ABB RELION REX640 Technical Manual page 1182

Protection related functions
Figure 674: Validation of PHIZ on sand
5.4.6 Application
PHIZ is used to detect a downed conductor dropping to a very resistive ground,
causing an earth fault which is very difficult to detect by a conventional protection
relay functionality. PHIZ is then targeted to be used with overhead lines. PHIZ must
be used in electrical networks with efficiently grounded or isolated neutral.
Electric power lines experience faults for many reasons. In most cases, electrical
faults manifest in mechanical damage, which must be repaired before returning the
line to service.
Most of the electrical network faults are earth faults. Conventional protection
systems based on overcurrent, impedance or other principles are suitable for
detecting relatively low-impedance faults which have a relatively high fault current.
However, a small percentage of the earth faults have a very large impedance.
They are comparable to load impedance and consequently have very little fault
current. These high-impedance faults do not pose imminent danger to power
system equipment. However, they are a considerable threat to people and property.
The IEEE Power System Relay Committee working group on High Impedance Fault
Detection Technology defines High Impedance Faults as those that 'do not produce
enough fault current to be detectable by conventional overcurrent relays or fuses.
High-impedance fault (PHIZ) detection requires a different approach than that for
conventional low-impedance faults. Reliable detection of PHIZ provides safety to
humans and animals. ABB has developed innovative technology for highimpedance
fault detection with over ten years of research resulting in many successful field
tests.
5.4.7 Signals
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PHIZ always needs sensitive Io measurement.
Figure 675: Validation of PHIZ on grass
1MRS759142 F
REX640
Technical Manual