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British Railways OLE, part four, Overlap

Clive Mortimore

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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, many have jumper wires from the wire leaving contacting it to the wire taking over and some have the wires that can be isolated from each other. These have the switchgear so that a length of wire can isolated 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 are where an overlap occurs. In viewing many of the tunnels on the West Coast Main Line there appears to be an overlap at each entrance


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


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 by harmonising the return feed with the power feed. They are not power transformers.










Twin track


Fixed tension 1500v DC and 6.25 or 25Kv converted from 1500v DC overlaps only take up two portal masts, 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. Booster transformers on Mk1 equipment is normally on an independent mast, the LTSR and GER lines has them mounted on the cantilever mast. On the WCML Mk1 overlap uses portal masts for the centre two, and the GER and LTSR twin cantilever masts are used, the same as for Mk3.


Slow speed lines may only use three masts. The central one supporting the contact wire for both the outgoing and incoming wires at the common contact point.




Curved Overlap


On curved tracks the overlap wires have to maintain that all-important contact with the pantograph. Like plain curved track the contact wire has to be on the outside of the curve at the registration point, therefore curved overlap has special mast.






I will do the masts drawings in the next few post.


Multiple track overlap will be delt with later.



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I will start the masts with a typical WCML Mk1 overlap. A quick note about the WCML mast drawings these are pre WCML upgrade when new power supplies were added and the registration arms were changed and some mast as well.










The next drawing is of the booster transformer mast that if in place sits between the second and third mast. This is a composite drawing showing the wiring arrangement for a booster transformer with an isolator on the left and one without on the right. Normally the arrangement was with or without isolator switches both sides.



If the overlap is one where isolator switches are needed then these are mounted on either the second or third portal.



As with curved track the overlap mast need to ensure the contact wire retain the all important presence with the pantograph, so like all curved track the pull off is on the out side and the push off is on the inside track of all four mast which means there is a different arrangement of registration arms etc. on the mast.









Curved overlaps can have isolating switches and booster transfomers.


At some locations an overlap is used to feed power to the overhead. I will explain power feeds later. This drawing is included just to show how impressive a overlap portal can look with a power feed.



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Keeping with the Mk 1 theme here are some round post overlap mast as found between Chelmsford and Colchester on the GER main line. Round mast post are found in other locations but are not that common.




On the next two drawings is the in line insulator as found normally on the compound cantenery but has been reused when this section of OLE was converted to simple cantenery.







Again booster transformers are carried on a separate mast. 




The isolators are carried on the twin arm mast.





In some locations the balance weights are on their own mast close to the mast with the registration arms. In the case of the overlap I based these drawings on it was close to a bridge and the encumberance or system height was reduced to allow the catenery wire to pass under the bridge.





The next two drawings needed are the second and third mast above. A booster mast could be added as well.















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On sections of track where Mk1 OLE is used but unlike the WCML portals are not used for the second and third mast for an overlap the registration arms differ from straight track, and still have maintain that interface between the contact wire and the pantograph. These drawings show round post but the cantilever arms are the same for H section mast.









For side views of mast 2 and 3 use the ones in the post covering the GER mast.


Booster mast and isolating switches can be added to this type of overlap.

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Mk3 did was not only placed above straight track but also curved track. These have been drawn with booster transformers, if modelling a overlap on a curve without the transformers the registartion arms are the same.










In the open post I mentioned slow speed overlaps only had three mast. To date I have not drawn the straight track ones but I have done some based on photos of OLE at hackney downs on a curve.







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