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In substations, protective equipments such as arresters and neutral transformer are being connected to substation ground grid to protect them against transient overvoltage, firing and other damages. But unfortunately, lack of grounding connection of electrical equipments or high resistance of grounding grid is one of the main reasons which cause risk for staffs and electrical equipments. In this paper, effect of increasing in grounding grid resistance on transient overvoltage which is caused by short circuit, switching and lightning on the interior equipments and safety in a grounding grid at substation is investigated. Effect of transient overvoltage with different short circuits is another subject that is considered. In this paper, indices for ground grid safety are expressed and grounding grid analysis will be done with CYMGrd software. Transient analysis simulations are carried out using DigSilent software and the results of investigation on a real system are presented.