Siemens siprotec SJ62 User Manual page 182

Time Constant τ
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
with
I
max prim
I
Nom Obj.
I
Nom CT prim
Example: Motor and transformer with the following data:
Permissible Continuous Current
Nominal Motor Current
Current Transformer
The overload protection tracks overtemperature progression, employing a thermal dif-
ferential equation whose steady state solution is an exponential function. The TIME
CONSTANT τ (set at address 4203) is used in the calculation to determine the thresh-
th
old of overtemperature and thus, the tripping temperature.
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 with the formula:
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 by the given duration. For example, if the 0.5-second
current rating is known:
It is important to note, however, that the longer the effective duration, the less accurate
the result.
2.10 Thermal Overload Protection 49
Permissible thermal primary current of the motor
Nominal current of the protected object
Nominal primary CT current
I
max prim
I
Nom Obj.
1200 A / 1 A
= 1.2 · I
Nom Obj.
= 1100 A
180