Instantaneous Derived Earth Fault Protection (50N); Figure 3.3-1 Logic Diagram: Derived Directional Earth Fault Element - Siemens 7SR21 Manual

Minimum Polarising Voltage
The 67N Minimum Voltage setting defines the minimum polarising voltage level. Where the measured polarising
voltage is below this level no directional output is given and operation of protection elements set as directional will
be inhibited. This prevents mal-operation under fuse failure/MCB tripped conditions where noise voltages can be
present.

Figure 3.3-1 Logic Diagram: Derived Directional Earth Fault Element

3.3.2

Instantaneous Derived Earth Fault Protection (50N)

Two instantaneous derived earth fault elements are provided in the 7SR21 relay. Four elements are provided in
the 7SR22 relay e.g. giving the option of using two elements set to forward and two to reverse.
Each instantaneous element has independent settings for pick-up current 50N-n Setting and a follower time
delay 50N-n Delay. The instantaneous elements have transient free operation.
Where directional elements are present the direction of operation can be set using 50N-n Dir. Control setting.
Directional logic is provided independently for each 50-n element.
Operation of the instantaneous earth fault elements can be inhibited from:
Inhibit 50N-n
79 E/F Inst Trips: 50N-n
50N-n VTSAction: Inhibit
50N-n Inrush Action: Inhibit
©2010 Siemens Protection Devices Limited
7SR21 & 7SR22 Argus Description of Operation
A binary or virtual input, or function key
When 'delayed' trips only are allowed in the auto-reclose sequence
(79 E/F Prot'n Trip n = Delayed).
Operation of the VT Supervision function (7SR22).
Operation of the current inrush detector function.
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