A couple years ago I was asked to commission the PLC logic and VFD’s for a conveyor line. The customer was a Rockwell house, but an end user forced them to change a job to Siemens. They had an S7-315PN/DP-2 and a stack of Profibus G120D VFD’s, along with a 10in MP HMI. When I went on-site to download the logic, they were fighting to bring up power to all the drives. I knew something was off when I felt current on the main panel door, and saw a spark when I swung it further open.

When I went to set an IP address to the PLC, I noticed that there were no lights on the PLC. All the breakers were on, and they insisted it was all powered up, as other 24vdc devices had power. The first thing (and looking back, luckiest thing) was that they had the PLC and HMI on a dedicated Siemens power supply. These were the only 24vdc devices not lit up. We decided the first thing to do was to pull and bench test the PLC and HMI, as well as check voltage on the dedicated supply with nothing connected to it. That’s when we noticed the next sign of a serious issue. With the 24vdc supply switched off, there was a spark when removing the 0vdc wire from the terminal. Both the electrician and I were quite surprised by this one, as there shouldn’t be any power left here. We determined something was back feeding. Now the problem was finding what. I headed up to the office to speak with their controls manager (as this was way out of my scope on a fixed cost project) while the electrician worked to locate the source.

A few minutes later, he walked up to the desk with an orange wire in his hands. He placed it on the manager’s desk and asks him to read the wire labels. One side was labeled with the PLC’s 0vdc wire number. The other with simply X1.

It seems a trainee on the line assumed you ground your 0vdc the same as you do in a car, then saw the ground line coming from the X1 terminal on the transformer diagram and decided it was easiest to just wire them together. The PLC, HMI, and power supply were all toast. Fortunately they were isolated and the rest of the control components were spared.

Lesson here; Always double check your dangerous wiring: transformers, overloads, safety circuits (more on this in another post). Even if you didn’t have a trainee wire it, a second pair of eyes can’t hurt when the risk is high!

Programmer and Controls Engineer with over 10 years experience in IT and Industrial Automation. Avid gamer and Geek with an interest in nearly all things technical.