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Electrical connection Page 14/ 47
5.2 Maximum Length of the Connection Cable
The permissible total resistance of the cable and the load (see chapter 5.1.1 ) must not be
exceeded.
The cable (length and cross-section) must be chosen so that the supply voltage
will not fall below the sensor-specific minimum voltage (12 V) in the case of
the highest current consumption (21.5 mA).
Always make sure that the permissible external capacitance (C
o
) and inductance
(L
o
) of the associated apparatus are not exceeded (refer to the electrical data in
the EU-Type Examination Certificate of the apparatus) if the intrinsically safe
level sensor is used in a potentially explosive atmosphere.
The following table shows the resistances of different copper wire cross-sections:
Cable cross-section Q [mm²] Cable resistance R
Q
per m copper cable [ Ω/ m]
0.5 0.0356
1.0 0.0178
1.5 0.0119
Example: A 1000 m long, cross- section 0.5 mm² copper cable with outgoing and return
lines (2 -core) has a cable resistance of approximately 70 Ω .
The maximum cable length is calculated as follows:
L = (((U – U
min
) / I
max
) – R
B
) / R
Q
L = Cable length [m]
U = Supply voltage [V] (with negative tolerance value -
5 %)
U
min
= Minimum supply voltage [V] = 12 V
DC
I
max
= Maximum power consumption [A] = 0,0215 A
R
B
= Load
R
Q
= Cable resistance per m cable [ Ω/ m] for cable cross- section Q [mm²]
Example:
Supply voltage = 24 V
DC
(± 5 %), supply voltage U = 22.8 V
DC
(24 V
DC
- 5 %),
minimum supply voltage U
min
= 12 V
DC
, current consumption I
max
= 0.0215 A,
load R
B
= 100 Ω, cable cross- section Q = 0.5 mm², cable resistance R
Q
= 0.0356 Ω/ m
A copper cable with outgoing and return lines (2 - core) could theoretically be up to 5650 m
long with the parameters of this example.