Wire Color

Dumb question, Hooking up two wire reed switches out in the field. They are wired into sinking input cards. Once the wires leave the panel, do both wires become blue to and from the device back to the panel, or does is go out wht/blu to the device and come back as blue? Or out and back as wht/blu? Thanks, we are trying to settle an argument.

We would use blue going to the switch and blue going back.

We mainly use Allen Bradley, but if I remember correctly some controllers define sinking and sourcing differently. So my rule of thumb is to look for how it is hooked up. If I need to wire my -24VDC (common) to the common terminal of the card my wires coming in from the inputs are blue, if I need to put my +24VDC to the common terminal of the card my wires coming in from my inputs will be wht/blue.

Sinking input means the reed switch provides +24 to the PLC input.

That would either be blue for both leads, or if you use brown for +24 then brown to the switch and blue from it. I commonly use 2 conductor cable for a lot of field devices that is Brown/Blue & brown is + usually.

Depends entirely on your wire colour standard.

For most of my customers, the wire colour would be determined by the voltage intended to be applied to it when there is voltage applied to it. If I wire 24VDC through a switching device to a PNP input, the wire going to the switching device and the wire coming back are coloured for 24VDC, as that's the voltage that will be applied to it by the switching device. It makes no difference what the switching device is; pushbutton, reed switch, whatever. It only matters what voltage is applied to the wire.

Likewise if I wire 0VDC through a switching device to an NPN input, the wire going to the switching device and the wire coming back are coloured for 0VDC. When the wire has voltage applied to it, that applied voltage is 0VDC. Sure, you may measure another voltage on it when the reed switch is off, but that's not the intended voltage of the wire, that's just the "floating" voltage because it's not connected to its intended voltage source.

And then I have one customer who specifies that 24VDC is one colour, 0VDC is another colour, digital inputs are a different colour again, and digital outputs are a fourth different colour. So in their case, the wire coming back from a switching device to a digital input would be the same colour regardless of whether it was switching 24VDC or 0VDC.

That led me to an interesting discussion with a colleague the other day, about what the correct wire colour should be for a safety test pulse on a safety input module, according to that colour scheme.

The panel builders thought they should be the digital input colour, since it was connected to a digital input module and associated with a digital input.

Someone else countered that it's not a digital input, it's the source voltage that is switched to the digital input, and the voltage in question is 24VDC, so it really should be the 24VDC colour.

To which I noted "Well Ackshually", the 24VDC is pulsed on and off in a certain pattern so it's not strictly a 24VDC wire, it's probably techically a digital output of sorts. So if we were to be as technically correct as possible, it should be the colour of a digital output.

But then we all agreed that using the digital output colour on a wire that ran from a digital input module to an e/stop would be confusing as **** for the maintenance electricians, and that the most logical option was to use the 24VDC option.

ASF said:

...
But then we all agreed that using the digital output colour on a wire that ran from a digital input module to an e/stop would be confusing as **** for the maintenance electricians, and that the most logical option was to use the 24VDC option.

Click to expand...

It might make sense to use both colors, like the digital output as the base color on the wire and digital input as a stripe/tracer. Or suggest, like I've seen done, a completely different color for safety wiring.

Blue only in auto fields.