Schmersal, in conjunction with its partner-distributor, Parmley Graham, has supplied a number of bi-directional position limit switches for 35 bespoke powered access gantries, built by Alternative Access for permanent installation at Heathrow Airport's new Terminal 5 building. Thirty one of these gantries, each 15m long, will be used within the terminal building for maintenance access to the roof soffits, while the remaining four will be used for external window cleaning and general maintenance operations.The internal gantries are fitted with two Schmersal T4VH 355 bi-directional switches, one at either end, to prevent them from being operated while under excess load. The switches are also used to prevent any conflict in the direction of the cross-travel crab; for example, if the gantry is moving left to right, the switch isolates the signal to prevent it from moving right to left.Schmersal switches are also fitted to 23 of the terminal's 160m long bus bars, forming a three-phase multiple terminal rail that provides power for the access gantries. The switches detect when the gantry has reached the end of the bus bar and are also used to isolate the power while the gantry is moved from one bus bar to another on a different level.