What is a Common Wire

What is a common wire? I have seen the Neutral of a 120 volt ac circuit called a common and I have also seen the power wire going to a field device called a common.

Like a limit switch in the field which has a 120 ac wire going to it and the wire coming back to the plc would be labeled as the input it goes to and the description may be "Door Closed Switch" and the 120 volt wire going to the limit switch would be labeled with the correct wire number but may have a description of "Door Closed Switch Common"

Which is correct or is it both?

Probably depends on who you talk to,.

To me in a PNP system, 0V is common, in a NPN system 24V is common..

To me "common" just means a wire or signal shared by more than one device. Limit switches generally have a "common" terminal that is shared by both the NO and NC contacts and is usually at a voltage above zero. A neutral wire can also be common to several devices.

The term can cause problems. I once had an electrician argue that he could use white for the 120 VAC to limit switches because it was a "common" and that made it OK. (Of course, he screwed up a lot of things on that project!)

The common wire is the one shared among multiple devices. In some cases it could be the positive side of the DC circuit, in other cases the negative side. I some AC circuits it could be the hot side, in others the neutral.

Tim not sure if this is what you meant mate but this might help,

Often limit switches will have a set of contacts, there can be normally open and normally closed contacts, or just one or the other.
If you had both contacts (normally open and closed) you would have a "common" to both contacts.(In this situation there would be three terminals) The common would be where you would connect the wire you intended to switch and come out of either the normally open or closed contact depending on which one suits your diagram.

This applies to most switches not just limit switches.

Lemming, what i have been taught is that a PNP system switches the Positive and that a NPN system switches the Negative. I think that would mean that i have been taught the opposite to you as i see it in PNP the 24v would be the common and in NPN the 0v would be the common.

Tboller said:

Lemming, what i have been taught is that a PNP system switches the Positive and that a NPN system switches the Negative. I think that would mean that i have been taught the opposite to you as i see it in PNP the 24v would be the common and in NPN the 0v would be the common.

Click to expand...

My interpretation of common is whatever is not being supplied to switch out at the field devices. ie, a PNP system where a 24Vdc signal is switched back onto the PLC input, the common is the 0V to the PLC module...

Common = the starting/direction point of Conventional flow

or

Common = the starting/direction point of electron flow

I'm going with Tom Jenkin's answer. The 'common' must be combined with another descriptor ("AC 0 volt common" for example) to have any meaning. It just means a potential which is connected to a wide group of items. Another help would be an identical wire number for all of these connections.

Your conclusion ?

Lemming said:

Probably depends on who you talk to,.

To me in a PNP system, 0V is common, in a NPN system 24V is common..

Click to expand...

0V is 0V, 24V is 24V.