Rig floor crew ware preparing for changing tubing tong jaw. The driller left the driller console after securing the travelling block hanging @3m above floor using draw works brake handle chained down without noticing the chain was stuck in the grating platform. The traveling block started descending down to the rig floor and continued to drag towards the V-door side and fell on the ground. All crew members safely came down using ODS (Off Driller side) ladder