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2.10.5 Definite Time Overcurrent element OC and EF
The OC and EF elements measure the phase currents and the residual current respectively.
2.10.6 Thermal Overload Element THR
Thermal overload element THR has a characteristic based on thermal replica according to the IEC
60255-8 standard (see Appendix N), which evaluates the phase current (A-phase) of the CT
secondary circuits. Figure 2.10.5 shows the characteristic of THR element. The element has trip
and alarm stages.
Trip stage:
t = τ x Ln
I
2
− Ip
2
I
2
− (k x I
B
)
2
Alarm stage:
t = τ x Ln
(I
2
− Ip
2
) x (1 − T
A
/τ)
I
2
− (k x I
B
)
2
where
t : operating time
τ : thermal time constant
I : load current
kxI
B
: allowable overload current as specified in IEC 60255-8 (refer to Appendix N)
I
B
: basic current of transformer (rated current)
k : constant (allowable overload current / I
B
)
Ip : prior load current before the overload occurs
T
A
: time for alarm
(Alarming time = t (operating time) – T
A (setting time)
Ln : natural logarithm
Figure 2.10.6 shows the thermal curve for a range of time constant settings in the cold state when
the prior load current Ip is zero.