Prior to 1950s, direct current (DC) was only form of electrical energy used for traction purposes in UK

650 V DC Third Rail System

1. System provides nominal 650/750 V d. c. supplies to trains via a third rail mounted outside the running rails. Return path for traction current is via either or both running rails

2. Was implemented on major networks, such as London Underground, Liverpool (Merseyrail), Euston Watford and North London lines, former Southern Region

3. Limitation – Exposed nature of conductor rails necessitated voltage being relatively low, and hence high currents were needed to provide required power requirements. Voltage drop, was therefore more

1500 V DC

Use of overhead catenary wires enabled supply voltage to be increased to 1500V, and number of these systems were introduced in 1930s and 1940s, e.g. Manchester and Sheffield.

Limitation – Necessary size of overhead conductor and structures to support it proved very expensive

1. With the introduction of 25 KV AC to electrification schemes in the 1950s, the current requirements and resultant voltage drop were dramatically reduced for similar power levels, resulting in – reduction in size of overhead conductor and supporting structures

  – supply substations could now be sited at much greater distances apart, reducing overall costs

2. All overhead 1500 V DC systems in UK have now been converted to 25 KV AC. Only ‘conductor rail d c system’ remain as their replacement would require huge investment