When it is necessary or desirable to work on a part of a hydrogen system while another part of the system remains in operation or in standby condition. An example is a system with two compressors, where one is normally operating and the other acts as an installed spare. The two block valves are closed, and the bleed (vent) valve is open. This arrangement assures that any hydrogen leaking through the valve on the pressurized side will be vented to atmosphere rather than entering the part of the system being maintained.

The need for DBB is generally identified during the hazard analysis. Important variables include liquid or gas service, pressures, and procedures for operations and maintenance. Some organizations have a matrix for isolation required based on risk, frequency of maintenance activity, and work permit requirements.  High severity or frequent maintenance will usually be DBB. Low frequency, low hazard or where additional protective measures are present will not.