Third Rail - How does it work?!

[New Haven Neil]

Came across a photo elsewhere (can't post due  copyright) of a three wire system in my home town (South Shields).  One wire for the trams, two for the trolley buses that were slowly replacing them.  I can see issues if you put the wrong pickup on the wrong wire?

[melmerby]

1 hour ago, New Haven Neil said:

Came across a photo elsewhere (can't post due  copyright) of a three wire system in my home town (South Shields).  One wire for the trams, two for the trolley buses that were slowly replacing them.  I can see issues if you put the wrong pickup on the wrong wire?

That's unusual as usually trams and trolley buses ormally used a common feed, the return being the other wire for the trolley bus and the steel track for the tram.

i've a picture in a Tramway book of South Shields No.52 at the entrance to the depot, there are just two wires.

(It notes that the tram cost £2500, whereas a replacement Trolley buses was £2000!)

This appears to be the depot:

https://goo.gl/maps/Uhe6svzvpg72FokV7

 

Which has this on the side:

https://goo.gl/maps/sDbPXRMaw75X1uc68

Edited May 12, 2019 by melmerby

[BR60103]

Cincinatti street car (tram) system used 2 poles IIRC because of concerns about return through the rails. They were converted to one pole when Toronto bought them around 1952.

 

Toronto also had a few places where trolley buses and street cars shared wires. I remember one where they had to separate at the junction at the end of the street. There were a few other places where the street car wire was in the center of the street and the trolley bus wire was over the other lane.

 

[Pandora]

The 750VDC of the conductor rail is the simple part,  it is the the running rails which is the complex part,  the traction return current passes from the train into the running rails back to the electreical substation or TP hut,  and therefore the traction return current shares the running rails with AC rack circuits in the running rails, ( track circuits operate the signals ).

As an ex S&T I would not know where to start  an explanation,  here are a few terms: single rail and double rail AC track circuits,  res bonds, ally plates,  sideleads, black dogs,   impedance bonds,  fishplates: Benklers,   Edilons,  our raili, their rail  ie to describe if S&T or Pee Way are responsible for the electrical connections to the rail.

[Compound2632]

4 minutes ago, Pandora said:

The 750VDC of the conductor rail is the simple part,  it is the the running rails which is the complex part,  the traction return current passes from the train into the running rails back to the electreical substation or TP hut,  and therefore the traction return current shares the running rails with AC rack circuits in the running rails, ( track circuits operate the signals ).

As an ex S&T I would not know where to start  an explanation,  here are a few terms: single rail and double rail AC track circuits,  res bonds, ally plates,  sideleads, black dogs,   impedance bonds,  fishplates: Benklers,   Edilons,  our raili, their rail  ie to describe if S&T or Pee Way are responsible for the electrical connections to the rail.

 

As I understand track circuits, as a layman, they have to use a section of track that is electrically isolated from the rest of the line. Thinking about this as a physicst, I can see that there could be ways of making a section isolated at a suitably high ac frequency whilst connected to ground at dc - presumably by inserting some inductance across the isolating rail joint.

 

I'm hoping that gives Pandora the opening to start an expert's explanation for beginners. I've often found that the best way of eliciting a correct explanation is to propose an inadequate one.

[Pandora]

The magnetic field of the traction current travelling in the running rails is significant, AWS magnets in the four foot are stronger on the Southern,   identified by the green casing, (yellow elsewhere).

Point machines are different too, HW1000 for DC third rail, HW2000 for 25kV,  the machines have to be immune to electrical noise which could cause the point to attempt to self motor and unlock the switch

Edited August 7, 2019 by Pandora

[phil-b259]

33 minutes ago, Compound2632 said:

 

As I understand track circuits, as a layman, they have to use a section of track that is electrically isolated from the rest of the line. Thinking about this as a physicst, I can see that there could be ways of making a section isolated at a suitably high ac frequency whilst connected to ground at dc - presumably by inserting some inductance across the isolating rail joint.

 

 

Which is what those big lumps in the 4ft with chunky leads connected to the rails and long aluminium plates sticking out are (i.e. a big inductor / coil of wire)!

 

You combine them with capacitors to ‘tune’ them like you do a radio so the track circuit electricity (be it the bog standard 50Hz AC stuff or the very high KHz audio frequency based stuff) is blocked but your DC traction electricity can pass through unaffected.

 

To understand how traction return and track circuit based signalling can co-exist on the 3rd rail you need to first have a basic knowledge of AC electrical theory and the appreciate the concept that unlike DC theory, things like resistance are directly related to the electrical frequency, inductance and capacitance - not a simply a product of voltage and current.

 

 

 

[Compound2632]

2 minutes ago, phil-b259 said:

 

To understand how traction return and track circuit based signalling can co-exist on the 3rd rail you need to first have a basic knowledge of AC electrical theory and the appreciate the concept that unlike DC theory, things like resistance are directly related to the electrical frequency, inductance and capacitance - not a simply a product of voltage and current.

 

I'm there. Keep going...

[Nearholmer]

This contains a pretty good, easy to follow, explanation - keep going beyond the introductory part. https://www.noratelpowerengineering.com/impedance-bonds