The Transmission part of the power sector is very much important as it deals with the huge losses of the complete power sector. Transmission substations play a key role in the power transmission process since the power generation at the power plants to the final stage at the consumer end utilization. The utmost care should be taken to protect these transmission substations as same as the care is taken to reduce the losses. The substation needs to be protected from blackouts which is the big problem of substations. For this, the substation high voltage equipment’s like GIS & Power Transformers are to be properly installed & energized according to the standards & care is taken for its maintenance. So, during the commissioning process which is after installation & before energization, there are different tests which are performed on high voltage equipment’s, especially the Gas Insulated Switchgear, in which the High Voltage & Partial discharge has huge impact as the ageing and the life time of GIS can be easily assessed & the GIS can be saved from the damage, tripping and the blackout of a substation. The procedure of the high voltage test and the partial discharge tests are clearly explained in this paper.