Final answer:
In the event of automatic circuit failure on heavy machinery, the hydraulic selector switch should be set to manual override or an equivalent manual setting, allowing direct operation of the hydraulic systems.
Step-by-step explanation:
When automatic circuits have failed on heavy machinery and operators need to resort to manual controls such as MAN H.A. (Manual High Amplitude) and MAN M.F. (Manual Moderate Frequency), the position used on the hydraulic selector switch typically is manual override or an equivalent setting which allows for direct manual operation of the hydraulic systems. This manual position bypasses the failed automatic controls and enables the operator to control the hydraulic functions directly. Hydraulic and mechanical lever systems are crucial in the operation of heavy machinery like a backhoe, and manual control becomes essential in the event of automatic system failure.