Signal Main Frequencies

Just to start a discussion on the 33.33 Hz  and 125Hz signal mains.

When Lots Road was first put into operation it generated at 33.33Hz, Neasden was the same. Greenwich being a tramway generator was 25Hz, later converted to 50Hz, but never supplied power to the underground until the Eastern End of the Central Line was built.

So from the outset, the signal mains were 33.33Hz, no issues with 50Hz contamination on the track or unlikely false operation as the dual element vane (DEV)* was almost bomb-proof.

The Western End of the Central Line was supplied with a Great Western Railway standard 50Hz Industrial Frequency distribution, so to keep the standard 33.33Hz LT signalling equipment, frequncy convertors were installed at each traction substation.

In the 1960's Lots Road was modernised and 50Hz generators installed, so to keep the 33.33Hz signal mains going, frequency changers were installed in virtually all traction substations.

The Piccadilly Line eastern extension was supplied from the National Grid at 50Hz and the signal mains were 50Hz, and no frequency changers installed at this time.

The eastern end of the district line was different again, it had 2 mains both at 33.33Hz the 2 mains were at 600v and 400v nominal but electrically 90 degrees out of phase, so a Dual element vane relay could be operated from each of the mains without the need for track capacitirs. (Resistor fed track circuits would operate perfectly satisfactorily.)

I think that this was due to the concern of interference with the adjacent 25kV 50Hz overhead electricification of the London Tilbury and Southend Line.

Sometime later these 5600v and 400v mains were refered to as "Clean" and "Dirty" mains, can't remember which as which.  I would imagine that power such as signal aspects were from the higher capacity 600v main and the 400V main reserved for relay operation.
At some stage, this ended up as standard capacitor-fed track circuits.

The frequency changers in the substations had one motor and two genetators mechanically coupled to produce the 90 degree phase shift, and the shafts were correspondingly marked so that the phasing could be maintained following major overhauls. If the generators were mechanically coupled the wrong way round the DEVs would be driven in the opposite direction with all the signals going to red, even if all the track circuits were unoccupied.

Not sure how 125Hz came into being, but guessing that if you are going to have frequency changers then ypu might asd well have a frequency that gives you some economic advantages as well, higher frequencies are more economic in transformer and generator construction, however, volt-drop in signal mains increase.



Fast Forward to SSL upgrade, the parts of the Met and District lines had to converted from 33.33Hz to 125Hz to avoid interference from the new "S" stock trains, and the track circuits were converted to audio frequency.
 A further 50Hz main was then installed to supply the audio-frequency cabinets.

Thus on the North end of the Met, a failure of the 33.33, 50 or 125Hz supplies would bring the service to a halt. This situation exists to this day, on the basis that it was a temporary installation and would be all swept away when the TBCT was installed a short while later (This was in about 2008)


I wonder if any other members can add any further information to this?

*The DEV is effectivly a 2-phase motor, with the operating torque being provided by a 90 degree phase shift between the 2 windings, which both must be of the same identical frequency. If windings are eneregised by different frequencies, a very feeble and eractic torque would be produced which would not cause significant rotation of the vane.