Tom Jenkins
Lifetime Supporting Member
I don't know if using 24 VDC started in Europe, but it became dominant there before it was common in the US. Remember, in Europe normal AC is 230 Volt. That makes the safety difference at 24 VDC more significant.
I have been using 24 VDC as much as possible for several years. A few engineering firms call for 120 VAC in their specs, but none of them can give a vaild reason. "24 VDC can't give accurate switching on long wire runs." Bull**** - just size the wire properly. "Our existing inventory is 120 VAC." Nonesense - most I/O cards are availale in stock from suppliers and I/O cards are cheap to inventory. "Our standard is 120 VAC" Dumbest reason of all - doing something because you always do it that way isn't engineering, it's superstition.
Advantages of 24 VDC:
1) Safety
2) For mciros and small PLCs I use 24 VDC for CPU power because it is one more layer keeping spikes, surges, and brownouts from my PLC
3) I can run DC and analog sgnals in the same conduit.
4) The hardware is usually less expensive.
5) Safety again
I have been using 24 VDC as much as possible for several years. A few engineering firms call for 120 VAC in their specs, but none of them can give a vaild reason. "24 VDC can't give accurate switching on long wire runs." Bull**** - just size the wire properly. "Our existing inventory is 120 VAC." Nonesense - most I/O cards are availale in stock from suppliers and I/O cards are cheap to inventory. "Our standard is 120 VAC" Dumbest reason of all - doing something because you always do it that way isn't engineering, it's superstition.
Advantages of 24 VDC:
1) Safety
2) For mciros and small PLCs I use 24 VDC for CPU power because it is one more layer keeping spikes, surges, and brownouts from my PLC
3) I can run DC and analog sgnals in the same conduit.
4) The hardware is usually less expensive.
5) Safety again