The sensor I had was the model LJ12A3-4-Z/BX . It has three wires which are colored brown, blue, and black. The device was wired as such:
brown: +24VDC
blue: E0 STOP GND
black: cathode of diode, anode to E0 STOP STP
I can't remember which wire was responsible for the arcs but it was contacting the Duet board in the area between the +5 and +3.3 VDC LEDs and the E0 / E1 Heater headers. But like I said, the board appears to be working fine except for the E0 Stop.