse time overcurrent
This protection function can be set by using the following parameters
:
Function Description Minimum Maximum Step Unit Default
51N
Neutral inverse time overcurrent
Permission - - Yes/No - No
Curve - - (1*) - Extremely Inverse
Dial 0,05 1,25 0,01 - 1,25
Tap 0,10 7,00 0,01 Inominal 0,50
Operating time 0,02 300,0 0,01 s 0,02
(1*) Inverse, Very inverse, Extremely inverse, Defined time
If the option "Defined time" is selected for the curve setting, the unit behaves like an instantaneous
overcurrent unit. In this case, the unit operating time is adjusted by using the parameter "Operating
time".
If a curve (inverse, very inverse or extremely inverse) is selected for the curve setting, the operating
time depends on the curve, dial and tap settings.
If the unit operates as defined time, the function is activated at 100% of the set tap value, and it
deactivates at 95%.
If the unit operates with a curve, the function is activated at 120% of the set tap value, and it
deactivates at 100%. The reset is instantaneous in both cases.
The activation time is accurate to ±5% or ±30ms, whichever is higher, of the theoretical activation
time.
The curves used are IEC255-4/BS-142, which are described in the "Curves" section.
4.6. Trip blocking protection for the switchgear
Some transformation centres use a combination of switchgear and fuses for cutting out. Switchgears
have a limited opening current. As a result, the fuses are responsible for cutting out the circuit for
high current short circuits, as the switchgear would be destroyed if opened in this situation. In order
to deal with these situations, tripping is blocked when the phase current exceeds a preset value.
Group Description Minimum Maximum Step Unit Default
Breaker blocking
Blocking - - Yes/No - Yes
Blocking limit 7,00 20,00 0,01 I nominal 7,00