Overlap

Overlap is a very important part of the overhead system. It is where one contact wire ends and the other starts. The contact wire that is starting always comes in as a trailing wire.  As the locomotive travels between the two masts supporting both wires the pantograph comes into contact with the new wire as it loses contact with the old one. 

Overlaps are very varied, some have the wires isolated from each other, and many have jumper wires from the wire leaving contact to the wire gaining contact. Some can be isolated and have the switchgear to isolate a length of wire when maintenance is being under taken. Very often booster transformers are located at overlaps, these too can have isolating switches as well. Before the introduction of the ceramic bead neutral sections, a series overlaps would form the neutral section.

Overlap is not normally found in stations but is frequently located just outside. Some power feed stations, especially Mk3 equipment can be found where overlap occurs.

 

An overlap model would be a very nice feature on a section of plain OHLE equipped line especially it has to be equipped with isolator switches and booster transformers.  If your layout has more than 14 masts then either an overlap or mid point anchor is required

 

Twin track

 

Fixed tension 1500v DC and 6.25 or 25Kv converted from 1500v DC overlaps only take up two portal mast, boosters are carried on independent booster mast. 

 

Automatic tensioned contact wires are carried on four masts. The outer two support the balance weights and the inner two each carry a contact wire and out of running wire. An oddity of Mk1 overlap sections is the WCML uses portal mast for the centre two, and the GER and LTSR use cantilevers the same as for Mk3.

 

Above are samples of mast found at overlap sections.
Slow speed lines can only use three masts. The central one supporting the contact wire for the out going and in coming wires at the common contact point.

Overlap with isolating switches

Booster transformers

 

These are sited about every two miles or every two overlaps. Booster Transformers form part of the system for minimising electro-magnetic interference. They are not power transformers. Booster transformers on Mk1 equipment is normally on an independent mast, the LTSR line has them mounted on the cantilever mast.