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I've tried googling this but to no avail. I've been trying to figure out how streetcars can draw power from a short circuit.

For reference:

If I imagine the power lines simplified as a shorted battery like so

and we assume the streetcar acts like a resistor like so

Would it not just reduce back to the first diagram with the short circuit?

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1  
Um, how did you conclude that the distribution system was shorted? (I'm not trying to poke fun, but trying to understand your thought process on this.) –  Adam Lawrence Mar 9 at 18:30
    
Ah, I was simply assuming ideal components, ie wires have zero resistance. Of course, there must be resistance in the wires, however, I'm just assuming it to be negligible such that 99% of the current would flow through it and only 1% would flow through the large resistor. –  Drahcir Mar 9 at 18:53
    
@Drahcir It seems that you're confusing series and parallel connection. In what you have drawn, all the current would be going through the short and current would be going through the resistor. In reality, the resistor would be a streetcar and the resistance of the wires would be modeled by a resistor in series with the streetcar. –  AndrejaKo Mar 9 at 18:56

3 Answers 3

up vote 11 down vote accepted

Power feeds in from the overhead wire and connects to a pantograph pick-up on the roof. This routes the power through to a controller and then to the motor and the return path for current travels along the rails: -

enter image description here

I used to remember a thing called a trolley bus in my home town when I was a kid - it was a regular bus with rubber wheels but had a double overhead cable pick-up - positive (it was a dc feed) was on one wire and negative (return) on the other. Ye olde trolley bus: -

enter image description here

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Ah, from the diagram, the rails act as the return circuit, ie the -ve part of my circuit, and the top wire acts as the +ve part of the circuit. Without the train, it would be an open circuit. Follow-up question: If someone were to wire up the top wire and the rail, would all of the streetcars in Toronto stop working because of the short? –  Drahcir Mar 9 at 18:50
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@Drahcir I've actually witnessed that happening in my town. Overhead line broke and fell on a rail. All street cars at one end of the line lost power, due to fuses popping, while cars on the other end of the line continued working. –  AndrejaKo Mar 9 at 18:54
3  
@Drahcir as AndrejaKo implies - there are separate circuits for different sections. There will also be several "feed-in" points. –  Andy aka Mar 9 at 18:55
    
Toronto also had trolley buses at one time, but they were phased out in 1993. transit.toronto.on.ca/trolleybus/9003.shtml upload.wikimedia.org/wikipedia/commons/1/19/… (round headlights!) –  Spehro Pefhany Mar 9 at 19:09
    
@SpehroPefhany Due to my age (LOL) I remember them more like these old things - sandtoft.org.uk/index.html - this is actually a running museum near my home town. I'm gushing now boo hoo –  Andy aka Mar 9 at 19:24

Obviously the power lines are not like a shorted battery, as no meaningful power could be drawn if all of it is being 'burned' into a short circuit, so to speak. Therefore, since streetcars are able to draw power and run, the distribution system cannot be a short circuit. If there are no streetcars on a line, it's essentially an open circuit - each streetcar is a load, and multiple streetcars on a line are loads in parallel.

Wikipedia explains the concept of a trolley pole very simply:

When used on a trolley car or tram, i.e., a railway vehicle, a single trolley pole usually collects current from the overhead wire, and the steel rails on the tracks act as the electrical return.

I'm confident that an actual short circuit would trip a protection device at the DC distribution source and disconnect the line, leaving it unpowered until the fault was corrected. (Aside: there are 52 substations providing DC power to Toronto transit vehicles)

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The overhead lines carry +600VDC (isolated into regions that are supplied by individual traction power substations). Return current is through the rails, which are held at a voltage close to earth potential.

The streetcar appears as a load from the wires (through the trolley pole) to the rails.

See, for example: http://www.vivanext.com/files/EnvironmentalAssessments/YongeExtension/Conceptual%20Design%20Report/3277670%20Conceptual%20Design%20Report%20Appendix%20C.pdf

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