7SR242 Duobias Description Of Operation
©2010 Siemens Protection Devices Limited Chapter 1 Page 31 of 52
3.5 Current Protection: High Impedance Restricted Earth
Fault (64H)
Two high impedance restricted earth fault elements are provided, one for each transformer winding
The relay utilises fundamental current measurement values for this function.
The single phase current input is derived from the residual output of line/neutral CTs connected in parallel. An
external stabilising resistor must be connected in series with this input to ensure that this element provides a high
impedance path.
64H Current Setting sets the pick-up current level. An output is given after elapse of the 64H Delay setting.
An external series stabilising resistor and a parallel connected voltage limiting non-linear resistor are used with
this function. See ‘Applications Guide’ for advice in specifying suitable component values.
Operation of the high impedance element can be inhibited from:
Inhibit 64H A binary or virtual input.
>
c
64H Current Setting
Inhibit 64H
64H Element
Enabled
Disabled
&
I
G
64H
64H Delay
Figure 3-11 Logic Diagram: High Impedance REF (64H)
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