The Case of the Smoking Control Cabinet

THE MYSTERY

The ACES CSI (Control System Investigator) was just finishing up a service call at a wastewater plant. He was testing the work he’d done on a controller, which runs a coagulation unit removing heavy metals from water.

This is a high-current, low-voltage application. He set the max output current to 85ADC and fired up the unit for 20 minutes. Everything looked good — until smoke started streaming out of the control cabinet.

THE CLUES

When the CSI opened the cabinet, it was immediately obvious that the smoke was coming from two output fuses on the rectifier. Upon closer examination, he ascertained that the fuses had been raised off of the cables with a hex nut and washer.

THE PERP

Someone had rigged the fuse holders out of 5/16” bolts and nuts. Next to the insulator there was a multi-layered stack including a nut, a copper terminal on the wire, then another nut. There was a small amount of exposed threads on the bolt, then a nut, washer, a bolt in fuse and finally a washer, and yet another nut.

Although the person building this rig at the factory made it look very nice, it’s obvious they hadn’t engineered this solution: the bolt was much too small to carry the current in the circuit. They could have used their fuse holder design if they had stacked the fuse directly on top of the copper terminals, eliminating a couple of nuts and washers.

The CSI explains: “Never use a bolt for a conductor. They’re steel and not copper and they have lots of resistance. The bolt itself was getting hot enough to smoke — and that’s hot.”

The bolts had probably been smoking all along, unnoticed. “You’re losing energy across that joint that should be going somewhere else. You get some funny problems when you’ve got extra resistance on a line, and then the controller thinks it’s doing one thing, but you’re getting a different result in the end.”

THE SOLUTION

The CSI called the factory and they said, “Well, we don’t really need those fuses.” So, with permission from the manufacturer, he rerouted the wires and removed the fuses entirely. Now, with the mysterious bolts cleared away, the coagulation unit runs smoothly and the control cabinet is smoke free.

CASE CLOSED