Trusted 40 Channel 120 Vac Digital Input FTA
4. Cable Capacitance
Rockwell Automation Publication ICSTT-RM290F-EN-P
Issue 7
9
4.
Cable Capacitance
With long field cables the capacitive coupling of the cable can cause a significant voltage to
be seen at the T8424 input when the field switch is open. This causes the T8424 to report
that the field switch is still on when it is turned off.
Revision E T8824 FTAs have lower loop resistance, which reduces the capacitive noise and
increases the available loop length.
The table below shows some estimated maximum lengths for different cables and mains
supplies. Practical distances will be less than this due to other noise and for switching safety
margins.
Revision E
Earlier revisions
15 pF/m, 110 V, 50 Hz
2940 m
890 m
30 pF/m, 110 V, 50 Hz
1470 m
470 m
15 pF/m, 110 V, 60 Hz
2450 m
790 m
30 pF/m, 110 V, 60 Hz
1220 m
390 m
15 pF/m, 120 V, 50 Hz
2620 m
850 m
30 pF/m, 120 V, 50 Hz
1310 m
420 m
15 pF/m, 120 V, 60 Hz
2190 m
700 m
30 pF/m, 120 V, 60 Hz
1090 m
350 m
Table 1 Theoretical maximum cable lengths
