24
Earth Fault Protection
Earth fault protection is available on M-PRO 30 and M-PRO 40
trip units. Any combination of unrestricted earth fault (UEF),
restricted earth fault (REF) and standby earthfault (SEF) may
be specified for factory configuration. Measurement is
achieved by neutral Rogowski coil (mounted internally or
externally) and/or earth leg CT. Details of standard circuit
configurations and measurement device location are available
on request.
The tables below summarise the recommended measurement
techniques:
3 – wire systems
Earth fault
protection
required
Rogowski
coil mounting
CT
mounting
UEF
Not required
Not required
REF
Not required
Earth Leg
ELP
Not required
Earth Leg
UEF & REF
Not required
Earth Leg
A summary of the recommended measurement techniques for
a 4-wire system is:
4 – wire systems
Earth fault
protection
required
Rogowski
coil mounting
CT
mounting
UEF (option 1)
Unrestricted
neutral
Not required
UEF (option 2)
Restricted
neutral
Earth Leg
REF (option 1)
Restricted
neutral
Not required
REF (option 2)
Unrestricted
neutral
Earth Leg
ELP
Not required
Earth Leg
UEF & REF
Unrestricted
neutral
Earth Leg
UEF & REF
Restricted
neutral
Earth Leg
Earth Fault Current
All earth fault protection pick-up levels are adjustable as
follows: Off, 10 to 100% x In in steps of 1% of In. Earth fault
current pick-up can be set for Restricted Earth Fault,
Unrestricted Earth Fault and Standby Earth Fault depending
on the options specified at the time of ordering.
Earth Fault Trip Time Delay
fixed time delay for earth fault protection can be set on the M-
PRO. Adjustable steps are as follows: Instantaneous to 1 sec
in steps of 0.1 sec.
Earth fault trip time delays apply only to Unrestricted Earth
Fault and Standby Earth Fault. REF is factory set to
instantaneous.
Earth Fault Cropping
A cropping factor can be applied to the earth fault protection
curves. The factor may be set by choosing the multiple, Cr of
the pick-up current at which the fixed time delay operates. The
tripping time is calculated as follows:
t = t
f
x C
r
x 1
p
1
t
=
time to trip
t
f
=
fixed time delay
C
r
=
cropping multiple
I
p
=
pick-up current
I
=
measure current
Selectable settings for Cr are OFF, 1.5, 2, 2.5, 3, 4, 5 and 6.
Note that the cropping feature does not apply to instantaneous
trips.
Earth Fault cropping applies only to Unrestricted Earth Fault
and Standby Earth Fault.
Earth Fault Inter-trip and alarm contacts
Earth fault protection is available on M-PRO 30 and M-PRO 40
trip units; these units have digital outputs fitted as standard. All
earth faults energise output 3, however restricted earth faults
may be configured NOT to energise the output. This feature is
useful for automatic changeover systems.
Restricted earth faults always energise digital output 4 to
provide a means of inter-tripping the upstream (usually high
voltage breaker). Standby earth faults may be configured to
also activate this output.
Current
Current
Current
Time
Time
Time
