December 19, 2019

Disruption at Victoria and London Bridge:
how it happened and what we are doing about it

John Halsall, Managing Director, Southern region

On Wednesday night, the signalling system that controls the safe movement of trains through Croydon shut itself down to protect itself from an external power supply problem.

A power surge from our external provider put too many volts into our supply for around 20 seconds.

Our system is designed to fail-safe, so every signal – the traffic lights of the railway – went red and every train was brought to a stand.

The result was that for almost an hour we were unable to run services on the busiest section of the UK’s railway, causing disruption that lasted until Thursday morning.

So what happened?

Three areas of signal control shut themselves down:

  • East Croydon
  • Selhurst
  • Norwood Junction

The signalling power supply at Streatham also shut itself down.

Our investigations show that four separate systems, from one that is 18-months old, to another that is 10 years old, locked themselves out to stop themselves from being damaged by the surge.

These include the signalling control system, an advanced reconfigurable power network and an older control system called TDM (time division multiplex) that allows control centres to talk to signals on the ground.

We sent technicians out to multiple sites to reset the systems manually and the system was back online in just over an hour.

East Croydon map
The map shows the area where every signal went red and we were unable to run trains.
East Croydon control
The local control panel at East Croydon when it is working correctly. This is where the electrical signals from the signalling centre are transmitted to the equipment on the ground to make signals change colour, points to move and trains to run. On Wednesday night, this panel was blank and switches on the right hand side had to be manually reset.
The signalling image shows trains stacked up waiting to get into Norwood Junction, held at red signals and unable to move. The Xs are where the signals were held at red by the power supply problem.
The signalling image shows trains stacked up waiting to get into Norwood Junction, held at red signals and unable to move. The Xs are where the signals were held at red by the power supply problem.

Why is there no backup?

There is backup. We have three different power supplies that we can switch between: two railway supplies that can be fed independently, and the domestic power supply to people’s houses.

Because there was no actual power failure – the power was never interrupted – these systems were not triggered.

I thought you said there was a power failure?

There was no power cut, however, there was a power surge (called an Over Volt) for around 20 seconds. Our systems can cope with momentary power surges, which happen from time to time, but 20 seconds is very unusual.

Had our systems not protected themselves the disruption would have been far worse as we would have had to replace damaged equipment at multiple remote sites across Croydon. It is likely this would have meant the failure would have continued throughout Wednesday night and we would have been unlikely to run a full service on Thursday.

What are you doing about it?

We are working with UK Power Networks to better understand the nature of the power surge.

In addition, we are working with multiple suppliers of our signalling and power supply equipment to investigate how the different systems responded to the surge and how we can improve their resilience so we can stop this from happening in future.

We are investigating whether fitting Uninterruptable Power Supplies to each signalling location would have prevented this failure, although it is also possible they would have had to shut down to protect themselves too, as happened at another location during the same event.

We recognise it would be easy to look for others to blame in these situations but the reality is that while our power supply is very reliable, we still have to review our equipment to make us more resilient in future.

Thank you for your understanding and patience.

John Halsall

John Halsall, Managing Director, Southern region