BIL 20 / BIL 20 Au, CAT I 4mm Chassis Mount Test Sockets. Hirschmann Test and Measurement BIL 20 / BIL 20 Au, CAT I 4mm gold plated and nickel plated chassis mount test sockets with insulated head and ring, solder connection and M6 threaded body. These BIL 20 series 4mm test sockets are ideally suited for use in equipment chassis and switch panels with a wall thickness of up to 2mm.. Rated voltage 30 VAC/ 60 VDC Rated Current 32A CAT I Screw mounting M6x0.75mm Solder termination