NPN or PNP PLC inputs


Thread Starter


I have seen the views on the PNP & NPN inputs at however to take this dicussion further if we had a sinking type input and the hot wire (let us say ...+24VDC in this case) which run all around the plant gets shorted and if there were no Input fuses at individual channels, then would trip the 24VDC power supply which feed to various other inputs modules connected to it.
Wouldn't it cause a nuisance tripping of the plant?

What are your views if we have higher DC Voltages like 48VDC or 110VDC is it safe to run the hot wire in the plant to various sensors in the field?
When do we say we need sourcing type or sinking types considering the safety prespective?
Are there any standards governing the same?
plz provide your views.

Curt Wuollet

It's not a good idea to run wire from a single supply "all around the plant" for many reasons, this being one of them. Returns should be common, but it's not a bad idea to localize DC power for just that reason and so you only have to look at one mile of wire to find a short. Many small supplies also prevent burning wires and lighting fires you get when your sensor shorts a 50 A supply. I hate a gooey mess in a 100 ft run of conduit. It's strange that most people do the right thing for wire protection with AC (per code) but you'll see 20 ga. wiring directly bussed to a 24V 20 A supply.

The assumption is that the negative side of your 24VDC is grounded. If not, there's no short.

Of course, the supply should be referenced to ground, so you could ground the positive, or ground the negative through a circuit breaker or 24VDC relay coil so that you knew when a ground fault occured without shutting down the system.

Another simple way to do the same thing is to connect each of the positive and negative to ground through a lamp. In normal operation, both lamps glow at low brilliance. When there is a fault, one is full brilliance, and the other goes out.

And I agree 100% with what the previous poster said.