GE D30 Instruction Manual page 384

CONTROL ELEMENTS
The reset time of the thermal overload protection element is also time delayed using following formula:
where
τ = thermal protection trip time constant
rst
T = a minimum reset time setting
min
5
The thermal overload protection element estimates accumulated thermal energy E using the following equations
calculated each power cycle. When current is greater than the pickup level, I
thermal energy:
5-252
 (
kI
B
------------------------ -
t = τ × ln 
rst rst
2

I (
–
Figure 5-142: IEC 255-8 sample operate and reset curves
E = E + ------------- -
n n 1
– t
2
) 
 + T
min
2

kI )
B
> k × I , element starts increasing the
n B
t Δ
op In ( )
D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
Eq. 5-23
Eq. 5-24