Electrical Troubleshooting Quiz, July 15, 2010

Your factory has an automated shipping dock system, with 10 bays. A conveyor system feeds 200-lb product boxes to a chute for each bay. When the chute for a bay is full, the indicator light outside the bay door turns from red to green so truck drivers know which bay to pull up to.

You come in on second shift to find a note saying the Bay 6 light never turns green when the bay is full. The tech replaced the analog output module in the morning, solving the problem. However, just before quitting time, the problem came back.

What's your next step?

First, determine if the replacement module went bad. This is probably the case, so let's assume it did. Do you really want to just stick another module in there with a two out of three failure rate already? No.

Most likely, something is amiss that's causing the conveyor drive motors to introduce power anomalies into the power distribution system. Put a power analyzer on the panel that serves this system to see what you're up against.

You need to solve for those specific power anomalies, but the solution may not be power conditioning. Perform these two steps next:

1. Check the grounding (bonding) connections on the PLC system, the conveyor itself, and the drive motors.
2. Watch how the boxes enter and move through the conveyor system. If the boxes move anything but smoothly, the solution is mechanical.