CHAPTER 5: SETTINGS
CONTROL ELEMENTS
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
5-295
5
Figure 5-164: IEC 255-8 sample operate and reset curves
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
n
> k × I
B
, element starts increasing the
thermal energy:
Eq. 5-30
When current is less than the dropout level, I
n
> 0.97 × k × I
B
, the element starts decreasing the thermal energy:
Eq. 5-31
where
∆
t is the power cycle duration
n is the power cycle index
t
op(In)
is the trip time calculated at index n as per the IEC255-8 cold curve or hot curve equations
t
rst(In)
is the reset time calculated at index n as per the reset time equation
0.1
1
10
100
I / Ipkp
t (min)
0.01
0.1
1
10
100
τ
= 30
rst
τ
= 30
op
T
= 10
min
827724A1.CDR
E
n
E
n 1
–
t
Δ
t
op In
( )
--------------
+
=
E
n
E
n 1
–
t
Δ
t
rst In
( )
--------------
–
=
