Siemens Alternating Current Block Instrument

The Siemens AC block instrument was invented by Dr Carl Frischen in 1871, and was widely used everywhere East of the Rhine, as far as Russia, in many thousands of examples. It is used to support the block system by providing a visual indication of block occupancy, as well as enforcing safe and regular operation of the system. With the addition of a few accessories, it provides a system that was known as "Lock and Block" in the US and UK. Siemens and Halske licensed the design to many alternative suppliers, such as AEG and Ericsson. The block instruments discussed here are only one type of Siemens and Halske instruments, but are a late and well developed example. Another type is discussed in the article on the block system

Its basic operation, animated in the illustration at the left, was as follows. One instrument was used at each end of a block, connected by a single line wire. Normally the instrument at the entrance of the block was in the "unblocked" state, while that at the exit was in the "blocked" state. Both instruments showed white vanes in the circular apertures on the front of the instruments. The unblocked state at the entrance instrument allowed a route lever to be moved that permitted the starting signal to be cleared by means of its own lever. When a train accepted the clear starting signal and entered the block, after restoring the starting signal and the route lever, the signalman would press down on the plunger of the instrument and rotate the crank of the magneto at the side of the row of block instruments from ten to fifteen turns. This caused a red vane to appear in place of the white one, and the instrument was then in the "blocked" state. The starting signal was then locked normal, and could not be cleared again. Simultaneously, the instrument at the other end of the block was "unblocked" and also showed a red vane. This advised the signalman of the approach of the train and permitted him to clear the home signal at the exit of the block. When the train had passed out of the block the home signal was restored, and then the signalman could block his instrument, just as the other signalman had done when the train entered the block. This would also unblock the instrument at the block entrance, and both would show white vanes. Even with no added apparatus, proper block operation is enforced. For example, the starting signal can be cleared only if line clear has been received.

Increased safety can be provided by means of track circuits or treadles detecting the movement of trains. The starting signal can be provided with an electrical slot that automatically restores the signal when the track circuit is occupied or the treadle is depressed. If the circuit is not re-established until a new line clear is received, it is clear that only one train can be admitted to the block for each line clear. At the exit end of the block, an electric lock may prevent the plunger from being depressed to block the instrument unless a train has occupied the track circuit, or depressed the treadle, while the home signal is clear. This assures that a train that has entered a block must be proved to have left it before line clear can be given again (In the UK, this is called a Welwyn Control).

The construction of the instrument is shown diagramatically at the right, in the "blocked" state. If this instrument is at the block entrance, it shows a red vane indicating "Train on Line". It is connected to a similar instrument at the other end of the block, which is in the "unblocked" state, but since the red and white sectors are interchanged on that instrument, also shows a red vane.

The electrical parts on the right-hand side consist of a magneto M, contacts O, and magnet coils F of a polar relay. When the shoe S at the end of the vertical rod PS is depressed, it closes the contacts to connect the magneto in series with the magnet coils on the two instruments that are connected. When the magneto handle is rotated, an alternating current is produced (nominally 20 Hz) that vibrates the armatures of the polar relays back and forth. This allows the anchor escapement R, just as in a mechanical clock, to allow the sector Q move in small steps in whichever direction it is pushed.

In case the instruments get out of phase, the window in front of the sector Q can be opened after breaking a seal, and the escapement anchor can be moved by hand until the instrument is in the desired state.

The plunger P, if it is free to be depressed, compresses the spring G while causing the piece D attached to it to press down the piece E attached to rod N, compressing the spring H and moving the dog L that prevents rotation of the route lever axis. These parts are shown as when depressed. When P is raised again (by the spring G) the catch C snaps to the position shown, preventing P from again being depressed. Since now S cannot be moved downward, the magneto cannot be connected to this instrument, guaranteeing that the instrument can only be cleared by an alternating current signal from the distant instrument at the other end of the block.

Before P was depressed, the catch B was swung to the left clear of the projection at the top of rod N. However, when rod N moved downwards, it caught the lip at the bottom of B and rotated it into the position shown, where it prevents rod N from rising again. When this happened, the sector Q was rotated anticlockwise from the position shown so that the top of catch B cleared the relieved area of the axis of Q. When in this position, with P depressed, rotation of the magneto handle permitted S to descend under its own weight to the position shown. Rotation of the axis A then trapped catch B as shown.

In the state shown, spring E is pressing upwards on a stud connected to sector S, but motion is prevented by the teeth on the periphery of S. When an alternating current is received from the distant instrument, the sector rotates upward by small steps. Eventually, the axle A rotates so that catch B is permitted to move to the left, freeing rod N which then is moved upwards by its own spring H.

Operation by alternating currents produced by the magnetos completely removes any danger of foreign currents affecting the instruments, as they can easily do with direct current instruments, and even more easily with instruments operated by instantaneous currents. There are no batteries to wear out or be serviced, and only one line wire is required for each block. The mechanical structure is rugged, and not liable to get out of order. For these reasons, the Siemens block appeared ideal when compared to its competitors, explaining its wide use.

The Siemens block was often seen as rows of four instruments in a common case above the interlocking frame, with the magneto handle on the right. The end instruments had electric locks above the plungers, showing red or white vanes to show their state. This was appropriate for a double track line with a unidirectional block on each line.

D. Wurmser, Signaux Mécaniques, Tome III (Grenoble: Presses et Editions Ferroviaires, 2008), pp. 92-94.

Composed by J. B. Calvert
Created 21 November 2008
Last revised