Editing Great Eastern Main Line (section)

====Historic 6.25kV/25kV voltage changeover====
Since trains would through-run between 6.25kV and 25kV sections in regular operation, all [[Electric multiple unit|EMUs]] built for the Anglia region between 1960 and 1980 were fitted for dual AC voltage with equipment to automatically switch between the two power systems while running.<ref name=":7">{{Cite book |last=Cowley |first=Ian |title=Anglia East |publisher=David & Charles |year=1987 |isbn=0-7153-8978-5 |location=Newton Abbot |pages=70-86}}</ref> The main [[transformer]] on the train had four identical primary windings which could be connected in series for 25 kV working, or in parallel for 6.25kV working, by means of a "supply changeover switch". It was vital that the changeover switch was in the correct position since, if the voltage was too low, the train could not operate, while if the voltage was too high, serious damage could occur. For this reason, the switchover had to be made automatically by way of voltage sensing equipment rather than rely on the driver operating controls.

At the points where the two systems met – such as on the main line just east of Shenfield – there was a short length of overhead line which was electrically neutral. Neutral sections were installed either at breaks between supply areas or where the two voltages met. Whether or not the voltage was different on the other side of the neutral section, the on-train equipment operated in exactly the same way. At each end of the neutral section there were track-mounted permeant magnets referred to as Automatic Power Control (APC) magnets.<ref name=":8">{{Cite book |last=Keenor |first=Garry |title=Overhead Line Electrification for Railways |year=2021 |edition=6th |pages=93}}</ref>

On board the train, the operation of the dual-voltage mechanism depended on the action of the main air-blast [[circuit breaker]] and voltage-sensing relays. When passing the first APC magnet, detection equipment fitted to the bogies would activate the circuit breaker cutting all power to the train. Once the train had passed the neutral section, the second set of APC magnets are detected by the bogie-mounted inducers which causes a lock to be released on the circuit breaker meaning its reclosing would be under the control of the voltage-detection equipment. This consisted of four voltage relays that would allow for a line voltage between 4.1 and 6.9kV for the low-voltage mode, and 16.5 to 27.5kV for the high-voltage mode. If the first relay was tripped, the changeover switch at the transformer was set to the low-voltage mode and the circuit breaker closed. All four relays were activated if the voltage was above 16.5kV. In that case, the current was sent to a magnetic switch that would set the changeover switch at the transformer to the 25kV position.

The other relays were installed to stop the changeover switch from being activated to low-voltage mode if the line voltage fell under 16.5kV but greater than 6.9kV. The changeover system was interlocked with the circuit breaker and the action of the APC magnets since any movement of the changeover switch while the circuit breaker was closed and connecting the system to any live current would cause destructive arcing for the internal components. Following testing and some in service incidents, modifications were made to the changeover equipment such that a switch to the low-voltage mode could only be made when the line voltage had dropped to below 3kV and then risen to 4.1kV, and that the re-closing of the circuit breaker is only made after a two-second wait following the detection of the second APC magnet to ensure a correct reading of the voltage.<ref>{{cite book |title=British Rail Locomotives and other motive power: Combined volume |year=1968 |publisher=[[Ian Allan Publishing|Ian Allan]] |location=London |isbn=0-7110-0008-5 |page=11 }}</ref><ref name=":9">{{cite report |url=https://www.railwaysarchive.co.uk/docsummary.php?docID=732 |title=Failures of Multiple-unit Electric Trains on British Railways: Final Report |last=Langley |first=C.A. |date=17 March 1962 |via=The Railways Archive |access-date=23 February 2025 |at=paragraphs 24, 44, 65, 68, 142–144; figures 1 & 2; map 2 }}</ref>

As well as new EMUs introduced after 1960, the previous [[British Rail Class 306|class 306]] and [[British Rail Class 307|307]] units originally built for the 1.5kV DC system underwent conversion to become 6.25/25kV dual-mode EMUs. The conversion of both these classes of train involved significant interventions particularly the relocating of the pantograph to a different carriage and the associated internal layout changes this caused.<ref name=":7" /> The full 92-unit fleet of class 306s and the 32 units of class 307 fleet underwent alterations at [[Stratford Works]] with new equipment provided by Associated Electrical Industries at Manchester.<ref name=":9" />

Neutral sections are, or course, still required on the modern 25kV system and APC magnets are still used to automatically trigger the opening of the train's circuit breaker so that an arc is not caused at OLE when the pantograph travels between live and earthed wire.<ref name=":8" />