P44x/EN AP/Hb
MiCOM P40 Agile P442, P444
(AP) 5-
Figure 87: Residual voltage, solidly earthed system
As can be seen in the previous figure, the residual voltage measured by a relay for an earth
fault on a solidly earthed system is solely depending on the ratio of source impedance
behind the relay to line impedance in front of the relay, up to the point of fault. For a remote
fault, the ZS/ZL ratio will be small, resulting in a correspondingly small residual voltage. As
such, depending upon the relay setting, such a relay would only operate for faults up to a
certain distance along the system. The value of residual voltage generated for an earth fault
condition is given by the general formula shown.
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