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Post by GentlemanJim on Jun 27, 2013 10:42:42 GMT
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Post by railtechnician on Jun 27, 2013 12:28:53 GMT
I worked on the T/T systems on all lines for more than 25 years and was the chief maintainer of the Picc line T/T system from 1996 until 2004. The article is particularly interesting to me in that I was unaware of any of the history of the T/T prior to the 1920s. I will say that as good as the article is, quite a bit is missing from the later history in terms of circuitry, for instance the system mentioned as the 1950s system was known as 'common control' due to the common wires multipled to the sections with an individual wire per section. One of my former bosses was the designer of the new system known as the 'loop control' system which he designed for the Victoria line. This system used a pair per section rather than an individual and commons and also included additional gap sections at current rail gaps to prevent 'bridging' if a train straddled a section gap, there was also a third wire (in a cable, not open) between stick and reset ends per section. This was subsequently used on Stage 1 Jubilee line with some minor differences in terms of cabling between stick and reset ends (a fourth wire in a cable) and included gap sections which were installed but not commissioned. These types of T/T circuit perform all functions over the section pair (the loop) using different voltages/polarities to achieve the necessary controls, trip indication and speech between driver and controller. A completely new T/T system was designed for the 1990s Central line resignalling using BR type Q relays and new circuitry which introduced another relay to ensure a double end trip of the circuit (on the older circuits a single end trip could occur in some circumstances). For the JLE another new system was designed and introduced also using Q relays but a different concept entirely, a departure from previous circuits which were adjustable to suit changes in environmental conditions which altered the insulation resistance of the tunnel wires from time to time. The new T/T either worked or it didn't so maintenance was more a matter of 'go/nogo' using a calibrated test unit rather than the 'test/adjust/retest' of the earlier T/T circuit types. The JLE T/T circuit was later installed on the Northern line as part of its upgrade. Interestingly most of the DRICO equipment was removed in the early 1990s but in 2004 I recovered the last of the DRICO equipment in the Earls Court control complex which had been dormant ever since Picc & District lines train radio was brought into use. The old T/T annunicator panels were still extant in many of the substations when I retired although they were decommissioned when 'enhanced line clear' was introduced in order to prevent battery drain during engineering hours. Another consequence of enhanced line clear was that we had to go to all T/T relay rooms and install current limiters to prevent excessive batttery discharge during engineering hours. Around the same time another project involved the removal of all lead acid batteries from T/T circuits and replacement with either sealed rechargeable batteries charged from the domestic 240v/50Hz supply (originally batteries were charged from various voltage/frequency sources according to what was available at site) or from the substation batteries via 50v/24v DC-DC converters. The T/T system was/is a discipline in itself and although called a 'Telephone' system was/is so very different from all forms of conventional telephony. The only real similarity is the T/T instruments themselves, the original telephones being standard central battery types as used by the GPO and NTC in the early days of telephony in the UK. The oldest phone I came across on the system was when I replaced and rewired the entire Bakerloo line T/T system in the late 1980s, the wooden cased phone with brass fittings and ceramic receiver had a label bearing the date 1919. Most of the Bakerloo line T/T phones that I replaced were of similar type and age. Stringing T/T wires is a dying skill now which I was still doing right up until retirement and of course still occurs today when corrosion in wet tunnels causes breakage. Of course back in the day it was common as normal public telephone circuits were carried on open wires on poles all over the country. One other point is that the author mentioned 'overriders' used to enable traction to be charged when the T/T circuit is out of commission and in other circumstances such as failure investigation during traffic hours. There are also 'inhibitors' (CB Inhibits) which are used in engineering hours to prevent the charging of a section while the T/T section is being maintained or otherwise worked on. These were also introduced with enhanced line clear as an additional safety measure for all staff working in tunnels.
Of course enhanced line clear never was fully invoked but the measures and changes introduced to accommodate it have provided enhanced safety for anyone working on the system and did improve the older T/T systems too. |
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