EE & coworker were to change screen of dryer. To remove upper cover of dryer, the chute above dryer must be released & moved to allow cover to be raised. EE climbed onto top of dryer to unlatch chute, when dismounting he placed foot on lid of effluent box. Lid moved causing him to lose balance & fall 38"" to concrete floor onto lt. arm resulting in simple fracture to wrist.