Rail Mail in London.

[Karhedron]

I had a trip on the new Mail Rail in London this week and can thoroughly recommend it to anyone in, near or visiting London.

Thanks for the tip. I would like to take the kids on this, possibly over half-term. Is there a museum attached or is it just the ride. Would it keep them entertained for long or is it just a quick visit sort of place?

[Suzie]

If the contact is right it is possible to get burns from 12V. Just resting your arm inadvertently on a 3.5mm jack plug on a live PSU is enough to get a long lasting scar from the resultant ulceration if you don't notice (I didn't notice!). Swallowing a button cell is almost always fatal for a small child and that is just a volt or so.

 

Up to 50V is usually manageable to not cause injury or death by inadvertent contact while invoking common sense, but higher needs a little more care.

 

I suspect that 100V line PA speakers and telephone bell voltage are the few situations where you are likely to come across exposed terminals with more than 50V on them with little tamper protection  - but that would probably not be allowed if it were invented today. 

[Nearholmer]

Yes, Keith, Suzie has one answer: short-distance conduction across the surface of the skin. Another way of getting a burn from a battery is to accidentally short the terminals through a piece of jewellery, such as a finger ring, although that might technically not be an electrical burn.

[TomJ]

Button batteries are very very dangerous to small children. They are just the right (wrong?) size to lodge in the oesophagus. There the current causes ph change and local burns to the surrounding tissues. Modern 3v are worse than the older 1.5v. This cause ulceration and perforation (holes) in the oesophagus which causes very very serious injury and even death. Also as they corrode they leak, causing further tissue damage.

 

So public health message of the day - keep them well out the way of children. If you have ANY suspicion they have swallowed one take them to A&E immediately. it's too late to wait till they complain of symptoms.

[phil-b259]

Thank you, Edwin.  I get the bit about using higher voltages to minimise losses between supply stations, but wasn't aware of the Skin Effect.

 

The thinking behind my original question is that d.c. is more dangerous in that if touched, it causes muscles to contract involuntarily and the person (or creature) cannot pull themselves away from the live rail - they remain a hazard until the current is switched off.  On the other hand, a.c. tends to throw those touching away from the danger.  Consequences in both cases are pretty serious - but perhaps (other things being equal) a greater chance of survival on a.c.  Or have I got it wrong?

 

You need to remember that the tendons in your body which moves your arms, fingers, etc and allow you to grip / release things are all controlled by electrical signals from the brain. Effectively to grip something an electrical charge is applied.

 

When you get a DC electric shock then your tendons will react in a single direction - and until the electrical voltage stops they stay locked in that position preventing you from letting go!

 

By contrast an AC voltage, which by definition alternates and usually passes through the 'zero volts' point at some stage will not apply a constant voltage. While at some parts of the waveform it may tighten your grip, a split second later the electrical signal passes through zero and is of the opposite polarity causing the tendons in your body to relax.

 

The net effect of this is an AC voltage will tend to 'throw' you off - which would make it more survivable were it not for the fact that AC systems tend to have a very high voltage that will kill you outright anyway!

[PhilJ W]

I remember as a child that putting ones fingers across any two of the rails of live Trix Twin track would result in blisters where you came in contact with the track.

[EddieB]

Eddie

There is a great, big, detailed CEN document setting out the affects of electric currents on the human body, and the upshot is that it is the voltage (and hence the current flowing through the body) that is important, along with the route through the body (across the heart is particularly bad, even at surprisingly low voltage/current).

In short(!) ac or dc is not to be messed with at voltages above a few tens, and even if it doesn't get you by electric shock, it can get you by burns ....... think of a 12V car battery, and the electrical burns that can cause, for instance.

Kevin

You need to remember that the tendons in your body which moves your arms, fingers, etc and allow you to grip / release things are all controlled by electrical signals from the brain. Effectively to grip something an electrical charge is applied.

 

When you get a DC electric shock then your tendons will react in a single direction - and until the electrical voltage stops they stay locked in that position preventing you from letting go!

 

By contrast an AC voltage, which by definition alternates and usually passes through the 'zero volts' point at some stage will not apply a constant voltage. While at some parts of the waveform it may tighten your grip, a split second later the electrical signal passes through zero and is of the opposite polarity causing the tendons in your body to relax.

 

The net effect of this is an AC voltage will tend to 'throw' you off - which would make it more survivable were it not for the fact that AC systems tend to have a very high voltage that will kill you outright anyway!

Both of which points were covered in my school physics and reinforce the point behind my question, namely that if we were to take comparable a.c. and d.c. systems, then the a.c. appears to be safer; so why not more examples of bottom contact a.c.?

 

As an aside, I think it might have been Stephen Jay Gould in one of his “this view of life” essays who wrote about the arguments that raged just over a hundred years ago over a.c. or d.c. being used to execute in the electric chair in the USA.

[melmerby]

Both of which points were covered in my school physics and reinforce the point behind my question, namely that if we were to take comparable a.c. and d.c. systems, then the a.c. appears to be safer; so why not more examples of bottom contact a.c.?

 

 

Their is at least as much evidence that AC is more dangerous than DC, but both are dangerous at "Traction" voltages.

 

Keith

[melmerby]

If the contact is right it is possible to get burns from 12V. Just resting your arm inadvertently on a 3.5mm jack plug on a live PSU is enough to get a long lasting scar from the resultant ulceration if you don't notice (I didn't notice!).

You must have something wrong with your arm!

The resistance of my arm is just too high to get any sort of current flow, which is required to cause skin damage

 

I just tried it. The contact resistance was over 2M ohms, even pushing the sharp prods into the skin about 1cm apart didn't get below 100k ohm.

However if you have some coating on the skin which makes it more conductive, that is another matter entirely.

 

Keith

[phil-b259]

Both of which points were covered in my school physics and reinforce the point behind my question, namely that if we were to take comparable a.c. and d.c. systems, then the a.c. appears to be safer; so why not more examples of bottom contact a.c.?

 

 

Because 'safety is not the only parameter to consider when it comes to electricity - Efficiency and practicality are just as important.

 

If bottom contact is used (and thus the rail can be enclosed then it doesn't matter what voltage you use) the risk of getting an electric shock is minimal - but as was highlighted in a previous post, the laws of physics will favour DC over AC in this instance.

 

In an age before sophisticated electronics a DC traction motor was the only realistic choice. In a age where the only way of transforming AC to DC was either large mechanical motor generator sets (rotary converters) or fragile mercury arc rectifiers then the final delivery of power to the train had to be DC. In an age where death and injury were dismissed as an 'occupational hazard' of working on the railway and where society did not feel the need to protect idiots from harming themselves, exposed DC conductor rails had a lot going fort them.

 

If the semiconductor revolution had not taken place in the 1960s making the provision of train mounted AC - DC conversion possible at 'point of use' then we would still be installing DC electrification (though at 1500VDC in OHL) - not 25KV AC to power trains.

[jjb1970]

Both of which points were covered in my school physics and reinforce the point behind my question, namely that if we were to take comparable a.c. and d.c. systems, then the a.c. appears to be safer; so why not more examples of bottom contact a.c.?

 

As an aside, I think it might have been Stephen Jay Gould in one of his “this view of life” essays who wrote about the arguments that raged just over a hundred years ago over a.c. or d.c. being used to execute in the electric chair in the USA.

Didn't Edison promote the electric chair to undermine AC and tried calling it the Westinghouse? The idea being to equate AC with getting fried and ending up dead. Edison was a fine engineer and businessman but there were some rather unpleasant sides to him too.

 

Somewhat ironically given the popular truism that AC is more efficient for transmission and distribution modern systems are increasingly ultra high voltage DC and even marine power systems onboard ships are starting to adopt DC networks. However it is quite a different DC concept.

[Nearholmer]

Yes, Edison tried to 'trash' a.c. by all sorts of devious news-management, including promoting an ac electric chair, electrocuting dogs, and electrocuting elephants. IIRC his first business venture was as editor of a tabloid newspaper, and he had the instincts that went with it.

 

Eddie - can I recommend "electric railways 1880-1990" by professor Duffy.

 

It's not an 'enthusiast' book, but it is written in a very easy to follow way, and explains all of the 'why' as well as the 'what'. The chapter about the BR electrification policy is the best quick summary of BR motive power policy in the 1950s that I've read, and the book has really good coverage of 'thermo-electric' as well as straight electric motive power.

 

Kevin

[mezzoman253]

Thanks for the tip. I would like to take the kids on this, possibly over half-term. Is there a museum attached or is it just the ride. Would it keep them entertained for long or is it just a quick visit sort of place?

We went with a U3A group shortly after it first opened. The museum is relatively small but very interesting. It's on the other side of the road, and a little further along, to the train ride.There's a cafe/restuarant and the food was very good..

 

We were advised not to take unecessary items on the train.There are small lockers, in both museum and train area to put loose items in. They took an old pound coin, probably a new one now..It's a tight fit on the train. But I'm 6' 3" and managed to fit in I'm also claustrophobic, but it wasn't as bad as I'd imagined. There are other exhibits in the train area whilst waiting for the train. It's 20 minutes between trains.

 

https://www.postalmuseum.org/ for details.

 

Rob

Edited October 17, 2017 by mezzoman253

[melmerby]

 

If the semiconductor revolution had not taken place in the 1960s making the provision of train mounted AC - DC conversion possible at 'point of use' then we would still be installing DC electrification (though at 1500VDC in OHL) - not 25KV AC to power trains.

Unfortunately not completely accurate.

There were ways of rectifying high voltages before semiconductors

As part of the 25kV electrification scheme at first BR AC class 83 electric locos had mercury arc rectifiers.

They later got replaced with solid state devices.

 

Keith

[phil-b259]

Unfortunately not completely accurate.

There were ways of rectifying high voltages before semiconductors

As part of the 25kV electrification scheme at first BR AC class 83 electric locos had mercury arc rectifiers.

They later got replaced with solid state devices.

 

Keith

Which was why I made reference to mercuary arc rectifiers in my post! - the Southern Railways late 1930s electrification used them in their 'raft style' substations I believe.

 

The point is that while such devices were a vast improvement on the rotary converters previously required, in general such rectifiers were not only bulky but they didn't like being shaken about as it increases the chance of the vacuum being lost and them burning out.

 

Therefore putting them in locos was generally not thought to be a good idea - hence the general use of DC overheads in the 1930s.

[Nearholmer]

Yes, in this country, both LT and SR installed MARs in the 1930s; I'm less sure about what the LNER and LMS schemes used, but I think they too used MAR.

 

But, on train, mercury devices were pretty fragile, even the single-phase ones, and there were other, slightly weird and wonderful, arrangements tried, including rotary single to polyphase convertors, before diodes reached sufficient ratings and everybody breathed a sigh of relief.

Edited October 17, 2017 by Nearholmer

[jjb1970]

Rotary converters can still have their uses. I was involved in a design as recently as three years ago which reverted to rotary conversion to supply a certain sensitive distribution system on a..ahem...military vessel when none of the normal techniques using solid state power electronics and filtering could sufficiently isolate the system from system harmonics and frequency disturbances feeding back from the main power and propulsion system. Eventually somebody suggested trying rotary converters and after the laughter died down it was tried more out of desperation than anything else but it worked.

[melmerby]

Which was why I made reference to mercuary arc rectifiers in my post! - the Southern Railways late 1930s electrification used them in their 'raft style' substations I believe.

 

The point is that while such devices were a vast improvement on the rotary converters previously required, in general such rectifiers were not only bulky but they didn't like being shaken about as it increases the chance of the vacuum being lost and them burning out.

 

Therefore putting them in locos was generally not thought to be a good idea - hence the general use of DC overheads in the 1930s.

I was replying on the basis that several suppliers of "modern" high voltage AC locos in the late 50s (classes 81-84) thought they were a practical proposition to install in the locos rather than in the substations (as it turned out they were less than ideal!).

They worked reasonably enough on nice level track but were seriously disturbed when bounced over points and crossings.

However it wasn't until 1971-2 a start was made on replacing them with silicon devices, so many locos operated for 10 or more years with the mercury rectifiers.

 

Class 85 had Germanium rectifiers from the start

 

Keith

Edited October 17, 2017 by melmerby

[Nearholmer]

If you want to get really into the development of drives for single-phase 50Hz locos, there is a paper somewhere on the web (which, frustratingly, I can't find right now!) which gives technical details, and results of performance in service, the SNCF "smoothing iron" classes, 12000, 13000, 14000, and 14100, each of which had a different configuration. There were German and Hungarian systems pre-WW2, too.

 

All of which doesn't have a great deal to do with the GPO!

 

PS: this isn't the paper I was thinking of, but it's close https://www.j3ea.org/articles/j3ea/pdf/2008/01/j3ea08040.pdf

Edited October 17, 2017 by Nearholmer

[45669]

The visiting public at Mail Rail are kept back from the platform until the train has arrived and the passengers on it have de-trained and cleared the platform.  So there would be no risk if there was a third rail.  However, if the train was to fail in mid tour and the passengers had to get out and walk it would be a different matter... 

[45669]

Thanks for the tip. I would like to take the kids on this, possibly over half-term. Is there a museum attached or is it just the ride. Would it keep them entertained for long or is it just a quick visit sort of place?

 

Sorry not to have replied earlier.  The Postal Museum across the road is quite interesting  and there is also a small display relating to the Post Office Railway which one can look round either before or after the train ride.  We had our train ride, then went across the road to the café at the Postal Museum for lunch, had a look round the Postal Museum, and then went back to the railway again to look round the Mail Rail display. 

[Broadway Clive]

I visited this Wednesday morning with my wife and when we tried to buy tickets we were told the train was fully booked for the day, so its obviously necessary to book in advance.

[PhilJ W]

I visited this Wednesday morning with my wife and when we tried to buy tickets we were told the train was fully booked for the day, so its obviously necessary to book in advance.

A fellow MRC member is going next Tuesday. His tickets were provided by his grandkids but he had to change the start time to later so that he could use his twirly pass to get there.

[melmerby]

If you want to get really into the development of drives for single-phase 50Hz locos, there is a paper somewhere on the web (which, frustratingly, I can't find right now!) which gives technical details, and results of performance in service, the SNCF "smoothing iron" classes, 12000, 13000, 14000, and 14100, each of which had a different configuration. There were German and Hungarian systems pre-WW2, too.

 

All of which doesn't have a great deal to do with the GPO!

 

PS: this isn't the paper I was thinking of, but it's close https://www.j3ea.org/articles/j3ea/pdf/2008/01/j3ea08040.pdf

16 2/3 Hz (cycles/sec) was chosen initially so that DC motors could be run from an AC supply without them protesting too much, because as the frequency increases so the losses increase.

However providing this current for the OHLE is more complicated than systems which use 50Hz (or 60Hz) off the main grid.

Those countries that invested heavily in very low frequency AC are now pretty well stuck with it until all traction equipment can run off multiple supplies.

 

Keith

[Suzie]

The USA had 25Hz AC (at 11KV or 22KV) with replacements and extensions now being done with 60Hz (at 25KV) because the legacy low frequency only traction has all been retired and using industrial frequency and standard voltage makes life a lot easier.