I also agree with the overload theory. the next possible root cause could be a short circuit some where along the PLC or the PLC rack if any or any of the PLC input or output cards. please post the power supply, PLC make and model, and the input and output card modules that are in the PLC system rack if any. After that a check by process of elimination will discover the fault. take care!
Figure 1. A Wago modular PLC I/O system. Modules are assembled by sliding together from front. Blades on the left edge connect IO bus power from module to module while a set of gold-plated contacts near the top rear provide databus connection between the modules.
Figure 2. Module close-up. The 'system' side of the device is most likely confined to the area inside the red box. The remainder of the device will be opto-isolated from the system.
Definitions would help:
- System: The CPU side of the parts including the data bus.
- Supply: Usually the 24 V supply to the PLC / modules.
- Field: The sensors, transducers, etc., connected to the input module.
Isolation: 500 V system/supply for analogue i/o module.
It means that the CPU side of the system is isolated from the analog module external supply. There is no ground connection. The isolation is rated to 500 V.
Isolation: 500 V system/field for digital i/o module.
It means that the CPU side of the system is isolated from the analog module inputs. There is no ground connection. The isolation is rated to 500 V.
And how can i know the type of isolation (channel to channel or channel to earth).
The type - whether opto-isolated or transformer isolated you could only tell by opening a module and having a look for opto-isolators or a transformer.
Figure 3. A well laid out opto-isolator board with good creepage clearance. Source: Soft Solder.
Usually the isolation zone - no-man's land - is clearly visible.
Channel to channel isolation: To have a breakdown between channels it must breakdown between the channel and the 24 V supply or channel and system. It's still 500 V.
Channel to earth isolation: This is up to the user and how it is wired. You have the option to have, for example, the 24 V supply to the analog modules remain floating with no ground reference. You need to determine the best solution for this yourself, depending on your application.
Best Answer
What I believe you need here is a DAC to give 4-20mA output to feed your 4-20m4 inputs.
The more flexible and expensive solution is a real DAC. Usually 0-10V. Simply drive a current amplifier scaled to give a maximum 20mA. The analogue outputs of PLC can sometimes be configured as current sources as well.
The cheaper way is to use a voltage source and series resistors switched by some relays. Unless there is some compelling reason I would use 3 relays and resistors to give me 4mA, 12mA and 20mA sources. (Bottom, Middle and Top of the analogue range to be tested). This catches most issues I've seen in production. The relays can be driven by any suitable digital output of a PLC.