Consider the importance of flood alarms before disabling them
We received a call to help troubleshoot the PLC control system on a rail car unloader. “It used to work perfectly” was the start of the conversation, but when the owner tried to use it after it had been offline for a while, part of the system did not work at all. Their conclusion was that there must be something wrong with the PLC or perhaps with the scale controller that senses product being present and starts the rest of the system.
After a bit of troubleshooting we found that indeed signals from the PLC to certain field components were not getting to where they needed to go. The PLC was doing its job though.
We were able to gain remote access to the system and converse with the electrician on site to further investigate the issue. It was determined that the PLC was working as it should, but there seemed to be a problem with the wiring between the PLC and the field equipment. This wiring passed through a below grade junction box.
The conveyor pit where the junction box was located was equipped with a sump pump and flood alarm. Both of these had been disabled while the facility was temporarily not in use.
This picture was taken after the water was pumped out:
After the water was pumped out, it was clear which circuits had been energized when this system went under water as those were the wires that suffered most from electrolysis. They had corroded in half and had some discoloration.
Amazingly, once the electrical connections had been repaired and all of the blown fuses had been replaced, the system worked as it always had.
Sometimes sump pumps and high-water alarms are placed in certain locations for good reason and its always a good idea to investigate before disabling them.