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– 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
In order to detect a Pyrolyzer failure in the central unit [I
the Pyrolyzer in a fault situation (250 mA) combined with the cable cross-section [A]
and the shunt resistor [R
shunt
R
= (R
x I
) / 250 mA
line
shunt
F
L
= R
x A / (0.0175 Ohm x mm
max
line
Example:
The following is valid with a permitted error current [I
@ R
= 50 Ohm
shunt
A
L
max
≥18.23 m
2
0.50 mm
≥27.34 m
2
0.75 mm
≥54.69 m
2
1.50 mm
≥91.15 m
2
2.50 mm
In a given situation the cable length [L
of a resistor [R
] into the cable for the 4 to 20 mA signal.
M
R
= R
+ R
schunt new
shunt old
M
Following limitations have to be fulfilled:
R
+ R
= between 30 Ohm and 500 Ohm,
line
shunt
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 terminals +24 V and 4...20), see
page 8.
9023927 - 2nd edition - May 2007
] has to be taken in consideration.
line
F
] will result in a maximum cable length [L
2
/ m)
] of 3.15 mA:
F
@ R
= 100 Ohm
shunt
A
L
max
≥36.46 m
2
0.50 mm
≥54.69 m
2
0.75 mm
≥109.38 m
2
1.50 mm
≥182.30 m
2
2.50 mm
] can be increased by a series connection
max
] and the current drawn by
].
max
@ R
= 250 Ohm
shunt
A
L
max
≥91.15 m
2
0.50 mm
≥136.70 m
2
0.75 mm
≥273.45 m
2
1.50 mm
≥455.75 m
2
2.50 mm
Installing Dräger Polytron 7500
+
4 ... 20 mA
R
M
+
R
shunt
–
0 V
Central unit
Page 9 of 72 Pages
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