An insulator for separating two objects with different electrical potential, the insulator including an insulating body enclosing a first volume for housing a gas, the body being provided with an opening, and a sealing arrangement arranged to seal the opening in the body and including a first gas-tight sealing element for sealing the opening and provided with a first closable opening, and a second gas-tight sealing element for sealing the opening, and arranged inside the first sealing element so that the first and second sealing elements define a second volume significantly smaller than the first volume. The second sealing element is provided with a second closable opening which in cooperation with the first closable opening enable control of the gas pressure in the first volume. Methods for leakage detection of the insulator are also disclosed.