Siemens PLC Main Disconnect Trip

[Skoooot]

The past week we received a new piece of equipment from Germany which utilizes siemens controls. Typically in our company we use A.B. controls for 99% or all equipment. I am currently trying to troubleshoot an issue and having very little success.

The machinery has 3 drive motors on VFDs and all work fine. Start/Stop/pause/increase, decrease speed. However this machinery has a pump/motor (not on a VFD) which when selected on the HMI, it instantly trips the main panel disconnect. I have taken the motor leads off the contactor and still it trips the main disconnect instantly. It seems like the PLC is sensing an immediate ground fault however I can isolate line/lead cables from the contactor or the motors overload and it will still trip the main. The output card has correct voltage and seems to be working correctly. Could this be a programming issue?

 

[bkottaras]

The PLC nor program cannot trip your Main Breaker.
You've got something else going on.
I'd start by looking/checking into the wiring of your machine/system.

 

[parky]

If it only trips on the pump i.e. it does not trip when all other things are running it could be that something else is failing or drawing more current, I would check the current without the pump including the contactor it is possible that the extra load had this once similar problem, it turned out to be the main switch tripping, if I disconnected just one motor or contactor control it worked, however no matter which one I disconnected it would not trip, although the actual current drawn was about 30A the disconnect was rated at 64A, turned out it was the isolator faulty (magnetic trip circuit I assume) so it could be something you would think would not cause the trip.

 

[bkottaras]

If it only trips on the pump i.e. it does not trip when all other things are running it could be that something else is failing or drawing more current, I would check the current without the pump including the contactor it is possible that the extra load had this once similar problem, it turned out to be the main switch tripping, if I disconnected just one motor or contactor control it worked, however no matter which one I disconnected it would not trip, although the actual current drawn was about 30A the disconnect was rated at 64A, turned out it was the isolator faulty (magnetic trip circuit I assume) so it could be something you would think would not cause the trip.

He removed the motor leads and such.
The breaker still pops as soon as the contactor pulls in.
I bet it's a cross phase or something similar within the panel.

 

[joseph_e2]

I would probably lift and properly insulate the wires feeding the top of the motor contactor. You mentioned "motor/pump". If it's not a pump, is it a reversing starter? There could be something wonky going on allowing both fwd and rev to pull in together, which would definitely cause Bad Things to happen.

 

[JesperMP]

Does the main breaker have a trip coil ?
I sometimes see control panels where this is used to automatically trip the breaker, for whatever reason.
I have seen several italian control panels that were wired to trip the breaker if you opened the door to the cabinet.

edit: On an older german control cabinet I have seen this used to achieve 2 disconnects for the emergency stop. One being the main breaker, the other being the individual contactors for all the motors.

edit: To explain how this can have anything to do with the PLC, I speculate that this automatic trip is bypassed when single the motor without VFD is not selected.
Maybe the VFDs use integrated STO to stop when there is an emergency stop. The DOL motor being the only exception.

 

[drforsythe]

I agree with JesperMP. I would look for a shunt trip on the main disconnect. IF one exists, follow the wires back to the point of control and troubleshoot from there. If the shunt trip is controlled from a PLC output, then there could be a wiring issue with the PLC output and not actually an overcurrent issue.

 

[James Mcquade]

turn off the vfd motors and try to turn on the pump motor.
if it still trips. pull the wires to the pump contactor (forward / reverse if applicable).
try to start the pump.
if it still trips the main, you have a wiring issue.
if you know where the output card is located, remove the wire to the pump contactor at the output terminal.
if it does not trip, its a wiring issue.
if it trips, call the oem and have them fix it. it should be under warranty.
james

 

[mburtis]

Is the motor contactor pulling in when the pump is called for and the breaker trips? Unless it's a shunt trip breaker or theres some sort of phase monitoring relay that trips it, only thing I could think of would be to isolate the contactor and see if it still trips without power going through the contactor. If it does it has to be a shunt trip or other control issue. Maybe something came loose in the contactor during shipping and it's shorted.

 

[mburtis]

Got any schematics you can post ?

 

[Liam Moran]

I know this is really stupid, but is it possible that one of the terminals on the outgoing side is the wrong way around? Normally they are only insulated on 1 side, and if one is reversed on the mounting rail then the metal parts on 2 adjacent terminals can touch and cause a short circuit. I got caught with this a couple of times.

 

[Liam Moran]

 

[JesperMP]

No-one is thinking about why the circuit breaker inline with the contactor and the motor does not trip while the main circuit breaker at the panel infeed do trip ?
If the reason for the tripping is due to something at the motor, motor cable, contactor or any wiring after the circuit breaker, then there is serious problem with the selectivity between the circuit breaker in the motor starter, and the main circuit breaker.