Spectroscopic results on low-current micro-arcs between cadmium-tungsten contacts for intrinsic safety in explosion protection*

There exists a spark test apparatus to support the certification of electrical devices to fulfil requirements on explosion protection. This spark test apparatus generates low-current micro-arcs with a length of 150 µm on a time scale of 1000 µs at around 60 mA constant current. The standardised contact material combination consists of a bulk cadmium cathode and a tungsten wire with 100-200 µm diameter as anode. The physics of the contact discharge is not well understood. Radiation is dominated by metallic cadmium. Most reasonable explanation is an a-spot contact bridge explosion that initiates the discharges and that provides initial material with low ionization potential. Results from optical emission spectroscopy are presented to provide deeper insights into the mechanism of this micro-arc by spatial profiles of excitation temperatures.