PLC defective

Hi! i would like to ask about what are the common reasons why PLC or I/O modules become defective or damage??..

Shorted circuits for sure.

Have some installations experiencing brown outs, cpu does not like that to much either.

Also stupidity, drilling holes right over equipment. Using wrong size tools etc..

Here, it's water. For some reason the rubber/foam gaskets in the cabinets don't hold back water at 800 PSI very well. We also lose a lot of HMI's that way.

Will.

the CPU of the plc ok.. but the DI modules or DO relay modules were always defective..were replacing DI or DO modules always.

We used to have a lot of trouble with I/O cards that were used to connect to larger loads directly. For instance tying an output directly to a solenoid without an interposing relay in between the two. Your question is a bit generic, so I'm afraid the answers will be as well.

Faults in the field would probably be the number one cause of I/O module defects.

Beyond that...

For "hard contact" or relay output modules, I would say the number one cause of defects is the lack of surge suppression and/or the use of interposing relays. Usually, an installer will properly rate the modules against the loads, but may not always suppression inductive loads correctly. This can greatly increase the degradation of the module's contacts.

For input modules I would say that for normal wear and tear the input frequency is often a big factor. After that, shorts, spikes, and all in between are common reasons for them blowing.

For I/O in general, and depending on the platform, the chassis, or backplane should also be considered. Leaving spare slots uncovered is a very common practice and can allow dust and critters easy access to its circuitry.

If you have several output modules, of a particular brand/model going faulty on a regular basis, then I would first look to the loads they switch and whether they have or require interposing relays and the loads themselves, whether they have or require surge suppression.

It's good practice to just do it by default if switching noisy inductive loads.

Regards,
George

I've seen a PLC go bad due to the enclosure door being left open (for cooling!) at a gold mine. Gold flakes don't help the circuit board electrons go where they are supposed to.

http://accautomation.ca/get-rid-of-surges-that-are-destroying-your-plc-outputs/

DC Solenoids are the worse culprits for electrical surges on your system. The in-rush is what destroys the outputs of the PLC.

I would also look at your DC power supply. Does it have short circuit protection?

Regards,
Garry
http://www.accautomation.ca

gclshortt said:

http://accautomation.ca/get-rid-of-surges-that-are-destroying-your-plc-outputs/

DC Solenoids are the worse culprits for electrical surges on your system. The in-rush is what destroys the outputs of the PLC.

I would also look at your DC power supply. Does it have short circuit protection?

Regards,
Garry
http://www.accautomation.ca

Click to expand...

This is not quite right. There is no significant inrush current with a DC solenoid but there can be a very high voltage spike generated when the output turns back off which can damage PLC outputs. It is normal to fit suppressors or diodes across output contacts to combat this. What you do need to watch for, however, is solenoids which have a pull in configuration and a holding configuration. The pull in part of these coils will require a much larger current than the holding part which could be construed as an inrush.

You are correct Andybr. When the electrically generated field collapses an opposite polarity voltage is generated. This voltage spike can be high enough to weld the contacts on a PLC output relay.

Regards,
Garry
http://www.accautomation.ca