the enclosure 10 the tightness of the compartment 20 in relation to the outside being ensured at the level of the coupler 48 both by the closed valve 52 and by the sealing cap 56 the depressed push rods 60 also have double sealing limiting leakage risks the communications provided by the orifices 26 prevent any pressure or temperature differences between the compartment 20 and the enclosure 10 and the measurement of the capacitive divider 14 16 is not affected by such temperature or pressure variations if a capacitor 36 is chosen varying with the temperature in the same way as the capacitor 14 16 when an incident occurs for example failure of the low voltage capacitor 36 the sealing cap 56 is removed freeing the moving plate 66 which moves to the cleared position allowing the valves 28 of the communication orifices 26 to close the filling duct 74 is then fitted as shown in fig2 connecting the filling duct 74 causes the valve 52 of the filling orifice 50 to open the valves 28 of the communication orifices 26 remaining closed the gas in the compartment 20 can be evacuated via the filling duct 74 being either recovered or rejected into the atmosphere after the compartment 20 has been emptied the cover 32 can be removed by unscrewing the screws 34 to give access to the capacitor 36 the enclosure 10 remains filled during this operation and the installation can.