9023927 - 2nd edition - May 2007
Page 9 of 72 Pages
Installing Dräger Polytron 7500
– Connecting to the central unit with a central power supply
●
Connect shield to GND of central unit (e.g. housing, earth bar, etc.).
Maximum line lengths
Installing a remote central unit with power supply by 3-wire connection the voltage
drop due to the line resistance [R
line
] has to be taken in consideration.
In order to detect a Pyrolyzer failure in the central unit [I
F
] and the current drawn by
the Pyrolyzer in a fault situation (250 mA) combined with the cable cross-section [A]
and the shunt resistor [R
shunt
] will result in a maximum cable length [L
max
].
R
line
= (R
shunt
x I
F
) / 250 mA
L
max
= R
line
x A / (0.0175 Ohm x mm
2
/ m)
Example:
The following is valid with a permitted error current [I
F
] of 3.15 mA:
In a given situation the cable length [L
max
] can be increased by a series connection
of a resistor [R
M
] into the cable for the 4 to 20 mA signal.
R
schunt new
= R
shunt old
+ R
M
Following limitations have to be fulfilled:
R
line
+ R
shunt
= between 30 Ohm and 500 Ohm,
For currents of 3 and 22 mA, a DC voltage between 16.5 V DC (3 mA), or 8.0 V DC
(22 mA) must be present at the unit (between the ter24 V and 4...20), see
page 8.
@ R
shunt
= 50 Ohm
@ R
shunt
= 100 Ohm
@ R
shunt
= 250 Ohm
A
L
max
A
L
max
A
L
max
0.50 mm
2
≥
18.23 m
0.50 mm
2
≥
36.46 m
0.50 mm
2
≥
91.15 m
0.75 mm
2
≥
27.34 m
0.75 mm
2
≥
54.69 m
0.75 mm
2
≥
136.70 m
1.50 mm
2
≥
54.69 m
1.50 mm
2
≥
109.38 m
1.50 mm
2
≥
273.45 m
2.50 mm
2
≥
91.15 m
2.50 mm
2
≥
182.30 m
2.50 mm
2
≥
455.75 m
0042392
7_1_en.eps
Central unit
4 ... 20 mA
0 V
+
–
+
R
shunt
R
M
