A high-reach order picker in a massive, partially open distribution center had a bizarre, intermittent shutdown issue. The truck would drive perfectly for weeks, but after a heavy rainstorm, the operator would drive about twenty feet, the "Motor Fault" light would flash, and the truck would lock out. If they let it sit inside for a few hours, it would work perfectly again.
The maintenance team had replaced the main contactor and the motor controller, assuming moisture was getting into the electrical connections. They sprayed every plug with dielectric grease, but the rain problem persisted.
When an AC forklift throws a motor fault, the controller is usually looking for an imbalance in the electrical phases or a short to ground. We brought out a megohmmeter (megger), which puts 500 volts of electricity through a circuit to test the integrity of the insulation. We tested the AC motor windings in the dry shop, and they read perfectly-a massive number of megohms, meaning the copper wire was safely insulated.
We then had a worker spray a fine mist of water over the top of the motor housing with a spray bottle. We meggered the motor again, and the resistance dropped to near zero. The "T" phase was shorting directly to the motor casing the moment it got wet. We pulled the motor and cut the heavy varnish insulation off the stator windings. We found a microscopic, hairline crack in the insulation, right where the wire exited the stator core. The crack wasn't caused by water; it was caused by a previous voltage spike-likely from an old, arcing contactor that had been replaced months ago. The extreme heat of the electrical arc had cooked the varnish. In dry weather, the air acted as an insulator. In humid or rainy weather, the moisture bridged the gap, shorting the motor to ground. We replaced the motor, but the real lesson is that a megger test means nothing if you don't replicate the environmental conditions that cause the failure.
