ELCB failure or something else?

It all depends on the wiring of the RCD in this case.

- Where was it wired: before the compressor connection?

- Where was the operator standing and what was he holding on?

If the 3 phases (& N if that is used on the compressor) go through the RCD and then to the Plug (dangerous one...) and the operator was standing on the wooden pallet when starting the unit, with only one hand-finger pressing / actuating the starting mechanism (in contact with any metal), then he should not have been shocked.

the generator door touching the compressor frame and tripping the RCD C.B.(Correct??) proves that there was a leak to the compressor frame.

The Operator could have been shocked if the current leak was between the HAND and another part of his body touching the compressor frame and the RCD will not trip because it cannot see that leak, not returning to the generator from outside the RCD! It looks as if it was going trough the Compressor circuits, and that is OK for the RCD.

In Short, the compressor frame must be earthed to the generator Neutral line for the RCD to Work properly. The Genset Neutral will be the Ground (in your case it looks connected to the genset body frame.