Frolicing critters

Yes indeed nothing is truly infallible unless the default is regarded as live and dangerous! I have always found light sticks handy and have used them for many years but I can light mine up without putting it anywhere near an electrical source. They are very good for determining with reasonable accuracy which wire is which in a twin and earth without cutting the sheath. Best used as an aid to other methods of checking whether a circuit is fed or not.

The best way to ensure the correct circuit breaker is off is to turn on the light or other device to be isolated on and then trip and reset the breaker three times ensuring the circuit follows. Sometimes it is the only way to determine where a circuit is fed from. When doing Crossrail enabling works at a tube station in the mid 1990s I had trouble identifying which breakers fed which equipment from a main switchroom. In the end the only method was to trip out breakers in turn and seeing what went off, at one point plunging large areas of the station into complete darkness as I was identifying main fuseswitches. It was an oft used technique known as 'dipping' when cables and equipment were not fully labelled and the cables were impossible to hand trace where they were run 'bunched' through ducts etc. The idea was to be quick and efficient, trip, count 1,2, reset,count 1,2 repeat.

On the contrary, they are insulators, there to stop the current leaking to earth. The power is supplied to the rails through cables at the end of each length of conductor rail.

I have always understand the rattling of the rails to be vibrations in the rails caused by the approaching train. (As in the old Western movies, where native Americans would put their ear to the track to detect the approach of a train - for some reason it is much more audible on the Underground without risking a fried ear)

Which part of the train - (wheels?, collector shoes?), is making the noise I'm not sure.

There are lots of things that combine to make up the 'rattle' from the rails, expansion and contraction, lateral bogie movement, loose chair bolts, loose fishplate bolts, looses anchors, insufficiently supported sleepers etc.

The running rails are pre-stressed and so their movement through expansion and contraction is limited under normal weather conditions, nevertheless even in normal weather it is possible for the two rails ends at a rail joint to 'butt up'during the day, having contracted against the bolts at night. For current rails expansion and contraction means the rails sliding through the supports, all that keeps them in place is weight and anchors every so often to prevent 'rail creep' in either direction, the anchors also having insulators inline where they are bolted to the sleepers.
Even though the track is regularly patrolled there will always be places where vibration causes bolts to slacken, turnout switch rails and their anchors can be prone to it.
Track will over time become insufficiently supported as a result of vibration, think of every train as a whacker plate vibrating the ballast beneath the track, weakness in the ground below usually due to poor drainage will create voids and the ballast will settle into them. The track then becomes bouncy exacerbating the problem and can lead to other issues such as loose trainstop mounting bolts. Vibration will also through up ballast which makes that stone throwing sound that is sometimes heard.

Tight curves will see the lateral movement of bogies causing wheel flanges to contact check rails, generally a greaser will keep any noise at low volume but they do dry up or fail and then audible squealing is heard.

Arcing and popping as collector shoes bounce along the juice rails where the ride is bumpy add to the cacophony.

Sometimes the P-Way walker will be happily tapping the keys back into the chairs as he inspects the track between trains, I imagine that is heard less and less as the railway replaces more and more bullhead track with flat bottom, I can't recall a pandrol clip insertion making any noise at all!

There are so many possible sounds and there are different symphonies to be heard at different times of the day and different times of the year.

I have the same problem at home when EON or Western Power Distribution are doing maintenance or fault finding or otherwise following auto recharge events. This is despite having UPSs on all my Desktops, laptops, linux boxes and peripherals. Following the latest incident a couple of weeks ago I have just invested another large sum into two new UPS units.

I find it incredible that (I presume an office full of) 200 computers would not have been battery backed. Surely it is unthinkable not to provide standby power in this day and age. I find it hard to believe that any competent electrician would not know or recognise emergency supplies whether generator, inverter or alternative mains input even if not labelled.

Going back to my school days, in order for any electrical circuit to work there has to be a potential difference between the two conductors, so one of them is always +ve or -ve with relation to the other. As a service engineer albeit with low voltage equipment, there was always a mains supply, and then a rectified step down transformer. Where you have in most office blocks a 3 phase fuse board, the fuse/circuit breaker is always on the +ve feed, with the -ve usually connected to a common bus bar. Its normal practice for the +ve feed to be fused. In a domestic situation your 3 pin plug has the fuse in the +ve feed(brown) the earth being a local earth only.

The "Live" leg is positive with respect to earth on each half cycle, then negative 20 milliseconds later, positive again 20 ms later, and so on - that is what "alternating" means
The "Neutral" leg is nominally at 0V with respect to earth, hence the name, but as it is insulated from Earth, in practice it may be a few volts either side, alternating in sympathy with the live leg, and will of course be carrying the same (alternating) current as the live side.

" in order for any electrical circuit to work there has to be a potential difference between the two conductors, so one of them is always +ve or -ve with relation to the other."
True, but a difference is all that is needed - it works whether the conductors are at +9V and - 9V (as in a battery)
or +420 and minus210V (LU standard)
or 750V and 0v (NR third rail system)
or 240V and 0V one moment, and minus 240V and 0V 20milliseconds later (mains ac)

It would work if they were at +1000V and +500V - there's still a difference: and indeed our choice of zero is arbitrarily determined by the electrical potential of the earth - in the same way that our choice of a zero for longitude, temperature, or height of land is arbitrary but convenient. Objects insulated from the earth such as aircraft can have a different local potential - which is what is thought to be what did for the Hindenberg

(Indeed, "earth potential" in an absolute sense can vary, for example as a result of electrical storm activity - an absolute zero potential would be one in which a test probe would experience no electrostatic attraction or repulsion - unlikely to be found over any significant volume of space unless you have a supply of neutronium - in which case electrocution would be the least of your worries!

The second one is the sound of wheels going over a joint - the clue is the way they come in pairs - because there are two axles on each bogie. The sound is different as subsequent wheels reach the gap because of the weight of the leading bogie on the rail.

Observing tube trains over the weekend, it is noticeable that the rails start to rattle just as the train reaches the beginning of the nearest length of conductor rail, so the sound is it presuambly associated with that, and not the running rails. Presumably it's the shoe gear sliding along the rail that makes the noise, but I don't notice it on the Southern - so is it something about the way the negative shoe (which of course do not appear on SR trains, which have an earth return) is fitted that makes the distinctive sound?

Wow! Thanks Brian. I can see from looking at the document that I will need to put my thinking cap on! As always, you are the guy to go to for the knowledge.

Railtechnician, your right, I was trying not to be too technical, and got myself total jabbered!. Lets not even mention deltas and 120 degrees! Mia Culpa

I'm always baffled by the strange cacophony of sounds made by tube trains passing over the joints on open sections. I grew up with the familiar 'de-dum de-dum' of bogie carriages, and the frantic 'dum-dee dum-dee' of unbraked coal hoppers, but the multiple complex sounds made by tube stock eludes me. Are the rail joints staggered? or perhaps the shoes also beat against the conductor rail joints? Difficult to imagine, the contact surfaces being so smooth.

The sound is captured for posterity in the recent film adaptation of LeCarre's 'Tinker Tailor Soldier Spy'. Smiley's room at the Hotel Islay is seen from the outside, and the distinctive LT sound is heard, despite the setting being on the approach roads to Liverpool Street!

My memories of travelling into London on the Central line as a child, a decade or so ago, include a constant, regular "de-dun, de-dun...de-dun, de-dun...de-dun, de-dun" as you went along. This seems largely to have gone away these days, with it being a much quieter ride. Am I just imagining things, or is there actually something that's changed? I still here the "de-dun, de-dun" at Woodford, especially when a train heads into the siding.

I have a vague memory of reading something on here about a new preference for longer rails with fewer joints, which has the advantage of a smoother quieter ride, but is more prone to problems caused by flexing or something of that kind. Could that be anything to do with it? Or did I imagine that too?

"Norbiton" hit the nail on the head there, most "running line" track (in the London area anyway) seems to now be of the new Continuously Welded variety, however most sidings and less-frequently used lines still have the old jointed rails if you look on the National Rail network and probably on LU too.
Meanwhile standing on the platform at Hounslow Central you can often hear a continuous hissing sound which to me seems to be the electricity in the rails

So it is. But whoever's law it is, the current on live and neutral sides must be the same.

Ohm's law is the one that says the voltage across a given rresistance is proportional to current.

There is often confusion between electric potential and potential difference - both are measured in volts. The voltage drop between generator and load is very small, but the resistance is low so the current is large. This is true on both sides of the circuit, whether they are positive and negative (e.g +420 and minus 210) , or live (+240) and neutral. The voltage difference between the two legs is large (because the resistance through the load is much greater) so there is nevertheless a large potential difference between the live and neutral sides, which is what drives the current through the resistance.

The voltage drop and consequent power losses over long cable runs can be significant - it is this factor which limits broadband speed over copper cables, and - getting back on topic - why substations are needed every few miles on electric railways. One of the problems with the original CSLR was that the steep ramp up into King William Street was at the point furthest from the power station at Stockwell, and there was a significant drop in voltage between the power station and KWS - the energy being lost in heating the rails and cables.

Broadband isn't limited by power loss as much as by signal spread, line noise and standing wave reflections. ADSL is a surface bound high frequency signal that migrates along the surface of the copper bundle - if there are tiny breaks in the copper, it doesn't affect the POTS, as that just goes through another strand and brings all the copper up to the potential, but if the HF wave hits the broken end, it reflects and bounces back creating a ghostly shadow signal which, if there's enough of it, will mask the true signal. Tiny differences in path length and impurities in the copper spread the normally sharp edges of the signal out in time and mean that a signal has to be regenerated at certain points, i.e. converted into digital and rebroadcast over the next leg.