A fast closing valve (FCV) system has been built to protect the vacuum of the electron storage ring against sudden vacuum failures during synchrotron radiation experiments. An electronic control detects the failure, and closes a 140 Ã 17 mm aperture within 32 ms with a guillotine blade driven by pneumatic pressure. This pressure is gated by a magnetic valve which is operated by an explosive charge from a capacitor bank. The design of the FCV itself is very simple; it is operated directly by a single pneumatic piston. The FCV system has a long operating lifetime and the blade can be readily rearmed in 0.5 s.