A thermal overload will often not be detected by other protection functions and the
introduction of the thermal overload protection can allow the protected circuit to operate
closer to the thermal limits.
The three-phase current measuring protection has an I
2
t characteristic with settable time
constant and a thermal memory. The temperature is displayed in either in Celsius or in
Fahrenheit depending on whether the function used is Thermal overload protection one
time constant, Fahrenheit LFPTTR (26) or Celsius LCPTTR.
An alarm pickup gives early warning to allow operators to take action well before the line
is tripped.
Estimated time to trip before operation, and estimated time to reclose after operation are
presented.
6.6.3 Function block
LFPTTR (26)
I3P*
BLOCK
AMBTEMP
SENSFLT
RESET
TRIP
RI
ALARM
LOCKOUT
TEMP
TEMPAMB
TERMLOAD
ANSI11000246-1-en.vsd
ANSI11000246 V1 EN
LCPTTR (26)
I3P*
BLOCK
AMBTEMP
SENSFLT
RESET
TRIP
RI
ALARM
LOCKOUT
TEMP
TEMPAMB
TERMLOAD
ANSI08000038-2-en.vsd
ANSI08000038 V2 EN
Figure 50: LFPTTR/LCPTTR (26) function block
1MRK 511 287-UUS A Section 6
Current protection
129
Technical manual
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