SG Gas Discharge Tubes Littelfuse SG Gas Discharge Tubes (GDT) offer high surge ratings in a miniature package and are designed for surface mounting on PCB with a small size of 4.5mm x 3.2mm x 2.7mm. The GDTs' low insertion loss is perfectly suited to broadband equipment applications. Their capacitance does not vary with voltage, and will not cause operational problems with ADSL2+, where capacitance variation across Tip and Ring is undesirable. Littelfuse SG Gas Discharge Tubes are extremely robust and are able to divert a 1000A pulse without destruction.