welding on plc

[Johnny T]

Just as an aside (and slightly off topic) I've also seen an old Schleicer PLC turn itself off if you used your mobile phone too close to it...

I haven't seen any of the modern PLC's affected by this but I think these kind of incidents in the past have helped fuel the current "no mobile phones in the factory" policy that a lot of UK factories have...

As for the welding issue, rsdoran is spot on, its due to a lot of current backfeeding into the device. The earth clamp needs to go near where the welding is taking place, but even then, as is the nature of electrickery, you can never be sure there won't be a problem. I would guess that tired old PLC's would be more susceptible than a newer model. Plus, newer models tend to build in backfeed protection (I've been told)...

 

[TConnolly]

I've never seen the PLC taken out but I have seen some servo hydraulic valves taken out by welding on the machine.

 

[mgvol]

I agree regarding poor grounding of welders/workpieces. I worked on a large construction project about 11 years ago where an electricican came running into my office yelling something about all of the ground wires melting in the J-Box he was wiring. Turns out that the machine structure had not yet been bonded, and all of the welders were hooking their ground clamps to the structure. All of the I & C wiring grounds had been bonded to the structure and back at the main ground bus in the control room. So the ground path for about a dozen arc-welders was back to ground thru some AWG 12 - 16 instrument ground wires.

 

[bamatki]

Have never seen this happen to a PLC but when I have worked on any type of machine with any type of electronic controller or computer I was taught to remove the device prior to (SMAW) welding on the frame or chassis.

It would appear to me that if the PLC were part of the actual circuit being welded I would not feel comfortable with it.