8.3.3.4 High impedance earthed systems
M13853-18 v10
In high impedance earthed systems, earth faults cause a neutral voltage in the feeding
transformer neutral. Two step residual overvoltage protection ROV2PTOV is used to trip the
transformer, as a backup protection for the feeder earth-fault protection, and as a backup for
the transformer primary earth-fault protection. The setting must be above the highest
occurring "normal" residual voltage, and below the lowest occurring residual voltage during
the faults under consideration. A metallic single-phase earth fault causes a transformer neutral
to reach a voltage equal to the nominal phase-to-earth voltage.
The voltage transformers measuring the phase-to-earth voltages measure zero voltage in the
faulty phase. The two healthy phases will measure full phase-to-phase voltage, as the faulty
phase will be connected to earth. The residual overvoltage will be three times the phase-to-
earth voltage. See figure
115.
3U
0
UL1
IEC07000190-2-en.vsd
IEC07000190 V2 EN-US
Figure 115: Earth fault in Non-effectively earthed systems
8.3.3.5 Direct earthed system
GUID-EA622F55-7978-4D1C-9AF7-2BAB5628070A v8
In direct earthed systems, an earth fault on one phase is indicated by voltage collapse in that
phase. The other healthy phase will still have normal phase-to-earth voltage. The residual sum
will have the same value as the remaining phase-to-earth voltage, which is shown in Figure
116.
Section 8 1MRK 505 370-UEN D
Voltage protection
214 Busbar protection REB670
Application manual
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