What makes overhead lines so dangerous?

Look at some of the comments made on social media about the You vs Train campaign, and there are a few pointing out that 25,000 volts alone is not what will kill you. The average police taser delivers 50,000 volts and only causes temporary impairment, so it can’t just be down to voltage.

The danger of overhead lines comes from a combination of that high voltage and high current (up to several thousand amps, compared with 15 in a household plug socket).

When you consider that the overhead lines need to provide sufficient power for a 400-ton train to run at 125mph, it’s clear that the current must be high. The third rail only has 750V, and yet can still be lethal because of the high current required to accelerate a train.

The secondary danger comes from the ability of the electricity to ‘jump’ across a gap. In extreme conditions, 25,000 volts can jump up to three metres. And when it surges through the air, it creates enough heat to burn skin and ignite clothing.

RAIL’s Philip Haigh has a good analogy for this: “Think of voltage as similar to pressure and current as flow. If you stand in front of a garden hose you’ll get wet, but not harmed. Stand in front of a firefighter’s hose and you’ll get knocked over. The firefighter’s hose uses high pressure (voltage) and higher flow rates (current). The result is that you get hit by a lot more water in a short time. Power is energy divided by time, so a short burst of energy is more powerful than the same amount delivered over a longer period.”