P4084ENa
Time
Fuse
Earth fault delay
Current
Contactor
Breaking
capacity
Figure 47: Fuse characteristic
7.5.5 INSULATED SYSTEM
Principle
The adv
antage gained by running a power system, which is insulated from earth, is that during a single phase to
earth fault condition no earth fault current is allowed to flow. Therefore, you can maintain power flow on the
system even when an earth fault condition is present. This advantage is offset by the fact that the resultant steady
state and transient overvoltages on the sound phases can be very high.
Operational advantages can be gained by the use of insulated systems. However, it is vital that detection of the
fault is achieved. This is not possible through standard current operated earth fault protection. One possibility for
fault detection is through a residual overvoltage device. This functionality is included within the P24xM relay and is
detailed in the Residual Overvoltage Protection section. However, fully discriminative earth fault protection on this
type of system can be achieved by the application of a sensitive earth fault element. This type of relay is set to
detect the resultant imbalance in the system charging currents that occurs under earth fault conditions. Therefore,
it is essential that a core balance CT is used for this application.
Chapter 6 - Current Protection Functions P24xM
120 P24xM-TM-EN-2.1
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