A testing device is provided to allow the proper functioning of the emergency stop valve to be checked even during normal turbine operation.The principal components of the device are test piston (12) in the hydraulic section, a pres- sure oil connection H with a mini-valve, and a connection (13) for the test pressure gauge for observing the test pressure.If the mini-valve is opened, pressure oil will be admitted to the space behind the test posi- tion which will then be pushed against the piston (10) and will move the piston and the valve spindle towards the closed position.