F20
Twin Earth Fault Indicator
© Nortech Management Ltd, United Kingdom
Page 7 of 16
3
Operation worked examples
The examples below illustrate how applying electrical reset can affect
detection and indication of faults.
The examples assume the F20 is configured with a 3 hour indication period
and consider only one Earth Fault element.
The three faults occur at 2 hour intervals. After Fault 1 supplies are lost for 1
hour. After Fault 2 supplies are lost for 5 hours except for a few milliseconds
while Fault 3 is on the network.
3.1
Indication with no electrical reset
time
fault current
time
LED indication
and relay contacts
3 hours
3 hours
Fault 1
Fault 2
Fault 3
Unit starts
indication for
fault 1
Fault 2 not detected as
already indicating
Reset at end of 3 hour
timer
Indications with timer reset only
3.2
Indication with AC reset signal
time
fault current
time
LED indication
and relay contacts
time
AC supply status
1 hour
3 hours
1 hour
Fault 1
Fault 2
Fault 3
AC supply restored
Reset occurs 10 seconds
after AC is restored
No Reset as AC is on for
<10 seconds
AC supply lost when fault
occurs
AC supply restored
AC on for less than 10
seconds
Indications with AC reset connected
