GE L90 Instruction Manual page 536

CONTROL ELEMENTS
The thermal overload protection element corresponds to the IEC 255-8 standard and is used to detect thermal overload
conditions in protected power system elements. Choosing an appropriate time constant element can be used to protect
different elements of the power system. The cold curve characteristic is applied when the previous averaged load current
over the last 5 cycles is less than 10% of the base current. If this current is greater or equal than 10% than the base
current, then the hot curve characteristic is applied.
The IEC255-8 cold curve is defined as follows:
The IEC255-8 hot curve is defined as follows:
where
t = time to operate
op
τ = thermal protection trip time constant
op
I = measured overload RMS current
I = measured load RMS current before overload occurs
p
k = IEC 255-8 k-factor applied to I
I = protected element base (nominal) current
B
To ensure element accuracy for high overcurrent conditions, the maximum value of I/(k x I
5 realistically it is exceeding this value.
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
5-350
, defining maximum permissible current above nominal current
B
) is limited to 8, even when
B
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
Eq. 5-44
Eq. 5-45
Eq. 5-46