What would cause the following to happen:
In a high voltage switchyard, a three step platform near SF6 breaker has a build up of voltage i woul expect it resulting from induction from the nearby overhead exposed buses. The platform is grounded at one end.
Are you sure that this is actually induced potential in the platform? It sounds to me like this might be static build up and discharge. You stated the platform is grounded, so I imagine that it is actually a person approaching the platform that has the charge and in effect creates a potential between themselve and the platform.