Trolleybus - Trolleybus Wire Switch

Trolleybus Wire Switch

Trolleybus wire switches are used where a trolleybus line branches into two or where two lines join. A switch may be either in a "straight through" or "turnout" position; it normally remains in the "straight through" position unless it has been triggered, and reverts to it after a few seconds or after the pole shoe passes through and strikes a release lever. (In Boston, the resting or "default" position is the "leftmost" position.) Triggering is typically accomplished by a pair of contacts, one on each wire close to and before the switch assembly, which power a pair of electromagnets, one in each frog with diverging wires. ("Frog" generally refers to one fitting that guides one trolley wheel/shoe onto a desired wire or across one wire. Occasionally "frog" has been used to refer to the entire switch assembly.)

Multiple branches may be handled by installing more than one switch assembly. For example, to provide straight-through, left-turn or right-turn branches at an intersection, one switch is installed some distance from the intersection to choose the wires over the left-turn lane, and another switch is mounted closer to or in the intersection to choose between straight through and a right turn. (This would be the arrangement in countries such as the US, where traffic directionality is right-handed; in left-handed traffic countries such as Britain and New Zealand, the first switch (before the intersection) would be used to access the right-turn lanes, and the second switch (usually in the intersection) would be for the left-turn.)

Three common types of switches exist: Power-on/Power-off (the picture of a switch above is of this type), Selectric, and Fahslabend.

A Power-on/Power-off switch is triggered if the trolleybus is drawing considerable power from the overhead wires, usually by accelerating, at the moment the poles pass over the contacts. (The contacts are lined up on the wires in this case.) If the trolleybus "coasts" through the switch, the switch will not activate. Some trolleybuses, such as those in Philadelphia and Vancouver, have a manual "power-coast" toggle switch that turns the power on or off. This allows a switch to be triggered in situations that would otherwise be impossible, such as activating a switch while braking or accelerating through a switch without activating it. One variation of the toggle switch will simulate accelerating by causing a larger power draw (through a resistance grid) but will not simulate coasting and prevent activation of the switch by cutting the power.

A Selectric switch has a similar design, but the contacts on the wires are skewed, often at a 45-degree angle, rather than being lined up. This skew means that a trolleybus going straight through will not trigger the switch, but a trolleybus making a turn will have its poles match the contacts in a matching skew (with one pole shoe ahead of the other), which will trigger the switch regardless of power draw (accelerating versus coasting).

For a Fahslabend switch, the trolleybus's turn indicator control (or a separate driver-controlled switch) causes a coded radio signal to be sent from a transmitter, often attached to a trolley pole. The receiver is attached to the switch and causes it to trigger if the correct code is received. This has the advantage that the driver does not need to be accelerating the bus (as with a Power-on/Power-off switch) or trying to make a sharp turn (as with a Selectric switch).

Trailing switches (where two sets of wires merge) do not require action by the operator. The frog runners are pushed into the desired position by the trolley shoe, or the frog is shaped so the shoe is guided onto the exit wire without any moving parts.