A heavy-capacity electric forklift nearly killed a warehouse worker. The operator was driving, lifted a load, and when they took their foot off the accelerator, the truck kept driving forward. They turned the key off, but the truck didn't stop. A coworker had to run alongside and hit the emergency disconnect.
We pulled the main battery contactor. The heavy copper moving contacts were welded together in the closed position. This happens due to low battery voltage and arcing. A contactor relies on a magnetic coil to pull the contacts together, and a strong spring to snap them apart. When the contactor closes, there is a micro-second where the gap is tiny, causing an electrical arc. If the battery voltage is high and the spring is strong, the arc extinguishes instantly.
However, this truck had a severely depleted lead-acid battery (voltage under 40V on a 48V system). The weak voltage meant the magnetic coil barely had the strength to pull the contacts together, slowing the closure speed. The slower closure created a prolonged, massive arc that generated enough heat to melt the silver alloy on the contact tips, welding them together. When the operator let off the seat, the coil de-energized, but the spring was not strong enough to break the weld. The truck kept moving. Never run a heavy electric forklift on a deeply discharged battery; the low voltage will eventually weld the contactors, turning the truck into a runaway missile.
