IPM /G.D. tube is used in MDF for protection of exchange cards from surge voltage. However G.D. tubes can not be tested by any ordinary meter. In this specialized  GD tube tester modulated signal equivalent to lightening signal is internally generated. Which is applied to GD tube. This will forces the gas inside the GD tube  to discharge and we get alarm from the unit.  Any type of G.D. tube like Pouyet, E-10B , Crone type can be tested with this tester .Individual section of GD tube like  A limb or B limb is selected first & then it Is subjected to  low & high signal. For low signal gas should not discharge where as for high signal gas should discharge. Accordingly it is indicated on the meter. From this tester we will come to know whether GD tube is open or short or ok.