To provide a mechanism for preventing the operation of a mechanical component when a failure occurs in a hydraulic component connected to the mechanical component.

This hydraulic system comprises an isolator 72 built therein near a hydraulic actuator for preventing accidents. The isolator 72 has a first port connected to the hydraulic actuator, and a second port having an electrically operated isolation valve connected between the first and second ports. A pressure relief valve responds to the hydraulic actuator whose pressure exceeds a given level while releasing pressure, and the isolation valve is opened with no application of electricity to release pressure in the hydraulic actuator. A control valve assembly is located remote from the hydraulic actuator and connected to the second port for measuring the amount of a flow between a fluid source and the hydraulic actuator.