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Mrt - third rail

Mrt - third rail
Description:	location of "contact arm" & "third rail"
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Date:	20.01.2006 17:53
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Added by:	Samuel

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Samuel
Member

Join Date: 05.06.2004
Comments: 0

Oh yeah

feel free to addon man, it's good to learn something new! Hahahaha

Anyway once I was at the middle platform in JE station. The MRT enter, I think it had both side of the contact arm extended. I saw one of the contact arm hit the third rail, it actually sent off sparks!!

20.01.2006 19:08

Samuel
Member

Join Date: 05.06.2004
Comments: 0

But then again...

I still wonder how the MRT returns the current... through where?

20.01.2006 19:11

NickL
Member

Join Date: 16.11.2005
Comments: 0

Re: But then again...

Another contact arm.

20.01.2006 20:49

Samuel
Member

Join Date: 05.06.2004
Comments: 0

you sure about that

20.01.2006 20:59

NickL
Member

Join Date: 16.11.2005
Comments: 0

Yes. I think.

It must be a closed circuit for electricity to pass through, remember?

20.01.2006 21:02

k3nshiboi
Member

Join Date: 12.08.2005
Comments: 0

sparks???

i actually monitored it a few times, it dun comes from the third rail, instead is when crossover, the rails causes it instead of the third rail, i nv see such phenomenom at pasir ris or elsewhere...

21.01.2006 01:05

Samuel
Member

Join Date: 05.06.2004
Comments: 0

I remembered very clearly

because it just a few weeks ago,

when the other side (left) of the contact arm touched the third rail, i saw sparks from it...

21.01.2006 08:32

NickL
Member

Join Date: 16.11.2005
Comments: 0

Sparks...

can be caused by friction but also can be caused by contact that is too fast (the MRT travelling fast)? As for the sparks at the crossovers, it has to be along the older stretch of tracks of unused track like those emergency crossovers.

21.01.2006 08:41

Samuel
Member

Join Date: 05.06.2004
Comments: 0

return current...

If i recall correctly, because SMRT uses third rail, the voltage is very small (about 750V) so the return current is very high, so high that the cables use for the return current have to be very thick... so i think the thick cables by the side of the wall are for the return current.

29.05.2006 21:37

EMU12
Member

Join Date: 20.03.2004
Comments: 0

return current

is usually through wheels to the rails (earthed)...

08.07.2006 08:52

NickL
Member

Join Date: 16.11.2005
Comments: 0

I just noticed...

When I was at the end of Yishun MRT Station, both contact arms are extended (cannot be retracted) and are on the bogies.

08.07.2006 16:35

Westinghouse
Member

Join Date: 15.08.2006
Comments: 0

3rd rail "pick-up"

It's alwayz in the sprung-up position. Its spring loaded (upwards) so that firm contact could be achieved wherever a 3rd rail exists. It can be "disabled mechanically" or put down (no contact with 3rd rail during maintenance.

4 pick-ups in every car. All bonded to one point, so that continous 750 vdc supply can be garnered. Possible of a flash, or what is termed as "an arcing" will happen, if any fuse from one pick-up is blown. So, when the car passes on a 3rd rail gap (normally at crossings),high current, which will result in an arcing will take place. This is because either one of the pick-up fuse is blown and the other working one, is taking momentary power supply loss. This does not happen normally, as the fuses are regularly checked.

12.09.2006 22:36

kelvintwt
Member

Join Date: 28.01.2007
Comments: 0

sparks

i remembered i once seen a very bright spark coming from below the last door of the 3rd or 4th coach with a very loud "bang"..on NS line while entering braddell station

06.12.2007 03:49

Westinghouse
Member

Join Date: 15.08.2006
Comments: 0

Loud Bang...

...is the instantaneous release of air from the brake cylinders (dump valve)... as the brake arms 'locked the wheels'...

At any time, current is picked up on either left or right side 3rd rail...When any one of the fuse is blown (faulty), power is tapped from the other working fuse.

In Braddell (and many other locations too), there are 3rd rail 'gaps' bcoz it can't be continous all the time, due to power distribution design.

Relevant to the previous explanation, when that car was totally 'isolated electrically' from the 3rd rail...max friction brakes will apply as the 'regenerative brakes' was momentarily cut-off since no continuity on the 3rd rail.

Within split of a second when 3rd rail power continue to come in contact (as the train moves at speed), it'll restores regen braking and ALL FRICTION BRAKES RELEASED at once! the 'bang/s' comes from the discharge of compressed air in the brake cylinders.....via the dump valves.....

Can you follow?
It would be easier for those who have knowledge of regen braking, rheostatic braking and pneumatic braking....to understand this as it is quite complex...but be don't despaired....

I am willing to share with you if you have the interest and patience to know more abt this....

Knowledge is for ALL to share...

06.12.2007 09:34

Westinghouse
Member

Join Date: 15.08.2006
Comments: 0

Application of brakes

...application of brakes were taking place because a train will apply its brakes at all station. When you notice the arcing and loud sound, the train was in 'full braking mode'... regen brakes were in progress and brake arms were locked momentarily when regen brakes 'paused' for a while...

The arcing was a result of the other 'blown fuse' not picking up seamless 3rd rail.

06.12.2007 11:17