Abstract: An air-tight molded resin enclosure for containing a fluid under pressure is disclosed. The enclosure includes a safety membrane which can rupture at a fluid pressure less than the pressure at which the enclosure will rupture. The enclosure may contain an electric device such as a circuit breaker and a pressurized insulating fluid such as sulfur-hexafluoride (SF.sub.6) gas. When an electric arc occurs in the circuit breaker, the pressure of the gas contained in the enclosure increases in magnitude. When a sizable over-pressure exists inside the air-tight enclosure, the safety membrane will rupture without damage to the circuit breaker or to any neighboring equipment. The safety membrane includes grooves which act as rupturing primers. The thickness of the safety membrane and the depth of the grooves determine the pressure at which the safety membrane will rupture.