Alarm
Applications/Failover Option
A-10
SIMATIC PCS 7 OSx 4.0.0 Hardware (PA RISC)
Connecting
Options: Failover Cir
cuit (continued)
To
calculate the maximum length of the wiring used for failover cable
wiring, you must know the resistance of the wire used. For example, use
28
A
WG cable and a specified resistance of 74 ohms/1000 ft., and calculate
as follows.
1.
Calculate the total current required for all OSx stations.
The worst case is 16 stations in a system, with the first station as
primary
. The remaining 15 stations require 14 mA of current at 24
volts for each station. The total current moving through the cable to the
remote stations is 210 mA.
2.
Calculate the resistance allowed for voltage drop:
W
ith a nominal voltage of 24 volts, and a minimum input of 15 volts,
the allowable drop is
24 volts –15 volts = 9 volts
3.
Calculate the allowable resistance:
9 volts / 210 mA = 42 ohms
4.
Calculate the length of cable allowed:
Distance (D) at 74 ohms per thousand feet:
(D / 42) = (1000 / 74)
D = (1000 / 74) * 42
D = 567 ft.
Since there are two wires involved in the closed-loop circuit, you divide the
resulting distance by two in order to get the length of the cable you need to
use:
D = 567 ft. / 2 = 283 ft.
If you need more cable length, use larger wire.
NOTE:
A
void routing the failover wiring near high voltage cables (480 volts
or higher), as well as other sources of noise that could adversely affect the
signals.
Failover Cir
cuit
Length Calculation
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