Allen-Bradley Guardmaster, User Manual

Page: 72 / 100

72 Rockwell Automation Publication 440R-UM013E-EN-P - March 2017
Chapter 11 Troubleshooting
Long Wire — Resistance Effect
Long input wiring adds resistance and decreases the input signal voltage.
Figure 67 shows the effects on the pulse tests. Notice that the shape of the
pulses has not changed, only the amplitude. If the wiring is too long, the safety
relay does not turn ON.
Figure 67 - Resistance Effect on Input Signal
GSR relays have a maximum input resistance of 900 ohms.
Table 16 shows the resistance of wire sizes that are typically used to connect to
safety devices. For example, if you used 200 m (656.2 ft) of a 4-wire cable that
contains 22 AWG wire, the wire resistance from the relay pulse test outputs to
the safety device and back to the relay inputs, would be:
200 m x 52.94 Ω / 1000 m x 2 directions = 21 Ω
As the example shows, long wires are not a likely cause of safety input issues.
Table 16 - Wire Resistance
ISO Cross Section [mm²] AWG Ω Per 1000 m Ω Per 1000 ft
0.33 22 52.94 16.14
0.5 20 33.30 10.15
0.75 18 20.95 6.386
1.5 16 13.18 4.016
2.5 14 8.28 2.525
4 12 5.21 1.588