When I watch videos of high speed trains I always see explosions of electricity near the top, or arcing. Why does that happen? I know that the Acela does it a lot but other high speed trains have it, too.

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    – Air
    Jun 6, 2017 at 16:49
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    $\begingroup$ It can also occur with 'low speed' trains. $\endgroup$
    – Karlo
    Jun 6, 2017 at 17:02
  • $\begingroup$ Actually I have seen this much more often on low speed trains than on fast ones. Does someone have data on whether it really happens more often on fast trains? $\endgroup$
    – Nobody
    Jun 6, 2017 at 20:57
  • $\begingroup$ @Nobody Arcs happen roughly equally on all trains, it's more of a maintenance cost trade-off. Low-speed overhead lines are usually in a much worse shape (so there are arcs even at lower speeds), but they don't require to be readjusted every couple of years like high-speed lines do. Of course, if you go at 300 kph on a low-speed line, you'll have an arc almost non-stop (that is, until your pantograph melts). $\endgroup$ Jun 7, 2017 at 16:41

7 Answers 7


Several factors influence this:

  • at high speed, there's a higher chance of the pantograph losing contact with the catenary wire: at higher speed, bumps in the wire cause a more violent excursion that can exceed the capability of the suspension of the pantograph.
  • Low-speed trains can show arcing too.
  • high-speed trains often use high voltage (15 or 25 kV), which is capable of arcing across longer distances than the lower voltage (e.g. 1500 V) used in older trains.

The point where the pantograph of an electric train makes contact with the trolley wire creates one of the most complex and challenging environments for rail component manufacturers and test engineers to understand, let alone predict and improve.

For trains to operate efficiently, the pantograph must maintain constant contact with trolley wires suspended from catenary systems. Yet these wires and their support structures exhibit different vertical stiffnesses along any given section. The catenary system zigzags in 30 to 100 meter intervals to prevent grooving. The force the pantograph applies to the wire must stay within a well-defined range (70N to 120N). If it is too low, loss of contact results in arcing, which not only causes the train to lose power but damages the trolley wire and the contact bar through etching and overheating. If the force is too high, the resulting friction wears down the wire and contact bar prematurely.

Delivering the right amount of force requires variable vertical motion. But when trains move at higher speeds, pantographs lose their ability to react appropriately. Even when the trolley wire is as flat as possible, it is only flat when it hangs undisturbed. When the pantograph lifts the wire, the resulting deformation creates a wave. If there is too much uplift, the pantograph creates a much larger waveform that causes contact problems for the next pantograph coming down the line.

The catenary wire isn't stationary: it gets moved around by trains and by the wind.

In general, when a pantograph runs underneath the catenary, it sets up a wave-like disturbance which travels down the wire with a speed determined by the tension in the wire and its mass per unit length. When a train approaches this critical speed, the pantograph catches up with the disturbance, resulting in dangerously large vertical displacements of the wire as well as contact interruptions. The top speed of the train is then limited by the critical speed of the catenary. This problem was central to the test runs, since it was desired to test set 325 at speeds well above the critical speed of standard TGV catenary.

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    $\begingroup$ In addition, a train that's accelerating or running fast draws a lot more current than when it is running slow. $\endgroup$
    – Dave Tweed
    Jun 7, 2017 at 11:40
  • $\begingroup$ @DaveTweed that's true, but arcing is almost entirely a function of the electric field strength, not the current behind it. Once an arc is struck, then certainly the available current affects the amount of damage (welding/cutting) that takes place. $\endgroup$ Jun 7, 2017 at 14:14
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    $\begingroup$ @CarlWitthoft: See the comment elsewhere about the inductive nature of the load. Higher current means higher voltage when that current is suddenly interrupted. $\endgroup$
    – Dave Tweed
    Jun 7, 2017 at 14:20
  • $\begingroup$ @CarlWitthoft The amount of light produced by the arc is proportional to the current in it, not to the voltage. Once the arc is lit, it only drops a fraction of the overhead line voltage. $\endgroup$ Jun 7, 2017 at 16:40
  • $\begingroup$ @DmitryGrigoryev Correct. I should have specified that it's the striking of the arc that depends on field strength (and dielectric constant) $\endgroup$ Jun 7, 2017 at 17:30

It is due to the high voltage still causing a connection when the contacts separate due to an irregularity (bump etc) between the contact and the wire.

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    $\begingroup$ With a prominent example being ice on the overhead wires. $\endgroup$
    – Ghanima
    Jun 6, 2017 at 19:04
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    $\begingroup$ High speed vastly exacerbates the problem - minuscule irregularities over which the pantograph would just slide smoothly, retaining contact at all times, its flexibility yielding and pushing following the surface, in case of high speed cause it to jump and create a gap - which causes sparking. $\endgroup$
    – SF.
    Jun 7, 2017 at 9:13

As others have posted, a temporary gap between pantograph and overhead conductor is part of the answer, however that's not the complete story. The other big factor is that the train's motors are an inductive load, which seriously complicates what happens when the circuit is interrupted.

When there is an interruption of a circuit with an inductive load, the current through the load cannot go to zero instantly. Instead, current continues to flow through the load, generating an voltage spike at the point of interruption. (The extra energy to do this is actually come from the inductive load.) The voltage rises suddenly until a breakdown (e.g. arc) occurs. One an arc has formed, the voltage drops, but less voltage is needed to sustain an arc because plasma is more conductive than air at typical temperatures.

The currents flowing for a high-speed train will typically be much higher than their low-speed counterparts, so the voltage developed when the circuit is interrupted will be higher.

  • $\begingroup$ Do you have a citation to show that the engine's internals don't have back-surge suppressors? $\endgroup$ Jun 7, 2017 at 14:15
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    $\begingroup$ This is wrong: motors are never connected directly to the pantograph circuit, except on toy trains. $\endgroup$ Jun 7, 2017 at 16:02
  • $\begingroup$ No economically viable power conditioning circuits between the pantograph and motors changes the fact that it's still effectively an inductive load. The fact that arcing may occur at the pantograph has been long accepted, so absorbing transients in electronics rather than at the pantograph seems economically unwise. @CarlWitthoft, do you have a citation showing otherwise? $\endgroup$ Jun 8, 2017 at 18:21
  • $\begingroup$ @Burt_Harris Train motors cannot run at 50/60 Hz all the time (which would mean constant speed), so the line voltage is first converted to DC, and the DC bus is managed to absorb the current generated during braking, if that's what you call "inductive load". May I ask what kind of experience you have with trains? $\endgroup$ Jun 13, 2017 at 9:54
  • $\begingroup$ @DmitryGrigoryev - in DC overhead systems, 1500v to 3000v, the motors in classic multiple units and locomotives can be started with resistance in series, and series/parallel switching, and then speed up until the motors are connected directly to the line, via protection such as circuit breakers of course. $\endgroup$ Dec 6, 2020 at 14:39

The upforce on a pantograph is 15-40 pounds, 60 pounds at the outside. (7-18kg, max 30 or so).

The trolley (contact) wire is made of solid bronze or copper, typically 4/0 to 400kcmil (107-200mm^2), with a stranded steel messenger (catenary) wire of 3/8-1/2" (10-13mm) diameter. The messenger wire is supported every 100-200 feet (30-60m) and it supports the contact wire every 6-10' (2-3m). So the contact wire is free to rise up even a foot (0.3m) as the train passes. It often has a stabilizer bar to keep it from moving laterally but is free to move vertically.

As discussed, any irregularity in the contact wire, or in how it's hung, can cause pantograph and wire to separate for a moment.

Wave action in the wire can also cause a momentary separation. Sufficient wire or train movement can cause the wire to move out onto the curved "horn" of the pantograph.

enter image description here enter image description here src

Irregularities in the pantograph's running surface can also cause arcing. There are typically inset copper or bronze slides; physical damage to a slide or simply a burned spot from arcing may cause the wire to lose contact.

Also a pantograph typically has two slides, fore and aft, and the pantograph has either linkage or strong springs to keep it level. If there is any binding or broken linkage or a fatigued or broken spring, it may not be level and may ride on its heel or toe, causing poor contact.

The arcing of course is caused by current. Current might remain continuous through the arc (that tendency being proportional to voltage, more likely on the high voltage systems used in high-speed rail) -- however high speed air movement is likely to snuff out the arc, severing power to the train momentarily. Talk about voltage spikes!

  • $\begingroup$ "Pan" is the slang term for pantograph. It was also sometimes spelled "pantagraph", depending on which company was supplying them. $\endgroup$ Jun 8, 2017 at 19:46

It's not about voltage*, it's about current.

*line voltage

When a high-current circuit is interrupted (especially an inductive one), an arc is formed between the contacts breaking away. The high current then sustains the arc: ohmic heating turns air into plasma, while the plasma conducts the current. It's the basis of arc welding, which uses hundreds of amps at voltage as low as 20V.

Welding at high speed 5000 fps (close-up arc, spatter visible)

Even low-voltage (usually 600-800V) trams moving at walking pace do produce arcing and sparks at breakpoints in catenary, while subway does same at the level of the power rail.

Subway Sparks | New York Snowstorm 2017

Because of the high-current requirement, sparks happen mostly when the train is accelerating (eg from a standstill), or when it draws lots of power to sustain high speed, but they never happen when it's coasting at idle power, despite voltage being the same.

In low-speed operation, this happens mostly when a break of contact is introduced to the wire-shoe system externally, eg by a physical gap separating different circuits or by contamination from ice, snow or leaves.

In high-speed operation, in addition to all the low-speed ones, extra breaks are created by the pantograph jumping on catenary irregularities, just like an off-road truck which gets it's wheel momentarily airborne when it goes too fast on bumps. Some of those irregularities are introduced by the pantograph itself: one can imagine pantograph on a catenary as an upside-down acrobat on a tightrope. Instead of gravity acting on the acrobat downwards, the pantograph pushes the catenary upwards via a spring, so the whole system jumps up and down when passing under suspension points.

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    $\begingroup$ Well, sort of... current is related, but it's actually a voltage spike that starts the arc in the case of a pantograph. See my answer re inductive loads. $\endgroup$ Jun 8, 2017 at 17:25
  • $\begingroup$ @Burt_Harris What I've meant is that line voltage doesn't matter, as 25kV train arces as much as 800V subway, and not even remotely as much as 20V welder. I kinda overlooked that welders are inductive and there is a voltage spike. $\endgroup$
    – Agent_L
    Jun 9, 2017 at 7:52

When I watch videos of high speed trains I always see explosions of electricity near the top, or arcing. Why does that happen?

There is a gap in the contact, the electrons shooting through the gap turn the air into a plasma and breakdown the air. Because the air is a plasma, it can conduct current, this happens at around 3kV/mm so you know there is some voltage involved.

  • $\begingroup$ 3kV/mm is very optimistic. You can easily get an arc of several cm with 25 kV. $\endgroup$ Jun 7, 2017 at 16:35
  • $\begingroup$ The plasma effect is only peculiar to that particular air. On a train moving fast, it is likely that air will be blown away by the wind force, tending to snuff the arc. In fact, blasts of air are often used to snuff arcs in large contactors. $\endgroup$ Jun 7, 2017 at 17:00
  • $\begingroup$ @Harper, yeah thats why I wrote around 3kV/mm instead of exactly 3kV/mm $\endgroup$ Jun 7, 2017 at 19:59

Another factor is that overhead line profile changes much more rapidly at high speed. The contact wire isn't at exactly the same distance from the rail all the time.

The pantograph is constantly readjusted to apply a constant pressure on the contact wire, but at high speed that doesn't happen fast enough. When the pressure on the contact wire is insufficient, a small bump is all it takes to send the pantograph down a few mm, creating a visible arc.

Just for reference, low-voltage trains are capable to create quite visible arcs just as well (the lower voltage is usually compensated by the fact that it's DC), if they go fast enough or the contact wire is in a bad shape.


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