Electrification of the London Underground

Question

There are two contact rails in the London Underground: one with 420 V DC and one with -210 V DC.

Why not use 630 V and 0 V? Wouldn't it make one of the rails safe for people?

Answer 1

The reason for using an insulated return system and not an insulated feed (third rail) and return through the running rails is historical, as the government in c. 1900 required total voltage drop to be limited to 7 volts to prevent electrolysis damage by stray currents to nearby buried metal structures (a great deal of the Underground is in tunnel). A fourth rail was cheaper than heavy cables and boosters.

Although the traction supply is not directly referenced to earth, each section feeder station has potential divider resistors. The positive and negative traction rails are connected to the continuous running rail (which is bonded to earth) via high wattage high value resistors (usually of the order of 7.5 to 10 kilohms) to reference positive 420V above earth and negative 210V below earth.

This arbitrary fixing of voltage enables earth fault relays in each main section to detect positive and negative traction earth faults and indicate them at the Network Operations Centre and Line Control Rooms. An earth fault on one traction rail will drive the other rail to a full 630V positive or negative. Trains in that section are unaffected by a single earth fault, and can still run.

A modernization programme is in place to increase traction supply to 750V (+500 and -250) and lines are being converted as newer compatible rolling stock is introduced.

Where sections of line are shared with conventional surface railways which have third-rail only rolling stock, the centre rail is earthed and bonded to the running rail, and the outer rail is fed at +630 volts at present. Both types of train can run using this arrangement.

There have been a couple of cases (the Great Northern and City line 1977, and the East London line 2010) where lines formerly operated by London Underground have been transferred to National Rail ownership and operated as part of mainly above-ground rail services. These have been converted from four-rail to standard National Rail three-rail supply. Since 1900, advances in technology (e.g. insulated supports for the running rail) have removed the buried metal corrosion risk. The cost of converting the whole London Underground supply system and all the rolling stock is much too great.

Answer 2

Neither of the power rails are earth referenced - i.e. they are floating with respect to earth (and the running tracks). If one of them was at 0V (by this I presume you mean earth referenced) it would mean that rail was essentially inert, but the other rail would then be extremely dangerous to touch. This would not be an improvement in safety.

The reason the voltages are different is that the positive rail sits higher above the running tracks than the negative rail. This allows it to have thicker insulators, and hence it can support a higher voltage differential before breakdown.

Answer 3

Using +630V instead of +420V is indeed a possible option, and it would make the railroad equipment somewhat simpler and safer. This is what is often done with household electricity in most countries, where only one wire is "live".

It would require better insulators to cope with a higher voltage though. Perhaps such insulators were not available or too expensive when the first railway sections were built, so the voltage was split. And once these first sections have set a de-facto standard, other sections were respecting it for compatibility, even if high voltage insulators became available.

Splitting equally (+/- 315V) would have been better from electrical standpoint, but it seems that the higher height of the 420V insulators is useful in itself, as it prevents the 420V collector shoes from accidentally touching the -210V rail, which is lower.

Answer 4

I believe an original reason was to be able to start the trains off 210 volts then switch to 420 volts then 630 volts as the motors sped up. Avoiding the need for such massive heat producing current limiting resistors onboard the trains.

According to an engineer working on the Underground, In the deep Underground lines, removing waste heat is a major engineering problem as ventilation is harder.