Direct Earthed System - ABB RELION 670 Series Applications Manual

1MRK 504 152-UEN B
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 277.
IEC07000190 V1 EN-US
Figure 277: Earth fault in Non-effectively earthed systems
9.3.3.5

Direct earthed system

In direct earthed systems, an earth fault on one phase indicates a voltage collapse in that
phase. The two healthy phases will have normal phase-to-earth voltages. The residual sum will
have the same value as the remaining phase-to-earth voltage. See figure 278.
Application manual
Section 9
Voltage protection
GUID-EA622F55-7978-4D1C-9AF7-2BAB5628070A v7
473