Arching on the Metropolitan line?

It is arcing. It's essentially the current jumping between the conductor rail and the shoe, ionising the air, giving the blue flash and the noise. Arcing will happen with just about any electric train, 4th rail, 3rd rail, or even OLE, so its as much of a problem for S stock as it was for A stock and is for every other electric train.

It is electricity jumping the gap between the collector shoe and the live rail. As the live rail is not continuous, particularly at pointwork, there have to be gaps in the rail, and as the shoe loses contact with a rail or approaches another there is going to be a point where the resistance of the air is low enough to allow electricity to jump across. At what point that happens depends on a number of factors - notably the line voltage, the resistivity of the air (which depends on how humid it is) etc. You can see some quite big sparks when the pantograph is raised or lowered on Thameslink trains at Farringdon, accompanied by a big buzzing noise. The voltages there are, of course, about 100 times bigger than those on the tube. But for really big flashes, watch a live rail train in wet or icy conditions. Ice is quite a good insulator, and causes the shoes to make intermittent contact with the rail. It is usually most noticeable towards the front of the train, as the leading shoes tend to clear the ice away and the later shoes make better contact and keep the train moving. In extreme cases, where the shoe is held by the ice just far enough away from the rail for an arc to penetrate the ice, the arcing can last quite a long time and damage the shoegear. I have even heard of shoegear getting welded to the rail.

Rain can also do cause arcing - especially if the shoes start aquaplaning.

I'm sure that the engineers do all that they can to reduce arcing, whether on third or fourth rail or overhead electric current systems, in the UK or at any point abroad. The fact that it still occurs sporadically would suggest that perhaps they've got it about as good as it can get. You can of course completely eliminate it by using diesel or steam trains. That'd not be a very economical move.


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What you need is a volunteer to stand up straight on a nylon carpet and wearing your rubber soled shoes you charge yourself up by wiping your feet a few times. You then point your index finger closely at the volunteer's ear lobe and should hear a click as he reacts to the zap. A darkened room is best and vigorous feet wiping should give a nice fat blue spark as the victim volunteer becomes animated, the effect is not dissimilar to the shock from holding a spark plug lead close to the spark plug with the engine idling.

Zapping one another in the sixth form at grammar school was a popular pastime back in 1969 !

My GSCE physics teacher had a good trick to play on the first years.

His first class was always a fun one as he demonstrated the Van Der Graff generator, on the bench would innocently be a Bunsen burner with the gas tap turned on, but isolated at the shut-off under his desk. With one hand on the generator he would turn the gas on with his foot (rubber shoe of course) and then point at the child sat the other side of the Bunsen and shout "you!" Of course the spark went from his finger to the earthed Bunsen and ignited the gas.

He then spent the rest of the lesson explaining that he didn't have the power to make fire erupt from his fingers!

Arcing will occur on 3rd/4th rail (and OLE) as long as we have frosty mornings. To prevent it the only option is to have under running 3rd rail such as the DLR. Incidently does arcing occur on the DLR?

One of the most spectacular places for arcing is under the bridge at Eastcote station. A conveniently placed gap in the conductor rail always let to a good amount of arcing from a departing Uxbridge bound train. Great watching an A Stock depart here. Is it still the same now? Might check this weekend when photographing the 'C' Stock tour.

Arcing can of course be very useful! In my days as a telephone engineer we used to use an arc to write our name or other identifying marks on our tools. All that was required was a standard lead pencil, HB was ideal, and the exchange 50v dc power supply. We'd connect positive (earthed side of supply) to the tool to be marked and the negative (fused feed side of supply) to one end of the pencil lead and then simply write on the tool leaving an indelible burnt on impression. It helped if the pencil was brand new rather than worn, the longer length lead giving greater resistance and less chance of blowing the battery supply fuse. Of course marking too many tools at once was a bad idea as the lead would get rather hot and apart from making the pencil difficult to hold on to would also cause the lead to burn the wood on the inside of the pencil shaft enough that it became loose within it.

The only time an arc would be dangerous for a customer would be if they were wearing something metallic and holding holding said object inches away from the live rail. Any staff working about the track have been trained to avoid arcs (or use short circuiting devices). The only real danger is from "arc eye" but given how rapid the arcing is, it is not particularly dangerous, and more often than not, the passengers won't be looking directly at the arc. Drivers are slightly more at risk, but again the exposure is low, so it's not particularly dangerous, but I know that some drivers see arcing as one of the biggest irritations of the job.

Going back a bit, but here is an example of OLE arcing: