Schweitzer Engineering Laboratories SEL-751 - Page 219

4.93
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
Lastly, the relay computes the ambient temperature factor, FAMB, according
to Equation 4.9:
Equation 4.9
The thermal capacity used is a percentage representation of how close to the
thermal limit the protected power apparatus is. It is calculated using
Equation 4.10.
Equation 4.10
If , the relay calculates the time
before trip using Equation 4.11. This indicates an overload condition before
the thermal level reaches the alarm or tripping threshold. The time indication
allows an operator to take appropriate actions ahead of time.
Equation 4.11
If , the relay reports 9999 seconds.
If the thermal lockout Relay Word bit THRLT is asserted and
THIEQ < IEQPU, the relay calculates the time to release the thermal element
using Equation 4.12.
Equation 4.12
Otherwise, the relay does not calculate the time to lockout release.
where:
= User-selectable maximum operating temperature of
the equipment
= Ambient temperature measurement from user-
selectable temperature probe
FAMB
TMAX 40C–
TMAX TAMB–
---------------------------------------=
TMAX
TAMB
THTCU
t
THRL
t
THLT 100
--------------------------------- 100=
FAMB THIEQ
t
IMC
2
THLT 100
THTRIP
t
TCONH x ln
FAMB THIEQ
t
IMC
2
 THRL
t
–
FAMB THIEQ
t
IMC
2
 THLT 100–
-----------------------------------------------------------------------------------------------------



=
FAMB THIEQ
t
IMC
2
 THLT 100
THRLS
t
TCONC x ln
THRL
t
THLTDR THLT 100
----------------------------------------------------------


=