possible ground loop problem?

Thread Starter

bonneville baby cakes

Joined Jan 13, 2009
i have a project at work. it is for my electronics hands-on practical exam and a promotion if i complete it. i am replacing a plc with another plc. i cannot access the program ladder logic to the old one, so i am mimicking the logic by watching the LEDs flash on and off.

it is an old cabinet. the power being used to run the old system, it's transducers, on/off, cabinet lights, everything in there is run from a single leg of 208 3 phase power. which is 110V ac.

neutral wires are everywhere and all neutral wires are tied in to the cabinet ground of the 3 phase 208.

shouldn't there be a transformer in there in which, all the stuff that needs 110V ac can get it off of the clean secondary side of the transformer?

i installed switches for the outputs in which, i can switch over to let my plc run the system, or switch it back and let the old one that's in place run it. my inputs are in parallel with the older plc, so i can watch the action and adjust timers, etc. the problem is, when i hit the switches to convert over this one single output, which activates a 110V ac solinoid, i lose power to the 110V ac input from the index conveyor. the solinoid turns the hydraulic motor on and off that indexes the conveyor. but the flap sensor loses power when i convert over. when i switch it back to the old system, everything fires up like there was nothing wrong.

last field test, i measured the output voltage from the sensor, which is the input to the plc. it was like 0.01V ac.

should i install the transformer and get everybody on the same potential. please advise. thank you.


Joined Nov 28, 2008
neutral wires are everywhere and all neutral wires are tied in to the cabinet ground of the 3 phase 208.
I don't fully understand your present problem but it seems someone has used the 208 3-phase power supply system neutral as GROUND in your PLC system.

Connecting the power neutral to cabinet ground is in violation of many supply rules in many countries and could be dangerous if the neutral of the incoming supply become open-circuited somewhere upstream.

Yes, a 110V/110V isolation transformer is needed and with that one can safely connect the 0V of this transformer secondary to cabinet ground, which I assume is also PLC signal common in your case.