Safety contact thermometers acc. to DIN 12 878 class 2 or acc. Gerstel are connected with a three wire cable, the test current flows through the contact thermometer.Safety function: If the test current is interrupted because of e.g. breakage of contact thermometer or flling out of the cable plug, the heating cuts off.The required plug allocation is indicated on a reference plate above the diode bushing.Contact thermometer without safety circuit acc. to DIN 12 878 class 0. The instrument only heats if the test current circuit is closed by an electrical connection of the plug pins 3 and 5.