Track Question

[Colin]

There's one thing that puzzles me with regard to track, even more so when steel sleepers are involved -

Water conducts electricity.
So how come when it rains, and there's clearly a puddle of water lying on a sleeper, and said water is clearly making a connection between the two running rails via their supporting chairs - why are the track circuits are not affected?

[Deleted]

if it was bad enough it does thats why we have to boost the track (voltage gain) to get the relay to pick up and energise. if the track gets totally flooded as it does in certain areas (elm park - dagenham east) and (hammersmith tunnel) it will knock out the breakers in the substation when the level raises to conductor rail height. this was the reason down at southfields where when it rained hard the track bed just held the water due to bad drainage and caused the track relay not to pick up properly thats why the signal down there used to bob (red-green-red) this has now been sorted. this was also the same reason why last month at wimbledon they could only use 1 platform this was also due to bad drainage and totally made the tracks fail

[Colin]

So it's basically to do with how the electricity in the track circuits is manipulated?

[Tom]

Boosting gives voltage gain?? Where were you during college Pat? ;D

Increase of capacitance leads to an increased capacative reactance (X_c). X_c increases, impedance of the whole circuit increases, voltage dropped across the capacitor increases, voltage on the track decreases but CURRENT to the TR increases. The vane driving further is because the coil has extra current to use, voltage is irrelevant.

[Phil]

Tom, are you sure you're right??

By my physics capacitative reactance, X_c =1/(2πfC), so the increase in capacitance REDUCES the total reactance

BTW π equals pi, f equals frequency

[Deleted]

Oh God, this takes me back to A-level Physics! I never could get my head around the electrical stuff.

[amershamsi]

rbruce1314 said:

Tom, are you sure you're right??

By my physics capacitative reactance, X_c =1/(2πfC), so the increase in capacitance REDUCES the total reactance

BTW π equals pi, f equals frequency

you forgot the j (or i) to change the phase by pi/2

Z=1/(j omega C) where Z is impedance j is root-1, omega = 2 times pi times the frequency.

for DC circuits, like a 3rd/4th rail electrification system Capacitors have no effect impedance wide, as they don't deal with imaginary impedance.

Simon

[stanmorek]

aetearlscourt said:

if it was bad enough it does thats why we have to boost the track (voltage gain) to get the relay to pick up and energise.

That's same the explanantion given to me by a signals manager. He said signal current is boosted to make up for leakage and prevent signal bobbing. When times were bad they were boosting it up to the maximum levels permitted by LU standards.

[stanmorek]

colin said:

Water conducts electricity.
So how come when it rains, and there's clearly a puddle of water lying on a sleeper, and said water is clearly making a connection between the two running rails via their supporting chairs - why are the track circuits are not affected?

This has been investigated before but I don't know if this ever resulted in a satifactory explanation. However, Metronet were more interested in how tunnel seepages on sub surface lines were causing problems as it was costing them a lot in penalty charges. The tunnel east of Kings Cross was known as the P-Way Shower! The theory was the mix of metal swarf, track grease, ballast fines with water was quite a potent mix. They eventually decided to spend more on improving drainage replacing sections of the six foot brick drain or tapping into the existing with cess drains.

I heard that a large concrete box (possibly a services duct) had been discovered buried by the track near Southfields which slowed up rainwater discharge. I guess no would own up to putting it there or it belonged to another PFI contractor that Metronet didn't know about.

[Deleted]

tom i never done any college was always down the pub ;D ;D ;D why does it show

so why is it then when you boost a converntial track circuit (do you remember what one of these are) you get a voltage gain

typical voltages are:

125 hz = 6V - 13V
33 1/3 hz = 12V - 30V

except met line which have very long track circuits so the voltages are nearly and some exceed the required limit
they have to parallel a track transformer in some cases to get the required current flow

[Colin]

Well I got lost from post #3 onwards!!

*I think* I get the jist of all that, but is there any chance of someone translating it into laymens terms?

[Tom]

rbruce1314 said:

Tom, are you sure you're right??

By my physics capacitative reactance, X_c =1/(2πfC), so the increase in capacitance REDUCES the total reactance

BTW π equals pi, f equals frequency

Been a while... need to think this one through. Do we (almost) agree current increases though?

[Deleted]

yeah the current increases aswell as the voltage

[Phil]

tom said:

rbruce1314 said:

Tom, are you sure you're right??

By my physics capacitative reactance, X_c =1/(2πfC), so the increase in capacitance REDUCES the total reactance

BTW π equals pi, f equals frequency

Been a while... need to think this one through. Do we (almost) agree current increases though?

Yes.

Increased capacitance=reduced reactance.

Since current equals voltage/ reactance (impedance actually, but let's not split hairs!!!) reduced reactance=increased current.

So, your memory of the effect was right, but not your physics in how to get there!!

[Deleted]

Eh, what? You've lost me, tell you that now.

This has got me thinking of those of you who drive proper trains with fuses. Whats it like to have a 30 amp fuse blow?

[Colin]

*clears throat* AHEM!!

colin said:

Well I got lost from post #3 onwards!!

*I think* I get the jist of all that, but is there any chance of someone translating it into laymens terms?

I'm still none the wiser!! ;D ;D ;D
All I want to know is, why dosen't water affect track circuits?

[Phil]

Put at its simplest, water conducts pretty poorly at under about 100volts. Stick a (sealed) car battery in the bath you're in and you won't notice it : drop a 240volt mains electric fire in and it will kill you!!!!!

[Deleted]

Surprisingly, ice conducts no electricity!

[Colin]

Thanks for that Phil (and Jim)

Now you've said that, i'm even more intriuged ;D ;D........

[russe]

Put at its simplest, water conducts pretty poorly at under about 100volts. Stick a (sealed) car battery in the bath you're in and you won't notice it : drop a 240volt mains electric fire in and it will kill you!!!!!

Water conductivity is not dependent on applied voltage. The potential difference applied to the skin however will greatly affect the body's ability to conduct current. Generally, the insulating properties of the subcutaneous layers under the dermis will start to breakdown and conduct, to an uncomfortable and potentially dangerous degree, when the voltage gets much over 80V. In itself, voltage does not kill. You can hang onto many million volts and not feel a thing. It is current that kills, and in excess of 10mA can be lethal, depending on the degree of wholebody exposure.

Surprisingly, ice conducts no electricity!

Yes it does, but not nearly so much as water, because of the lack of free ions within the ice lattice. Here's the general trend:



Russ

[Deleted]

It must be very little as a train on ice covered rails won't pick up any current at all.

[Deleted]

It should be noted that distilled water is an insulator - it is incapable of conducting electricity. The only reason why water does conduct electricity (and Phil's teacher knowledge might back me up on this too) is because mosty water is slightly alkaline, containing minor concentrations of various ions (like fluoride, chloride, and so on). The presence of these ions is what makes water conductive, and also explains why ice is not a conductor: when water freezes, the ions can't move and can't create an electricial circuit.

[Phil]

theonekea said:

It should be noted that distilled water is an insulator - it is incapable of conducting electricity. The only reason why water does conduct electricity (and Phil's teacher knowledge might back me up on this too) is because mosty water is slightly alkaline, containing minor concentrations of various ions (like fluoride, chloride, and so on). The presence of these ions is what makes water conductive, and also explains why ice is not a conductor: when water freezes, the ions can't move and can't create an electricial circuit.

Exactly true, TOK.

And while I'm replying, although Russ is quite right as far as he goes, basic Ohm's law applies, so as water has a fixed conductance (and resistance) the higher the voltage across the water the more current flows. Which is why a set amount of water allows more current at higher voltages (even though the phrase "conducts better" is misleading).

And all this is without the effects on the human body (as Russ says). To have a human body across the track as well as wet rails would be unlucky indeed.

Remember all you science students: it's volts that jolts, and amps that kill

[CSLR]

About eight years ago, I was driving a car in the rain when the whole world suddenly went white and there was the most unbelievable explosion followed by silence. I thought that it was a bomb. As things returned to normal, I realised that while my passenger was screaming neither of us appeared to be harmed.
At that point, the car door opened and a man (who I later heard had been the driver of the following car) told me that we had been struck by lightning.
From later research, I discovered that the average lightning strike is anything up to 300 million volts and many thousands of amps. Interestingly, the car was unmarked, the electrical system undamaged and the radio still worked.
What happened was a well-known effect in which an electrical charge flows around the outside of an object to reach earth. In this case the lightning was faced with an obstacle course of good and bad conductors which included rain, the air, the painted car body, bare metal and rubber tyres. In its desire to get to earth it simply wrapped itself around us and took the line of least resistance.
This phenomenon is known as external flashover and is another example to add to our rules. It is also a reason why (in some cases and with the correct voltages) electricity can flow across or around insulators. In instances where the ice or any other insulator is covered in water, the flow can also appear to breach the gap by flowing through the surface water.
If you speak to members of staff who work on railways, you will hear stories of electricity acting in all sorts of apparently strange ways. Some of these stories appear to break the very rules that we have discussed; yet for each there will be some rule that allows it to happen.

[Tom]

rbruce1314 said:

tom said:

Been a while... need to think this one through. Do we (almost) agree current increases though?

So, your memory of the effect was right, but not your physics in how to get there!!

Probably because it isn't taught in GCSE or A level physics (a friend of mine who did A level physics just about got to DC Circuit analysis, let alone any AC theory). And of course, when I was at school we studied combined science and I don't have any A levels.

Well, that's my excuse anyway, and I'm sticking to it.

[Phil]

Mes apologies Monsieur Tom! I assumed that someone in your job would have studied AC theory - it used to be A level till about 6 years ago. What the Hell DO they teach at Physics A level now?

[Tomcakes]

How we should all be more careful and choose environmentally friendly ways of generating A level (not quite, but GCSE at least).

[Tom]

I did study it - at both ONC and HNC but err, well, the maths started to take over at HNC. We did also look at it in the signal school, and some of us then didn't have an ONC.

[Phil]

tombeevers said:

How we should all be more careful and choose environmentally friendly ways of generating (not quite, but GCSE at least).

Which is nuclear - as all physical scientists have been saying ever since global warming became an issue. Why Gov'ts listen to a few deranged protesters as opposed to expert opinion I don't know, but at least we're back on the right agenda now.

Stops before he starts to sound like a certain ex-member whose name could be likened to old DR stock........

[Deleted]

Note to self: Electricity and porcelain insulators sometimes do not mix. I'll explain:

Many years ago, I was at Ashford in Kent, taking a few photos, when there was this enourmous bang; I turned round and there was this cloud of blue/grey smoke rising up from the tracks, accompanied by a loud humming noise. This turned out to be arcing from the conductor rail to the running rail. I later found out, that the arcing had melted a chunk from running rail where the 'flashover' occured. Apparently, the substation circuit breakers tripped for 7 miles either side of the station, along with 'lighting up' the track circuits for the entire vicinity of the station. It happened just before the rush hour. Whoops. I never found out what caused it.