Our industrial oven heater is powered through two contactors in series. The 1st contactor's coil is AC. The 2nd contactor's coil is DC. AC power goes through a couple of devices (probably rectifier and filter) that change the AC power to DC before it feeds the DC coil. One day the DC coil contactor went bad and we didn't have a replacement, so we just bypassed the rectifier circuit and used another AC contactor. It's been a month and the oven runs just fine with two AC contactors in series. I am wondering why the DC coil contactor was used in the 1st place in the original design?