How can a tram route cross a trolley bus route without short circuits

Trams draw their power from a single overhead wire at about 500 volts above earth. Trolley busses draw their power from two overhead wires with about 500 volts between them. So how can a tram route cross a trolley bus route without short circuits?

This example was photographed in Innsbruck, Oesterreich in 1997 September. A trolley bus is just passing through the crossing. In the second picture the important bits are coloured (see key below):

The view looking directly upward and an annotated version:

Actually this picture was taken in Bahnhofplatz, Bern, Schweiz in 2002 September.

The first requirement for this to work is that the trams must have pantograph-type power collection. Where the trams use a trolley pole to collect their power, a different crossing style must be used.

The trolley bus wires run continuously through the crossing - they need to anyway because of the trolley wheel pickup method. The tram conductors are slung a few centimetres lower than the trolley bus wires. Close to the junction on each side, the wire merges into a solid bar which is angled to run parallel to the trolley bus wires for about half a metre. Another bar similarly angled at its ends is hung between the trolley bus wires. This is electrically connected above to the tram wire's catenary cable. The tram's pantograph will easily bridge over between these different conductor sections providing it with a continuous pickup.

Where the tram wire crosses, the trolley bus wires are protected by an inverted trough of insulating material extending 2 or 3 cm below the level of the trolley bus wires. The tram pantograph raises the conductor wire a little as it passes under. These troughs are presumably to limit how far it can do that and to provide a backstop to prevent the tram pantograph ever touching the trolley bus wires.

Australia

Adelaide, South Australia once had trams and trolleybusses (and still has one tramline today) both using trolley-pole current collection. They used insulated crossovers which required tram drivers to put the controller into neutral and coast through. Trolleybus drivers had to either lift off the accelerator or switch to auxilliary power.

In Melbourne, Victoria tram drivers are still required to put the controller into neutral and coast through section insulators, this being indicated to drivers by insulator markings between the rails.