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Potential catenary danger?

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Potential catenary danger?
Posted by Enzoamps on Saturday, July 17, 2021 11:41 PM

OK, as I understand electric locomotives, the single wire catenary frovides current at a high voltage for the locomotive.  As I further understand it, the rails act as the return or ground for the circuit.  Electricity always needs a circuit.  If that is not correct, please fix me.

Hypothetically, if the electric loco winds up on the ground - ie wheels on the ties, not touching the rails - and the pantograph is still in contact with the catenary, would the wheels and/or chassis be electrically "hot"?   The wheels would no longer be grounded via the rails.   Without a complete circuit, no current would flow through the system so there would be no voltage drop.  (Ohm's Law) Meaning the full thousands of volts might be present on the wheels, waiting for some unfortunate soul to touch them.

Make any sense?

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Posted by blue streak 1 on Sunday, July 18, 2021 2:05 AM

In that case the engineer would make a quick call to dispatch power board in the meanwhile (  drop the pan.

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Posted by beaulieu on Sunday, July 18, 2021 2:08 AM

There are a lot of assumptions in that, The first would be how would the pantograph still be in contact with the wire if the locomotive was on its side. second would be that the crew would not open the main circuit breaker. The third would be how would the pantograph be undamaged as damaging the pantograph causes it to lower, as the pantograph is held up by air pressure through the pantograph head to cause the pantograph to lower in case of damage without any action by the crew. And finally that assumes that the Overheads circuit breaker wouldn't trip sensing a ground fault. Yes all that could happen and yes just like any overhead wiring you could have live power on a locomotive lying on the ground but it isn't likely. Never the less it is a good idea to call the railroad emergency number if you see an electric locomotive lying on its side.

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Posted by daveklepper on Sunday, July 18, 2021 9:34 AM

Both measures are standard procedure in any derailment.  With third rails, open the line-switch replaces lowering the pantograph.

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Posted by zugmann on Sunday, July 18, 2021 9:43 AM

beaulieu
There are a lot of assumptions in that, The first would be how would the pantograph still be in contact with the wire if the locomotive was on its side. second would be that the crew would not open the main circuit breaker.

When you're in that bad of a pileup in electrified territory, the major concern is that poles are knocked over and you have wires laying on the train.  Yes, everything and its cousin should trip and there should be no power, but you never assume. 

 

   The opinions expressed here represent my own and not those of my employer or any other railroad, company, or person.

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Posted by Enzoamps on Sunday, July 18, 2021 11:04 AM

In my scenario, the loco was not on its side, it was sitting on the ties, as stated.  A simple derailment, not a major wreck.   My question was/is a technical one, not a procedural one, not wondering what they would do in response to the situation.  Yes, they should immediately drop the pantograph, and so on.  I wouldn't think dropping six inches would be enough to lose pantograph contact with the catenary.

However, in the moment before the alert engineer does drop the pantograph, flips the breaker, and what-not, would the train frame or wheels represent  shock hazard to anyone touching it while themselves on the ground?

Would all this ever happen?  Likely not, but that isn't my point.  I am curious about the electrical nature of the system.

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Posted by zugmann on Sunday, July 18, 2021 11:06 AM

Enzoamps
In my scenario, the loco was not on its side, it was sitting on the ties, as stated.  A simple derailment, not a major wrec

If an engine's wheels drops off the rails, the frame, pilot, body, something is usually still against the rails. 

   The opinions expressed here represent my own and not those of my employer or any other railroad, company, or person.

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Posted by rdamon on Sunday, July 18, 2021 11:35 AM

Are automatic reclosers used in transit applications?  Or if a fault is detected do they need to be reset manually?

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Posted by MidlandMike on Sunday, July 18, 2021 9:30 PM

zugmann

 

 
Enzoamps
In my scenario, the loco was not on its side, it was sitting on the ties, as stated.  A simple derailment, not a major wrec

 

If an engine's wheels drops off the rails, the frame, pilot, body, something is usually still against the rails. 

 

Maybe the traction motor casings would be resting on a rail.

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Posted by Enzoamps on Sunday, July 18, 2021 10:12 PM

Ah, never mind, I don't want to belabor it.   yes, it is likely SOMETHING would be touching the ground rail.  Just wondering about the unlikely situation where nothing does. I was tying to ascertain what situation exists when nothing does.  Perhaps the systems on board have a sense system to detect loss of cicuit.  Some sort of auto-disconnect.

I appreciate the replies.

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Posted by Electroliner 1935 on Sunday, July 18, 2021 11:04 PM

Enzoamps

Ah, never mind, I don't want to belabor it.   yes, it is likely SOMETHING would be touching the ground rail.  Just wondering about the unlikely situation twhere nothing does. I was tying to ascertain what situation ex

ists when nothing does.  Perhaps the systems on board have a sense system to detect loss of cicuit.  Some sort of auto-disconnect.

I appreciate the replies.

The simple answer is if the Electric Loco is sitting on the ties, it could be disconnected from it's normal "ground" return path but in all likelyhood would still have a return ground path via the coupler to the following cars steel frame and wheels. It is always best to take the safe action and assume the beast is energized until you know its not. 

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Posted by M636C on Sunday, July 18, 2021 11:54 PM

I would be very surprised if a derailment would occur in the manner suggested by the original post.

I attach a report of an actual derailment, in this case of a commuter train.

https://www.onrsr.com.au/__data/assets/pdf_file/0003/19155/Waterfall-final-report-Volume-1.pdf

The result of the accident is seen in the frontispiece photo.

I have never seen or heard of an accident where all wheels of a vehicle left the track and the catenary and supports remained undamaged and powered up.

The report I link to was caused by the train driver dying of a heart attack. The dead (literally) weight of his legs were enough to hold the "dead man's pedal" in place. The train guard, having never had to do anything in many years of service, panicked and failed to stop the train.

The first report of the accident was from a schoolboy who was travelling to high school, uaing his mobile phone. He called 000 (The Australian equivalent of 911) but was told that since the train operator knew nothing about the accident, he must have been making a nusiance call and to go away. (I understand that an attempt was later made to delete the recording of that response.).

But nearly everyone involved managed to get it really wrong that day.

But that is what actually happens, becoming a lawyer's retirement fund benefit shortly afterwards.

Peter

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Posted by Overmod on Monday, July 19, 2021 9:30 AM

Electroliner 1935
The simple answer is if the Electric Loco is sitting on the ties, it could be disconnected from it's normal "ground" return path but in all likelyhood would still have a return ground path via the coupler to the following cars steel frame and wheels.

This is incredibly unlikely unless there has been massive internal damage or the pan has become shorted to the carbody structure.  It goes almost without saying that the ground return is carried via the cables and motors to the wheels, and these would obviously not be shorted or tied to the frame or carbody, let alone allowed into the draft gear to induce wild stray currents.

Now I think it will be highly likely that at least some of the derailed wheels in the example originally posed would have some defective ground return through the ballast, especially if it is contaminated and wet, and this might rise to a dangerous level relative to adjacent bonded return... if the power director does not react properly.

There were, in fact, some railroads that arranged to pass power from one locomotive or vehicle to another without fancy contacts in special couplers.  Reading MUs and the GN locomotives as sent to PRR to become FF2s are a couple of examples.  Presumably a pan left up on a derailed unit would keep the whole shebang 'ground returned' although I doubt this would not limit the ability of the AC potential to generate lethal current through a 'relatively high-resistance contact' of any local leakage from wreck damage plus human grounding...

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Posted by 7j43k on Monday, July 19, 2021 10:38 AM

I think it unlikely that the ground return for the 15-25 kva system is through the cabling and motors.  They are running at a much lower voltage.  That system may or may not be earth grounded--I see no reason to do so, but....

That said, I do wonder at the normal ground path for the high voltage of an electric locomotive.  I would be anxious to have it going through the axle bearings to get to the wheels, for example.  But perhaps my anxiety is unfounded.

 

Search, and you might find:

 

https://www.quora.com/How-do-electrical-locomotives-get-current-as-there-is-a-connection-of-train-with-single-wire-over-it-and-there-is-no-return-path-for-becoming-a-closed-circuit

 

"axle brush"---hmmm....

By the way, for a 5000 HP locomotive, a quick ball park calculation shows 150 amps going through the pantograph and thus through those axle brushes.

 

While EXTREMELY unlikely, I don't think it impossible for an electric locomotive to derail and keep its pan connected but be insulated from ground.  On that extremely unlucky day, I expect (if the engineer neglected to drop the pan, or etc) that the engineer would be dead just a moment before he touched the ground.

And "onlookers" would likely see his body and be warned of a difficult problem to solve.

 

Ed

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Posted by bogie_engineer on Monday, July 19, 2021 12:42 PM

Your concern for current traveling thru the axle bearings is well founded. It's not uncommon that besides using an axle brush that the bearing outer race is ceramic coated to insulate it. 

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Posted by Overmod on Monday, July 19, 2021 1:44 PM

Remember that there are two separate 'circuits' in an AC locomotive: one the HVAC through the primary; the other either the tapped AC going to universal motors or the rectified DC (either as DC-link to inverters or output from Ignitrons, motor-generators, etc.  Obviously there has to be a circuit through a rail for a single-phase AC supply using only one wire, even if that is technically more of a whopping earth ground; it is difficult to imagine how AC power could work without a 'matching' source/sink of the electron flow... and the only thing metal touching the rail is the wheels.  Easiest way at present (I almost said 'currently' but thought better of it!) to get to the rail/wheel contact patch is via a contact on the axle end, even if this does have the current going 'through' the center of the physical bearing... does anyone here have a detail picture or diagram of the early Budd Pioneer MU trucks with the outside disc brakes that shows where the axle brushes were located?

There is no enormous technical reason why you couldn't rig a separate pickup shoe to slide on the rail on a pantograph equivalent.  There are many practical reasons why that's not done, starting with contact pressure (or more significantly its momentary absence as the suspension works!) and ending with obligate methods to reduce sliding wear on the actual shoe contact patch.  You'll think of some of the others without too much trouble... Smile

7j43k
I do wonder at the normal ground path for the high voltage of an electric locomotive.  I would be anxious to have it going through the axle bearings to get to the wheels, for example.

Oh, you do NOT want any current whatsoever going through rolling-element bearings.  And that means NONE, not even that from tired induction.  Ever watch that YouTube video where some guy uses EDM principles to go right through a ball bearing?  It takes VERY little time for sparking to erode the surfaces of rollers and races, right through (and in fact enhanced by) the tribology.  No fun except to judgmental railfans will come of that!

I think I have told the story about the line of 2HP commercial treadmills that were built without a good third brush... but came to acquire them during warranty repairs.  The situation is far more extreme in a locomotive environment...

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Posted by timz on Tuesday, July 20, 2021 12:25 PM

Never occurred to me to wonder about that -- AC electric locomotives always have axle brushes? And always have had?

When RRs used jointed rail, did electrified RRs always have extra-large bond wires at each joint? Someone once described seeing them glowing red on the PRR main west of Philadelphia when an ore train went west.

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Posted by Erik_Mag on Tuesday, July 20, 2021 8:31 PM

timz

When RRs used jointed rail, did electrified RRs always have extra-large bond wires at each joint?

You bet your sweet bippy they did.... Bare minimum would be a 4/0 wire on each side.

Maintenance of the bonds was an on-going item back in the jointed rail days.

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Posted by CSSHEGEWISCH on Wednesday, July 21, 2021 10:07 AM

interurbans were often notorious for not maintaining bonds properly, which led to some interesting routings for the return current.

The daily commute is part of everyday life but I get two rides a day out of it. Paul
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Posted by Overmod on Wednesday, July 21, 2021 10:51 AM

timz
Never occurred to me to wonder about that...

There are going to be people who visualize the current as going between the overhead catenary and the rail 'directly', right through the motor and all, as in model trains with live overhead wire.  It might not occur to them that an AC system would involve a 'return' connection on the transformer-primary power even if the motor circuits were effectively 'isolated'.  

Even three-phase systems that used only two wires had traction current going through the rails... as erikem pointed out a few weeks ago, the older 'two-phase' system (that had the fields in quadrature by analogy with two-cylinder DA steam locomotives) also requires the track return.

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Posted by Erik_Mag on Wednesday, July 21, 2021 11:39 PM

CSSHEGEWISCH

interurbans were often notorious for not maintaining bonds properly, which led to some interesting routings for the return current.

A fair number of street railways fell into the same boat. There is quite a bit of literature dating from ca 1910 to ca 1930 with respect to electrolysis caused by poor bonding in street railway trackage. The includes at least two NBS publications. While interurbans had issues with "interesting routings for the return current", there was less infrastructure to be damaged in rural areas than builtup downtowns of big cities.

In a similar vein, a 1927 issue of General Electric Review was largely focused on the IC suburban electrification,where one of the articles was focused solely on bonding.

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Posted by blue streak 1 on Thursday, July 22, 2021 12:15 AM

Can we assume that tracks are always grounded to earth at locations where the current can go two ways to front and rear?  Otherwise in a derailment that occurred behind the loco that separated the rails would leave potential  voltage on the rails with no means to go to ground.. So if I am an engineer that has a sudden emergengy brake application is it prudent to lower the pan as soon as I am stopped ?

 

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Posted by blue streak 1 on Thursday, July 22, 2021 1:07 AM

CSSHEGEWISCH

interurbans were often notorious for not maintaining bonds properly, which led to some interesting routings for the return current.

How well are the various operating electric museums doing about keeping their bonds in good shape ?  I know if I visit any I certainly will take a look.

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Posted by daveklepper on Friday, July 23, 2021 8:25 AM

Welded Rail is not infinetely long.  There still are joints and required bonds.   Just a lot fewer of them.  

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Posted by Paul Milenkovic on Friday, July 23, 2021 10:51 AM

daveklepper

Welded Rail is not infinetely long.  There still are joints and required bonds.   Just a lot fewer of them.  

 

 

What ought to be done is to braze the joints between rails, braise a thick cable to the joint, and then pass the insulated portion of this cable through a hole in the ground, where there is an underground cavern where the cables from the track connect to a through connection using these gigantic "wire nuts" (solderless connectors).  And don't start with telling me I need to post this on the MR Forum! Laugh

If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
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Posted by Overmod on Friday, July 23, 2021 11:22 AM

Paul Milenkovic
And don't start with telling me I need to post this on the MR Forum!

The initial thing about this is that the 'functional' engineering alternative is to string a second constant-tension catenary,or parallel trolley in the 'three-wire overhead' system proposed for tramways.  Most of the expense of catenary is in the support structures, the cost and installation of which are only marginally affected if carrying multiple wire systems.

And why "insulate" a connection that is at nominal ground potential either side of the bond?  Presuming adequate cross-section, and good integrity of braze to the rails, there is little point in expensively 'protecting' against incidental contact with the better conductor of the bond metal itself... I doubt that even if substantial current is flowing in the bond to 'ground', a tap via a 'human' contact to local ground reference would produce lethal levels of voltage and current -- any more than standing with one foot on a running rail of the PRR electrification at Thorndale would...

Theoretically you could use one of those resistance-welding rigs for CWR as a sort of enormous soldering iron, and periodically submerge rail joints in flux and solder them solid.  When there are temperature swings substantially off the neutral temperature, send crews around to clamp either side of the joint and 'reflow' for a couple of minutes...

(Yes, I appreciated the humor, too Wink)

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Posted by Erik_Mag on Friday, July 23, 2021 9:46 PM

How about rigging up a large capacitive discharge welding system where you can run a few million amps between the rail ends? Also useful for doing certain kinds of controlled fusion experiments.

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Posted by Overmod on Friday, July 23, 2021 9:56 PM

Actually, no joke, I remember this being touted as a possible use for ultracapacitors in the early days "once Moore's Law price drops made it economical" -- think of them as an intermediate energy storage for the genset-based resistance welding rigs used for in situ CWR instead of Thermit(e) kits.

What's strange about that?

Rail welding is a sort of controlled fusion... Wink

But of course a welded rail joint won't need bonding... however, expansion jointing in CWR certainly would.  Someone should find and post pictures of some of the ingenious methods used 'overseas' to accomplish this on HSR and other electrified lines...

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Posted by SD70Dude on Friday, July 23, 2021 10:58 PM

Wires across rail joints are a normal sight even in non-electrified territory. 

I've also seen both them and insulated joints on industrial spurs where dangerous goods are handled, presumably to reduce the risk of static electricity sparks.

Thermite would indeed be difficult to control or stop once you have started the reaction.

Greetings from Alberta

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Posted by Overmod on Saturday, July 24, 2021 8:09 AM

Wires across rail joints are necessary for reliable operation of some types of signal system, or for mmost present types of grade-crossing protection.

I can see why certain explosion-sensitive facilities might want static bonding between rails, although this need be nowhere near the wire gage involved in traction bonding.

Thermite kits for field rail welding have been a staple of CWR for decades; there are even YouTube videos on how to set up and use them.  The amount of 'reactants' is carefully proportioned to get the right thermal profile; I believe the 'iron' is amplified with alloy constituents to better match the given rail steel... but yes, touch it off and the Goldschmidttery runs to completion.

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