38
TB MRI3-ITE(R) 12.00 E
7.4
Tripping characteristics
7.4.1
Thermal characteristic
From the equation for the temperature equivalent:
ϑ
ϑ
τ
( )
(
)
(
)
t
I
k I
I
k I
e
B
B
t
=
⋅
⋅
+
−
⋅
⋅
⋅
−
2
2
0
2
2
100%
100%
it is possible to deduce the tripping characteristics according to IEC 155-8.
The temperature equivalent of the preload
ϑ
0
equals the preload current I
P
:
ϑ
0
2
2
100%
=
⋅
⋅
I
k I
P
B
(
)
Tripping is realized as soon as the temperature equivalent
ϑ
= k
2
⋅
100% is reached. This temperature equivalent
equals a permanent load current of k
⋅
I
B
:
ϑ
Trip
= k
2
⋅
100%
For the tripping characteristic:
TRIP
t
2
B
2
2
B
2
P
2
B
2
TRIP
e
%
100
)
I
k
(
I
%
100
)
I
k
(
I
%
100
)
I
k
(
I
%
100
−
⋅
⋅
⋅
−
⋅
⋅
+
⋅
⋅
=
=
or:
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
)
k I
I
I
I
e
e
I
I
k I
I
I
I
I
k I
B
P
t
t
P
B
P
B
TRIP
TRIP
⋅
= +
−
⋅
=
−
⋅
−
=
−
− ⋅
−
2
2
2
2
2
2
2
2
2
2
2
2
τ
τ
transpose to
TRIP
t
e
(
)
(
)
t
I
I
I
k I
I
I
p
I
I
k
TRIP
P
B
B
B
= ⋅
−
− ⋅
= ⋅
−
−
τ
τ
ln
(
)
ln
2
2
2
2
2
2
2
2
2
2
with p
I
I
P
B
=
The result are the tripping characteristics according to IEC 255-8:
t
I
I
p
I
I
k
I
I
k
TRIP
B
B
B
= ⋅
−
−
<
∩
≤
τ
ln
for p
p
2
2
2
2
2
2
2
2
2
2
2
with
τ
= thermal time constant of the object to be protected
I
= relay current
I
B
= Basic current
I
P
= preload current
p = preload factor (p = 0 means cold condition of the electrical item)
k = constant
