Prox switches work if ground wires disconnected

Sorry for such a long message, but I need to provide a bit of background info by describing the equipment.

I have a series of machines which has several conveyors used to transport pallets through each machine in sequence There is one main control panel with all the I/O hard-wired to various j-boxes through Hoffman wiring trough. The conveyor movement is via a 1/2 HP motor on each individual conveyor section. The motor wiring is in a separate wiring trough from the I/O wiring.

Prox switches are 2 wire AC by big name manufacturer "X" and used to track pallets as they move on/off each conveyor section.

The prox switch wiring and motor wiring are separated very well. Leads may come within 4" or so of each other in the j-box, but they do not run parallel to each other. Each j-box had a ground bar where the motors and prox switches are all grounded. There is then a #14 wire connecting the ground bar back to the ground bar in the main panel.

Now for the issue. The prox switch(es) are unreliable and go into what I'll call a lockout condition (flashing green power light) when motors are jogged. I can create the lockout condition by manually pushing in the contactor with an insulated object. Only way to clear the lockout condition is to cycle power to the prox switch(es).

I have 3 other various prox switches by various other manufacturers (all different part numbers) that I have successfully tried with no issues. Unfortunately none are a viable solution as they are either too big diameter or too short of sensing range or wrong type of cable connector. (They were just extras I had laying around.) But the point is that all function as they should with no "lockout" conditions. I even took 15' of cable from one of the prox switches and stuffed it in the motor lead wiring trough with no adverse affects.

Today, I had the distributor and "X" factory rep in. For testing purposes, they had me disconnect a switch from the machine (so the body was not grounded) and then unhook the ground wire from the cordset. And then the switch works perfectly. They claim they have seen this before with this particular series of prox switch. My contention is that the other 3 part numbers from various manufacturers (one was actually the same brand, just a different part number) all work with no issues so I need them to take the unreliable switches back. So distributor and factory rep are currently looking for a suitable, reliable, readily-available replacement.

My question, has anybody else ever experienced anything like this? How did you fix it? I've tried separating the motor and prox switch grounds and running them individually back to the main control panel. Made no difference.

Are you sure the ground bar you mentioned in the Jboxes is tied to a good earth ground somewhere? My first suspicion is that it can't be. You mentioned the conveyor motors are on contactors but is there maybe a drive upstream of that? If not and the motors are across the line is the upstream transformer grounded at the star point?

the lasst thing you should ever do is use a ground for an electrical return wire.
Ground wire or EARTH Wires are designed to carry a fault current.
this current could be very high amps - you will blow up devices this way.
dont forget one other point
'we want to weld that brace' ok just put the ground on that....
I wonder why it don't work now....

Why don't you just test them in another setting with proven good grounds, if it's still behaving strange then it sounds like it is malfunctioning and the seller should replace them.

sorry but there is no such thing as a proven ground
I do know what you mean but, with machinery, the chances of someone disconnecting something is very high. and comon place is welding.
I have seen a small earth in a loom melted togetther with the rest of the wiring after an incorrect welding earth was used.
not a pretty site

Ofcourse the term "proven" is fuzzy and maybe my sentence was unclear but what i meant was to set it up at a different location where the risc of bad grounding is lesser.
Maybe plug it into a test bench at the electrical workshop or something similar.

allscott said:

Are you sure the ground bar you mentioned in the Jboxes is tied to a good earth ground somewhere? My first suspicion is that it can't be. You mentioned the conveyor motors are on contactors but is there maybe a drive upstream of that? If not and the motors are across the line is the upstream transformer grounded at the star point?

Click to expand...

Good point, maybe the other end of the ground wire isn't actually connected in the main panel. I will check.

There is no drive upsteam of the conveyor motor contactors.

There are some drives in the system (on some other motors) but they don't seem to be causing issues as I only experience problems when the FVR motors are started/stopped.

The other thing I just thought of is that the conveyor sections are all isolated. I will have them add lugs and bond them all to each other and to a good ground.

iant said:

the last thing you should ever do is use a ground for an electrical return wire.
Ground wire or EARTH Wires are designed to carry a fault current.
this current could be very high amps - you will blow up devices this way.

Click to expand...

Maybe I wasn't clear or don't understand what you wrote.

They are what the manufacturer calls 2 wire proxes but there is also a 3rd ground wire. One of the 2 prox switch wires is 120VAC (essentially on the secondary of my control transformer) and the other prox switch wire goes to the PLC input module. The input module common is on the other side of the transformer secondary. The transformer secondary is also grounded at the transformer (the same side that is connected to the input module common).

I was only disconnecting the 3rd ground wire. I am not using it for a return and/or signal wire.

I understand - it is terminology
the secondary of a or any single phase transformer is an AC supply
there is NO 'ground wire' although there may be a connection of one of the terminals to ground, it is called an Earth point connection here.(this is not always fitted)
normally the other wire on the earth point treminal is called either a Neutral or (N) or 0Vac
but not a ground wire