Overhead Line Arcing

[Katketty]

Hi there! This evening I inadvertently caught some arcing between the overhead lines and a train pantograph on camera. I was wondering how common this is and what if anything are the consequences? I am fairly ignorant on the subject so I apologise if I sound naive. I thought this was only a problem during icy conditions.

For reference it was about 10.20PM, very light on and off rain (bordering on just damp) and the temperature was approximately 10 degrees Celsius. The overhead lines were emitting a reasonably loud hum with persistent, low vibrations (more than usual) and the occasional crackle.

Short 8 second video of the event can be found here.

 

[carriageline]

Would say it's fairly common, with third rail arcing in places. It being dark would make you notice it more. Why it does it I'm not sure, perhaps momentarily losing contact with the wire? Or changing section, not entirely clued up on the technicalities of OHL. And for consequences, if it's bad enough it could trip the overheads. And your right, when it's wet the OHL does crackle and hum loudly, why I do not know!

 

[DarloRich]

That is quite normal - I am sure someone can offer a precise technical explanation as to why it happens.

I assume it to be a momentary break in the connection between the contact surface of the pantograph and the wire itself

 

[starrymarkb]

Arcing is a common sight on High Speed lines in damp conditions. IIRC it's condensation on the contact wire causing brief (fraction of a second) losses of contact

 

[455driver]

That probably happens about a million* times a day and is nothing to worry about.
Still looks good though.

*might be a slight exageration but you get the idea.

 

[Cherry_Picker]

Across the entire network, including third and fourth rail? I'd imagine a million times a day isn't far off sometimes! Where our trains run parallel to the Underground you can sometimes see electric arcing so frequently against the shoes it's impossible to keep count, especially if it's a bit damp or frosty.

 

[Crossover]

Yeah it is regular. Stood the other week at Lichfield TV on a fairly dry day, at night, every Pendo coming through giving a bit of a light display! Even in a short section you can sometimes get 3 or 4 separate flashes, too!

 

[DownSouth]

How much distance is covered by electric trains every day in Britain, including Underground? Add them up, then divide that number by a million to find out how often (in kilometres) each one would have to arc to achieve a million.

 

[bangor-toad]

Why does arcing occur?
I'll have a go at a simple explanation...

In normal circumstances the electrical current flows along the overhead wire or 3rd rail and it's picked up by the train collector - either by pantograph or 3rd rail shoe. When it's all perfectly good the collector remains in absolute contact with the energised wire or rail.

However, there's always some imperfections in the engineering. You'll never get perfect connections, especially at 100mph+, and there may be some small gaps between the wire/rail and the collector.

Normally you think of air as being an electrical insulator. However, when you have a high voltage (and overhead wire is at approx. 25,000 Volts) the air breaks down and electrical current can flow through that bit of air - that's the spark you see.

There is a whole bunch of physics and engineering you can look up - perhaps start with "breakdown voltage" if you're interested, but this is the absolute basics.

If the air contains a lot of moisture, or sometimes even very fine dust, then the ease at which these sparks or arc occur increases dramatically. In fact the most spectacular can be when the conditions are just right for most of the current to flow through the conductor/collector as normal but enough sort of leaks around the sides. Here you get a nearly permanent arcing. (The physics of it mean this won't really occur with the 3rd rail but can happen with the overhead wires)

What does it do? In almost all cases nothing. If any particular event though is particularly severe it could trip safety systems. Also arcing can have an effect of putting little pits into the materials used which over a long time can damage them.

In short, a bit of arcing is no problem and quite fascinating to watch. Lots of arcing can indicate that something isn't quite right.

Hopes this helps.
Mr Toad

 

[Bald Rick]

How much distance is covered by electric trains every day in Britain, including Underground? Add them up, then divide that number by a million to find out how often (in kilometres) each one would have to arc to achieve a million.

Just on the national DC network south of the Thames there are about 7000 electric passenger trains a day. Average journey say 40 miles, so that's getting on for half a million train km before you get to the AC network. Not forgetting Merseyrail. So a million times a day is quite realistic.

 

[Katketty]

Thank you for the answers! I wasn't aware this was such a common occurrence. I guess witnessing it at night time made it seem more dramatic than it actually is

 

[Lrd]

This thread reminded me of this. (Not the exact video I was looking for but still makes the point)

 

[chuffchuff]

Strood is a good one

http://youtu.be/kXXw7qQukjM

 

[carriageline]

Gillingham makes some pops and bangs, even under normal conditions!!

 

[Daniel Pyke]

OK this is an extreme example (due to the very high power being transferred), but if you watch the clip of the rail speed record I posted up rently then you'll see a lot of arcing in the pantagraph shots.

Link

 

[SpacePhoenix]

https://www.youtube.com/watch?v=szA3rsdvqv4

In snowy weather, is 3rd rail way more prone to arcing then overhead?

 

[GB]

I wouldn't say its more prone, Ive seen plenty of significant arching on OLE when its icy, the difference is I think that with 3rd rail there is more points of contact so makes it look worse (or better depending on view point).

 

[jopsuk]

in frosty, foggy, weather, a class 379 is a sight to behold in the pre dawn gloom, roof and track lights a glow and pantographs flickering merrily

 

[IKBrunel]

https://www.youtube.com/watch?v=szA3rsdvqv4

In snowy weather, is 3rd rail way more prone to arcing then overhead?

not sure which is more prone. At the same voltage DC current is more prone to arcing than AC, because with AC the ARC can get extinguished every 50th of a secondly when the voltage passes through zero. But of course with higher voltage the current can arc over a longer distance.

Of course the arcing you really don't want is between the conductor and gantries/ tunnels/etc. I gather the separation was decided by BR research division by running a steam train in a tunnel with OHLE.

 

[O L Leigh]

If you get one arc at a given location you are likely to get loads.

Broxbourne was a good place when Stansted Express services were still operated by Cl317s. They'd belt through at 80mph and, if it was a unit with a Stone Faively "bicycle frame" pantograph, you'd get a bright flash as the contact strip momentarily lost contact with the wire as it passed below the road bridge at the southern end of the station. Units with the Brecknell Willis high-speed pantograph never did it quite so dramatically, though.

It does also depend on how much load the train is drawing. Under full power you could get some bright flashes, but if you're just coasting then you'll get almost nothing at all.

O L Leigh

 

[snowball]

not sure which is more prone. At the same voltage DC current is more prone to arcing than AC, because with AC the ARC can get extinguished every 50th of a secondly when the voltage passes through zero.

Surely every 100th of a second. Twice per cycle.


Manchester Metrolink (750V DC overhead) has had serious problems with ice on the wires on a few occasions, especially on the Oldham and Rochdale line. I think it has a tram which has metal rather than carbon on its pantograph which on rare occasions has run up and down the line overnight to scrape the wire clear.

 

[Katada]

If you get one arc at a given location you are likely to get loads.

Broxbourne was a good place when Stansted Express services were still operated by Cl317s. They'd belt through at 80mph and, if it was a unit with a Stone Faively "bicycle frame" pantograph, you'd get a bright flash as the contact strip momentarily lost contact with the wire as it passed below the road bridge at the southern end of the station. Units with the Brecknell Willis high-speed pantograph never did it quite so dramatically, though.

It does also depend on how much load the train is drawing. Under full power you could get some bright flashes, but if you're just coasting then you'll get almost nothing at all.

O L Leigh

The 317s speeding through Broxbourne... I do miss that, having a 379 glide through quietly just isn't the same.

 

[SIGGY56]

There is a location on the Merseyrail underground at Mann Island Jct, James St when the units pass over a gap in the conductor rail over pointwork, also affects the lights inside the units and everyone is either kept dark for 5 seconds or so or the lights stay dim for few seconds - used to have better results off the old 503 units that once ran. Also arcing on 3rd or even 4th rail on London Underground is quite common.

 

[SpacePhoenix]

There is a location on the Merseyrail underground at Mann Island Jct, James St when the units pass over a gap in the conductor rail over pointwork, also affects the lights inside the units and everyone is either kept dark for 5 seconds or so or the lights stay dim for few seconds - used to have better results off the old 503 units that once ran. Also arcing on 3rd or even 4th rail on London Underground is quite common.

One thing that surprises me about dc units is that they only have pickups on the bogies at the ends of the units (the bogies under the driver's cab). It must give an increased risk of a unit getting stranded. The 442s when they ran on the Waterloo-Weymouth line had pick-ups on every coach apart from the buffet (motor) coach.

 

[jopsuk]

Not as far as I can tell from any pictures of them in NSE or SWT livery they didn't

 

[eMeS]

Arcing is caused by the electrical inductance of the supply system - acting as a sort of "electrical flywheel" which tends to try and maintain the value of the current. So, if for some reason, the current stops flowing, the inductance creates a rise in voltage which can be high enough to break down the resistance of the air, and restore the current. Inductance is normally associated with coils of wire (as in electrical chokes and transformers) and similar, but in the case of long wire distribution systems, the length of the wires themselves has an appreciable inductance.

 

[GatwickDepress]

iirc 442s had (have?) pickup shoes on the bogies of the TSO and TSW nearest to the MBLS, with shoes on both bogies under the driving cab on the DTSes.

So shoes on the 1st, 4th, 7th, and 10th bogies.

in frosty, foggy, weather, a class 379 is a sight to behold in the pre dawn gloom, roof and track lights a glow and pantographs flickering merrily

...with a cheery rattle as all the door buttons fall off.

 

[Elecman]

inductance is unlikely in a single wire, it can have voltage induced into it from an adjacent conductor, a long wire is more likely to be capacitive in nature

 

[SpacePhoenix]

Not as far as I can tell from any pictures of them in NSE or SWT livery they didn't

Found this pic which shows the pickups

 

[the sniper]

Yeah it is regular. Stood the other week at Lichfield TV on a fairly dry day, at night, every Pendo coming through giving a bit of a light display! Even in a short section you can sometimes get 3 or 4 separate flashes, too!

Got to agree, Pendos seem to particular lend themselves to arcing. When it's been raining/drizzling, they're almost constantly arcing to a greater or lesser degree when at speed. Particularly noticeable when it's dark and quiet.