Method for installing a cable into a duct having two extremities, the method comprising the steps of: - determining a maximum pressure (p max) based on a bursting or a service pressure of the duct, taking in account an installation temperature and/or an installation time, - setting a leaking mode of a leaking plug to start at a leaking pressure drop (Δ p leak) across the leaking plug determined to be equal to or lower than the maximum pressure (p max), - attaching the leaking plug to a foremost end of the cable, - introducing the foremost end of the cable into the duct at a first extremity, - supplying a pressurized liquid into the duct: at a supply port arranged between the foremost end equipped with the leaking plug and the first extremity, and at a supply pressure (p sup) equal to or higher than a predetermined pressure, so that the cable is pulled by the leaking plug operating in a non-leaking mode, - before the foremost end has reached the second extremity, exceeding the leaking pressure drop (Δ p leak) at a location close to the leaking plug, leading the leaking mode to start, characterized in that the duct presents a trajectory with non constant altitude, the method comprising a step, before the foremost end has reached the second extremity, of adjusting the supply pressure (p sup), so that all along the part of the duct comprised between the first extremity and the foremost end of the cable and filled with liquid, the pressure of the liquid is lower than the maximum pressure (p max), wherein the supply pressure is adjusted, so that at any point along the part of the duct comprised between the first extremity and the foremost end of the cable and filled with liquid: - for points at higher altitude than the first extremity, the supply pressure (p sup) subtracted with a pressure loss caused by liquid viscosity to reach said considered point, is greater than a hydrostatic pressure caused by the liquid density and a difference of altitude between the altitude of said considered point and the first extremity, and/or - for points at lower altitude than the first extremity, the supply pressure (p sup) subtracted with a pressure loss caused by liquid viscosity to reach said considered point, added to the hydrostatic pressure caused by the liquid density and a difference of altitude between the altitude of said considered point and the first extremity is larger than zero and lower than the maximum pressure (p max). - determining a maximum pressure (p max) based on a bursting or a service pressure of the duct, taking in account an installation temperature and/or an installation time, - setting a leaking mode of a leaking plug to start at a leaking pressure drop (Δ p leak) across the leaking plug determined to be equal to or lower than the maximum pressure (p max), - attaching the leaking plug to a foremost end of the cable, - introducing the foremost end of the cable into the duct at a first extremity, - supplying a pressurized liquid into the duct: at a supply port arranged between the foremost end equipped with the leaking plug and the first extremity, and at a supply pressure (p sup) equal to or higher than a predetermined pressure, so that the cable is pulled by the leaking plug operating in a non-leaking mode, at a supply port arranged between the foremost end equipped with the leaking plug and the first extremity, and at a supply pressure (p sup) equal to or higher than a predetermined pressure, so that the cable is pulled by the leaking plug operating in a non-leaking mode, - before the foremost end has reached the second extremity, exceeding the leaking pressure drop (Δ p leak) at a location close to the leaking plug, leading the leaking mode to start, - for points at higher altitude than the first extremity, the supply pressure (p sup) subtracted with a pressure loss caused by liquid viscosity to reach said considered point, is greater than a hydrostatic pressure caused by the liquid density and a difference of altitude between the altitude of said considered point and the first extremity, and/or - for points at lower altitude than the first extremity, the supply pressure (p sup) subtracted with a pressure loss caused by liquid viscosity to reach said considered point, added to the hydrostatic pressure caused by the liquid density and a difference of altitude between the altitude of said considered point and the first extremity is larger than zero and lower than the maximum pressure (p max).