Siemens SIPROTEC 7SJ62 Instruction Manual page 243

Time Constant τ
Warning Tempera-
ture Level
7SJ62 Manual
C53000-G1140-C121-1
The thermal overload protection element tracks excessive temperature progression,
employing a thermal differential equation whose solution is an exponential function.
The 7,0( &2167$17 τ (set at address ) is used in the calculation to determine
the operating temperature. This is expressed as a maximum allowable operating tem-
perature.
For cable protection, the heat-gain time constant τ is determined by cable specifica-
tions and by the cable environment. If no time-constant specification is available, it
may be determined from the short-term load capability of the cable. The 1-sec current
(i.e. the maximum current permissible for a one-second period of time), is often known
or available from tables. Then, the time constant may be calculated from the formula:
1
Set Value τ (min)
----- -
=
60
If the short-term load capability is given for an interval other than one sec, the corre-
sponding short-term current is used in the above formula instead of the 1-sec current,
and the result is multiplied times the given duration. For example, if the 0.5-second
current rating is known,
0.5
Set Value τ (min)
------- -
=
60
It is important to note, however, that the longer the effective duration, the less accurate
the result.
Example:
Cable and current transformer with the following data:
Continuous permissible current
Maximum current for 1 sec
Current transformer
The k factor and time constant are calculated as follows:
I
500 A
max
---------- - -------------- -
k = = =
I 600 A
N
τ
min
The settings are: .)$&725 (Address )
7,0( &2167$17
By setting the thermal warning level  Θ $/$50 at address , a warning mes-
sage can be issued prior to tripping, thus allowing time for load curtailment procedures
to be implemented. This warning level simultaneously represents the dropout level for
the tripping signal. In other words, the tripping signal is interrupted only when the cal-
2
I
×
1 sec
--------------------- -
I
max Prim
2
I
×
0.5 sec
--------------------- -
I
max Prim
I = 500 A @ 40
max
I1s = 45 * Imax = 22.5 kA
600 A / 1 A
0.833
I
 
1 1
2
1s
⋅ ⋅
----- - ---------- - ----- - 45
= =
 
60 I 60
max
= 
=  min
Functions
o
C
33.75 min
=
6-103