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3-30
Protection Functions
Earth fault protection
I
> (50N/51N)
0
857-UM001A-EN-P – July 2009
Undirectional earth fault protection is used to detect earth faults in
low impedance earthed networks. In high impedance earthed
networks, compensated networks and isolated networks undirectional
earth fault can be used as back-up protection.
The undirectional earth fault function is sensitive to the fundamental
frequency component of the residual current 3I
the third harmonic is more than 60 dB. Whenever this fundamental
value exceeds the user's pick-up setting of a particular stage, this
stage picks up and a start signal is issued. If the fault situation
remains on longer than the user's operation time delay setting, a trip
signal is issued.
Figure 3.18 – Block diagram of the earth fault stage I
Figure 3.19 – Block diagram of the earth fault stages I
Figure 3.18 shows a functional block diagram of the I
overcurrent stage with definite time and inverse time operation time.
Figure 3.19 shows a functional block diagram of the I
I
>>>> earth fault stages with definite time operation delay.
0
. The attenuation of
0
>
0
>>, I
>>> and I
>>>>
0
0
0
> earth
0
>>, I
>>> and
0
0
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