Where is Current drawn from when trains are in depots?
This question has arisen as several readers have noticed that on the inside roads in depots there are no traction current rails - only running rails, so the normal current shoes are 'off juice'. So where does the train draw its power from, should it need to be moved?
Firstly, it is worth considering how the 630v DC current is distributed around the train. The following diagram is for the D78 Stock train, but most London Underground stocks are configured similarly, though of course the number of cars varies from stock to stock, as does the physical location of some equipment.
(clicking on this image will produce a full size version)
It is worth looking at how the train is made up. Each six car train is actually two units, each of three cars coupled together. Our illustration shows two 'single ended' (a driving cab at one end) units coupled together; there are a number of 'double ended' (a driving cab at both ends) on the fleet, but for the purpose of this section I'm using the single ended configuration.
The train is made up as follows:
West Facing Unit
Driving Motor Car (DMC)<->Trailer Car (T)<->Uncoupling Driving Motor Car (UNDM)
East Facing Unit
Uncoupling Driving Motor Car (UNDM)<->Trailer Car (T)<->Driving Motor Car (DMC)
You will note therefore that when the two units are coupled together all the equipment is duplicated and you may also notice that between the two units there is no linking of the 630v DC current distribution as shown by the red and blue lines which represent positive and negative respectively.
For the purpose of this discussion, what we're interested in are the 'Shed Receptacle Boxes' which can be seen on cars 1,3,4 and 6. These are effectively like household sockets, though they work in reverse in that instead of distributing current, they receive it, in place of the normal shoes.
Externally this is what the Receptacle Box looks like (this was actually from the Museum's 1938 Tube Stock unit, but the equipment is generic):
This is the box as you may well see it when out and about on the Underground. This is in fact a hinged flap. When raised it looks like this:
(Thanks to my colleague Wayne Price for modelling his fingers in this picture!)
So, that's where the train gets its power, but where is it supplied from? Current is supplied from overhead cables (called Shed Leads) which are suspended from tracks above the trains as illustrated below.
Again, this is a 1938 Tube stock unit, but you can see the cables inserted into the train, if you follow the line of the leads towards the roof you can see the tracks on which they are able to run; another set of tracks is visible to the left, above the D78 Stock.
In the following photo by Chris Cobley taken in Neasden Depot the leads can be seen in the foreground hanging on their storage hooks.
Nowadays only depot staff insert and remove the leads and move trains with the leads attached. When a Train Operator receives a train for service it will have been drawn forward so that the front DMC is 'on juice' and therefore is receiving current conventionally and the leads to the rear of the train should have been removed. However, the Train Operator preparing the train for service must ensure that the leads have been removed; if a train leaves the shed with the leads attached substantial damage will occur to both the train and the shed structure.
To help avoid the situation on the D78 Stock the Train Monitoring System (TMS) senses the presence of a shed lead and warns the Train Operator by means of an illuminated light and a 'sonalert' (a audible warning).
If there was a shed plug inserted in the train the 'Shed Plug In' light would illuminate