Thermal Overload Element Thr - Toshiba GRT100 Series Instruction Manual

2.11.6 Thermal Overload Element THR

Thermal overload element THR has a characteristic based on thermal replica according to IEC
60255-8 standards (see Appendix N), which evaluates the phase current (A-phase) of CT
secondary circuits. Figure 2.11.4 shows the characteristic of THR element. The element has trip
and alarm stages.
Trip stage:
I 2
t =   Ln
 (k  I
I 2
Alarm stage:
 Ip
(I 2
t =   Ln
where
t : operating time
 :
thermal time constant
I : load current
kI 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 2.11.5 shows the thermal curve for a range of time constant settings under cold state when
the prior load current Ip is zero.
Figure 2.11.4 Characteristic of Thermal Overload Element
 Ip
2
) 2
B
)  (1  T A /)
2
 (k  I
I 2 ) 2
B
t
K  I B
0
 42 
T A
Trip
Alarm
T A
I
6 F 2 S 0 7 8 9