Railway Point Circuit

Railway Point Circuit Introduction 

The majority of points used on the London Underground are pneumatically operated, the most common types being Four Foots, Chair locks and Clamp locks.  On the Central Line, electrically driven points such as M63’s and HW1000’s are used as there is no compressed air main supply available. In some areas, Four Foot and Chair lock points have had their pneumatic motors replaced by electro-hydraulic units. The control circuits vary slightly from air worked points; however, the principles remain pretty much the same.

This Unit will deal with all the circuits associated with air worked Four Foot, Chairlock and Clamplock points controlled from a ‘V’ style lever frame. The circuit principles also apply to the older ‘N’ style frames even though these circuits differ very slightly, explanations are offered in the relevant sections. The circuitry associated with all air worked points over which passenger trains can run are derived from the Basic Principles looked at in Unit 1.

To recap, these are as follows:

1. A means of moving the points to one of two positions (either Normal or Reverse).

2. A lock to hold the switches in the required position (Facing Point Lock, this requirement is achieved mechanically).

3. Detection of both the open and closed switches and the FPL.

4. A means of preventing the points from being moved when a train is upon them (Track Locking)

In addition to these principles, extra circuitry is needed to operate and detect the WL if it is fitted to the points.

An electric lock should be fitted to the controlling lever to prevent the points from being thrown when a train is occupying the associated track circuit(s). An extra circuit that needs to be added is some form of visual indication of the position of the points.

If the controlling frame is a manual type then the indication is mounted on the frame to inform the signal operator of the point’s position. If it is a remotely controlled frame then the indications will be found on the equipment room line diagram.

The Controlling Lever

The ultimate control of the points is effected from the point lever which is situated in the relevant IMR or signal cabin. The point number corresponds with the lever number as discussed in Unit 1 section 1.2. All levers, regardless of their function, proscribe an arc of 60º from the fully Normal to the fully Reverse position and back again. Fig 1.1 shows how this 60º arc is broken up into distinct angular positions which correspond with the letters N-A-B-C-D-E-R to represent these angles.

The lever shaft has contacts attached to it, known as lever bands, which are designed to electrically bridge the contact fingers as shown in Fig 1.2, and makeup circuits when the lever is in a certain position. For example, an NC lever band will make contact with its respective fingers when the lever shaft is within the NC arc as shown in Fig 1.1.  It can therefore be seen that the lever bands will make at different times and electrically energise circuits in a certain sequence.

With point circuits, the main bands used are the N, NC, RC and R. The N and R bands are normally used to energise the point auxiliary valves. The NC and RC bands are used to energise the WL electro-pneumatic valve.

We will now look at all the electrical circuits used with points. There are five different circuits and these are as follows:

1. The Point Control circuit.
2. The Detection circuit.
3. The WL Control circuit.
4. The Lever Lock circuit.
5 The Indication circuit.

The Point Control Circuit

The point control circuit controls the throw of the points. This applies to both the Normal and Reverse positions and fulfils the first basic principle. On Four Foot and Chairlock points, the valves are energised at all times allowing the air motor to hold the escapement slide fully home and locked. This in turn ensures that the point switches remain locked until the points are thrown the opposite way.

Fig 1.3 shows a typical point control circuit. The outgoing control lines 33NW and 33RW are fed from the main 33W fuse via the Normal and Reverse contacts on the point lever shaft. The lever contacts are arranged so that only one can make them at any one time, thereby only one of the point auxiliary valves can energise at any given time. For this to occur, the lever shaft must be either fully Normal or fully Reverse.

A fault in this circuit could lead to a situation where neither valve is energised. To prevent the points from moving if this were to happen, the ‘D’ valve in the point auxiliary valve is designed to maintain air pressure to the points even when there is no electrical supply to either the NW or RW valves.

What is Point Detection?

Before a passenger train can pass over a set of points under a signalled move they must be electrically proved to be in their correct position and locked. In the case of Four Foot points, there is a single point and lock detector box (P&LD) connected to the escapement slide and both point switches.

This box usually contains six contacts (6 way), three for the Normal detection and three for the Reverse detection. The relevant contacts are made when the facing point lock (FPL) is proved in position, the open point switch is detected open sufficiently and the closed switch detected up to the stock rail within tolerance. This fulfils the third Basic Principle

If any of these three criteria are not met then all contacts in the box must remain broken. We are going to look at the main electrical components involved in the detection circuit, namely the P&LD box and the three-position WKR, before looking at the detection circuits.

The P&LD Box

The Four Foot point equipment can be located on either the left or right-hand side of the negative traction rail. The position of the positive traction rail governs the type of layout to be used as the majority of the equipment should be at continuous rail potential; where possible the continuous rail is always next to the positive rail.

Both diagrams are viewed from the tips of the switches looking into the points.

There are two types of six-way P&LD box available and are labelled according to the layout they are to be used with. A left-hand box is for a left-hand layout and a right-hand box for a right-hand layout. Although the boxes are functionally the same they are mechanically different and cannot be swapped. In a Left-hand box the contacts are made (Normal or Reverse) when the radial arms and parabola ball are paraLLeL to one another.

In a Right-hand box the contacts are made (Normal or Reverse) when the radial arms and parabola ball are at Right angles to one another.

The contact numbering in the P&LD box changes according to the turnout of the points. Standing at the tips of the point switches, starting with the contact furthest away the labelling is as follows

The AN and BR contacts are used in the control of the WL valve. Contacts 1N, 2N, 3R and 4R are associated with the point detection circuit.

A little ditty to remember this is: I (R)(AN) to (L)London (Bridge)

Right-hand turnout (AN) is furthest. Left-hand turnout (BR) is furthest.

A summary of the contact labelling is shown below

All points are shown in the Normal position and viewed from the tips of the switches, facing the points.

The Three Position WKR

The detection circuit uses a three-position Double Element Vane (D.E.V.) type relay which has two coils (‘Q’ & ‘R’) and twelve contacts (six Normal and six Reverse). Both coils operate at 100V A.C, the ‘Q’ coil being fed directly from a local bus bar and the ‘R’ coil via the P&LD box contacts. By altering the direction of the current passing through the ‘R’ coil the relay will drive either Normal or Reverse. When there is no supply to either of the coils, the relay returns to the mid-position, where all contacts are broken.

Figs 1.12 and 1.13 show a typical detection circuit for a single ended right-handed set of Four Foots.

To change the phasing of the ‘R’ side the BX and NX feeds need to be changed by 180º (R1 and R2 feeds swapped around). In the detection circuit this is achieved by using loops in the P&LD box as shown in Fig 1.12.

In Fig 1.12 the points are shown detected in the Normal position. The BX and NX supplies to the WKR are fed over contacts number one and two respectively. The arrows represent the direction that the current is flowing through the coil at a given moment in time (used for
explanation purpose only).

In Fig 1.13 the points are shown detected in the Reverse position. The BX is fed over contact number three and the NX over contact four. The arrow is now shown pointing in the opposite direction as the phase has been changed by 180º.

At older sites, the feed for the ‘R’ coil is taken from a fuse bay in a kiosk or location adjacent to the points. At newer sites the ‘R’ coil supply originates in the IMR, feeds out to the points and returns back to the relay in the IMR.

In the case of a double ended set of points, the WKR needs to detect that both ends are Normal or both ends are Reverse.

To differentiate between the two sets of points they are labelled with a suffix of ‘A’ or ‘B’. The ‘A’ points being situated nearest to the controlling IMR and the ‘B’ end furthest away. A double ended layout may typically be labelled 33A and 33B.

The detection circuit for a double ended set of points requires additional wiring to connect the ‘A’ and ‘B’ end P&LD boxes. The loops are now split between the two boxes, an example of this is shown in Fig 1.14.

The WL Control Circuit

The WL control circuit can be sub divided into two parts:
1. Track locking
2. WL valve control

Track locking

Every set of points which has a passenger move over them, must be track circuited throughout their entire length. The associated section of track is known as the point locking track circuit. Fig 1.15 shows AA track as the point locking track circuit.

In some situations such as double ended points more than one track circuit may be used. Fig 1.16 shows AA and BD tracks as the point locking track circuits.

When the track circuit(s) is (are) occupied, the WL cannot be energised and the points remain locked to prevent them from moving under a train. This fulfils the fourth requirement in the basic principles.

Fig 1.17 indicates how the point locking track circuit contacts are wired into the WL control circuit. The example shows two point locking tracks, for a single track, only one TR contact would be used.

WL Valve Control

When the point lever is moved from say N → R to throw the points over, there is a sequence of events that must occur to enable the points to operate correctly.

A simplified event list for this N → R movement is shown below:

1. Normal point auxiliary valve de-energises.
2. WL valve energises retracting the WL from its port.
3. Reverse point auxiliary valve energises and points move towards Reverse Position.
4. WL valve de-energises and WL dab drops and rests on the WL slide.
5. Points reach fully Reverse position.
6. WL dab enters Reverse port on the WL slide.

The control of the WL has two stages, it must be energised before the points are thrown and then de-energised to allow the dab to enter the destination port.

The ground lock (WL) needs a separate energy source to activate it and unlock the points, but does not need an energy source to re-lock the points.

Fig 1.18 is an example of the WL control circuit for a single end, right-hand point turnout. The RC and NC lever bands control the pick-up of the WL valve. The AN and BR contacts are used to control its de-energisation once the points start to move.

The Ground Lock (WL)

Engineering standard E7152 A4 (Maintenance of Points) defines the Ground lock (WL) as:  An independent point lock which keeps the points locked to guard against incorrect operation in the event of certain types of failure (generally only fitted to pneumatic systems).

Pneumatic points are held locked in position by air supplied to the motor from a ‘D’ valve in the point auxiliary valve. The design of the ‘D’ valve is such, that if the electrical supply is removed, the air supply is still maintained to the points to prevent them from moving.
If however the air supply is removed, there would be nothing to prevent possible movement of the escapement slide and consequently the points.

A train travelling over a trailing set of points that are not home and locked will cause damage but is likely to stay on the track and not de-rail. If facing points move under a train there is a high risk of derailment and the possibility that some carriages will ‘jack-knife’ (some take the left route and some take the right). With such serious consequences there is clearly a need to have a secondary means of locking facing points to protect them in the event of a loss of air.

The WL is an air operated piston controlled from an independent electro pneumatic valve. When the valve is de-energised, a dab on the end of the piston is pushed by a return spring into a port on the WL slide as shown in Fig 1.20.

If the slide starts to move, it will catch on the dab and the point escapement slide will not be able to move as shown in Fig 1.21.

The WL slide has a port at either end so that the WL can lock the points in the Normal or Reverse positions as shown in Fig 1.22.

When the WL is energised, the air motor retracts the dab from the port (compressing the spring) and the slide is free to be moved (Fig 1.23). The design of the electro pneumatic valve is such that a loss of the electrical or air supplies will leave the points in a fail-safe condition (WL engaged in the port).