WIKA TR10-W Mode D'emploi page 13

7. Notes regarding the safety for the different versions
WIKA recommends the power of the head-mounted transmitter is limited by a
matched pre-fuse in the 4 ... 20 mA current loop. In case of a fault in the head-
mounted transmitter, the pre-fuse will trip and break the electric circuit.
Example to size the pre-fuse for a maximum power of 0.8 watts at the
sensor:
The internal resistance of thermocouples is considerably lower than the tempera-
ture-dependent resistance of a Pt100 sensor and therefore the more unfavourable
value for the resistance thermometer is calculated.
P = (1.7 x I )² x Rw
adm s
I = Rated current for fuse
s
P = maximum power at the sensor = 0.8 W
adm
R = Sensor resistance (temperature dependent)
w
at 450 °C = 264.18 Ω per DIN EN 60 751 for Pt100.
The nominal current of the fuse is calculated as follows:
I = sqrt (P / R
s adm
I = sqrt (0.8 W / 265 Ω) / 1.7
s
I = 32.32 mA
s
From this, a rated current for a miniature fuse/fast acting fuse = 32 mA is calculated.
Information regarding the fuse calculation:
Always chose the next smallest fuse value in accordance with IEC 60127. The
breaking capacity must be matched to the power supply in a technically sound
way. Usual values for such miniature fuse/fast acting fuses range between a
rated breaking capacity of AC 20 A and AC 80 A.
The following value results from the max. power at the sensor of 0.5 W:
I = sqrt (0.5 W / 265 Ω) / 1.7
s
I = 25.55 mA
s
From this, a rated current for a miniature fuse/fast acting fuse = 25 mA is calculated.
If multiple sensors are used simultaneously, the sum of the individual powers
must not exceed the value of the maximum allowed power.
Internal resistance of TC measuring inserts Ø 6 mm: approx. 1.2 Ω/m
Internal resistance of TC measuring inserts Ø 3 mm: approx. 5.6 Ω/m
The measured values apply to room temperature.
7.2.3 Temperature classes, ambient temperatures
Heating in the connection head with Version 2 can occur due to a failure in the
electronics. The permissible ambient temperatures depend on the housing used
and the additional integral head-mounted transmitters.
WIKA operating instructions models TR10-W, TC10-W
) / 1.7
w
GB
13