A fail-safe shutter (13) for a "cross-stack" monitor, comprises a body member (14) having an aperture (1) through which, in use, an associated monitoring instrument is adapted to sight. A gate (3) is movable between a retracted, open position in which the aperture (1) is exposed, and a closed position in which the aperture (1) is closed-off by a portion of the gate (3), and means are provided to bias the gate (3) towards its closed position. An electric motor (20), a gearbox (19) and a magnetic clutch (16) are provided, the clutch (16), when energised, connecting the gate (3) to the motor (20) via the gearbox (19), and the motor (20), when energised, serving to drive the gate (3) via the gearbox (19) and the energised clutch (16) to an open position, with attainment of the latter de-energising the motor (20) only, and with the bias means (7) being insufficient to overcome the mechanical advantage of the gearbox (19), whereby the gate (3) is retained in its open position until such time as the clutch (16) is de-energised. Control circuitry (21) is provided to maintain and/or interrupt and/or return electrical supply to the clutch, and/or to the electric motor.
