WIKA TR10-W Betriebsanleitung Seite 19

8. Safety-related instructions for the different variants
Use with a thermowell at the partition to zone 0, marking II 1/2G Ex d IIC T1-T6 Ga/Gb
The flameproof housing or the connection head is in zone 1 (or zone 2). The sensor is
within a thermowell (min. wall thickness 1 mm) which extends into zone 0 via a process
connection.
The thermometer should be operated with a power-limiting circuit.
P : 2 W U : 30 V
max max
A power supply with Ex ia circuits meets these conditions, but is not required if the limits
can be achieved with other measures. The responsibility rests with the operator.
WIKA recommends realising the power limitation through a suitable fuse in the 4 ... 20 mA
current loop of the head-mounted transmitter. In the event of a failure of the head-mounted
transmitter, the circuit will be interrupted through the fuse tripping.
Example for calculating the fuse for a maximum power at the sensor of 0.8 Watt:
The internal resistance of thermocouples is significantly lower than the thermal resistance
of a Pt100 sensor, so the much less favourable case for a resistance thermometer has been
calculated.
P = (1.7 x Is)² x R
all w
I = Fuse rating
s
P = maximum power at sensor = 0.8 W
all
R = Resistance of the sensor (temperature-dependent)
w
at 450 °C = 264.18 Ω in accordance with DIN EN 60751 for Pt100
The following defines the fuse rating:
I = sqrt (P / R ) / 1.7
s all w
I = sqrt (0.8 W / 265 Ω) / 1.7
s
I = 32.32 mA
s
This results in a rated current for a fuse link = 32 mA
Notes for fuse calculation:
The next smallest fuse value, in accordance with IEC 60127, must always be chosen. The
breaking capacity must be matched, by sensible engineering, to the voltage supply. Usual
values for such fuse links lie between AC 20 A and AC 80 A rated breaking capacity.
For a maximum power at the sensor of 0.5 W the following value is given:
I = sqrt (0.5 W / 265 Ω) / 1.7
s
I = 25.55 mA
s
This results in a rated current for a fuse link = 25 mA
When using multiple sensors and simultaneous operation, the sum of the individual powers
must not exceed the value of the maximum permissible power.
Internal resistance of Ø 6 mm TC measuring inserts: approx. 1.2 Ω/m
Internal resistance of Ø 3 mm TC measuring inserts: approx. 5.6 Ω/m
These measured values are valid for room temperature.
WIKA operating instructions models TR10-W, TC10-W (Ex d)
GB
19