3rd Rail Protection

[Old-School]

NightStar, as requested , here is a Picture of a short circuit bar
Click on Image to Enlarge

 

[Wikipedia]

On the proper underground sections the 3rd rail is at plus 440 volts and the 4th rail is at minus 220 volts, so if you used our SC bar you would have the 4th rail still with a potential voltage in it.

Don't LUL have there own SC bar that shorts the 3rd to the 4th?

3rd Rail Explained Short circuit bar at 07:00!

4 Rail System. Short circuit bar at approx 08:40. Very interesting to watch in full mind!

 

[jopsuk]

Ok. Now everyone has me intrigued as to what one of these bars looks like? Anyone have any photos?

Robert
--- old post above --- --- new post below ---
Ok this may seem like a silly question? However I am going to ask anyway. I understand the third rail runs at 750 volts DC with some rails being at 1200 volts DC. However I have never heard anyone give any suggestion as to the amperes coursing through the conductor rails? I know some diesel freight engines here in the US have 1800 Amps at full power to the traction motors. However I am under the impression the third rails are higher? More like 3000 amperes I thought I heard mentioned somewhere one time?

Robert

I don't think the UK 3rd rail ystem is ever (that) much above 750V?

One of the most powerful 3rd rail units is the Class 444. I've seen these quoted as having a power OUTPUT of 1500kW. The actual power draw will of course be higher, but if we assume that it is 100% efficient with no secondary systems, to draw 1500kW from 750V would require 2000A (as Current = Power / Voltage )

A double set will draw, from a single section, double that power. On the assumption a 450 draws almost as much power, a 12-car 450 is drawing 6000A by my calculation.

That's before efficency losses and power for control and "hotel" systems (lighting, heating, AC etc)

Any care to pick apart my maths?

(pity they could not work a sensible way to demonstrate those short circuit paddles and bars on energised rail)

 

[Monty]

Ok. Now everyone has me intrigued as to what one of these bars looks like? Anyone have any photos?

Robert
--- old post above --- --- new post below ---
Ok this may seem like a silly question? However I am going to ask anyway. I understand the third rail runs at 750 volts DC with some rails being at 1200 volts DC. However I have never heard anyone give any suggestion as to the amperes coursing through the conductor rails? I know some diesel freight engines here in the US have 1800 Amps at full power to the traction motors. However I am under the impression the third rails are higher? More like 3000 amperes I thought I heard mentioned somewhere one time?

Robert

A Class 450 Desiro will typically draw about 3000 amps when pulling out of a station, though this can vary.

 

[notadriver]

I thought the software on these units imposed a current limit such that they don't develop the full 1500 kW on DC. They seem painfully slow accelerating. An Electrostar (4 car) is quicker than a 444/450.

 

[455driver]

444s and 450s draw 100% power up to about 8mph, power then tails off very quickly to about 70% (444) or 55% (450) at about 20mph

 

[Jonny]

I don't think the UK 3rd rail ystem is ever (that) much above 750V?

IIRC the Bury - Manchester Victoria line had a different 3rd rail system which ran at 1200V DC; IIRC 2 when it and/or its associated rolling stock became life-expired, that was one of the prompts to Metrolink conversion.

 

[HSTEd]

Then there were the 4REPs..... 2400kW from an 80m long unit.

3200A even at 750V, once you include voltage drop.....

 

[NightStar]

Old School, Thank you for the picture. I was having a hard time picturing what one of those things looked like till now. I thought they looked more like a cross between who knows what and a gauge bar.

As for the amperes guess. I really had no clue so I was just guessing? 6000 amperes is probably more accurate? Although I would say 10 or 12 thousand maybe closer? There has to be a limit somewhere.

Robert

 

[Old-School]

NightStar, if you visit the link to the video "4 Rail System" posted by Wikipedia and look at early type of short circuit bar used by the London Underground, You'll find you wasn't far off from your original thought of looking something "more like a cross between who knows what and a gauge bar."

 

[jopsuk]

A Class 450 Desiro will typically draw about 3000 amps when pulling out of a station, though this can vary.

So potentially 9000A for a 12-car formation?
--- old post above --- --- new post below ---

IIRC the Bury - Manchester Victoria line had a different 3rd rail system which ran at 1200V DC; IIRC 2 when it and/or its associated rolling stock became life-expired, that was one of the prompts to Metrolink conversion.

That was an unusual system, as it was "side contact"

 

[NightStar]

Old School, I have a slow connection so I was just skipping over the videos as they take awhile to buffer for me. I am surprised you say I was pretty close with my guess! I have to admit I was batting blind on that one.

Robert

 

[notadriver]

I think 3rd rail is limited to 4000 amps thus longer trains perform more poorly than shorter ones.

 

[DXMachina]

But bear in mind that with EMUs, doubling the length of the train doesn't double the power consumption even for the same performance. The leading unit needs significantly more power than those following it due to wind resistance.