INTERLOCKING SIGNAL CONTROL CIRCUITS Bob Braden DRAFT !!! 0. FORWARD This is the first try at a survey of US practice for interlocking circuitry. I have chosen an area that seems the most straight- forward and, in my experience, the most uniform: the control circuits for interlocking signals. Areas for future surveys might be: o Locking -- Signal Indication, Switch Indication, Section, Route, and Sectional-Route Locking o NX Interlocking Circuits o Relay-Coded CTC This survey was influenced by the book by Phillips [Phillips, Edmund J., "Railroad Operation and Railway Signalling", Simmons-Boardman, New York, 1942], but it is based upon fragments of the circuits of half a dozen interlocking plants on the former Reading RR and Pennsylvania RR. Although this is a limited sample, I hope it is somewhat representative of US practice over the past 60 years, as the US signalling technology was controlled by a few companies (GRS and USS) and was standardized through the efforts of the AAR Signal Division. I would be most interested to hear of different approaches to the same problems taken in other countries or by other US railroads. The general attempt here is to understand the circuit design principles. I wanted to: (1) understand and explain existing circuit designs, and (2) be able in principle to write a computer program able to design a plausible circuit, conforming to established signalling practice, given a particular configuration of tracks, switches, signals, etc. Bob Braden [Page 1] Interlocking Signal Control Circuits June 1991 1. INTRODUCTION This document surveys the circuits used to controlling signal aspects within an interlocking plant. It assumes a general knowledge of railway signalling practice. For our purposes, an interlocking plant controls train passage and routing within a region of a railroad line. The region, known as the "interlocking limits", typically contains crossovers, junctions, and/or crossings which provide the possibility for conflicting or othewise unsafe train movements. The controls over the switches and signals are interlocked to prevent such conflicts. Because of the safety requirement, signals within the interlocking limit almost always give absolute rather than permissive aspects. Even in the present day, interlocking circuits are generally relay circuits, presumably because these electromechanical devices have been engineered for extremely high reliability. In modern practice, CTC (Centralized Traffic Control) is often in use. With CTC, a human dispatcher at a central site is able to control signals and switches at remote interlocking plants. Control and indication messages between the central controller and the remote interlocking may be sent by a variety of means, including high tech channels like radio signals. However, the safety in the system depends upon locally enforced interlocking at trackside locations, and [I believe] this depends upon relay circuits of the type shown in this document. In this document, we will generally describe the circuits as if they were under control of a local towerman operating an interlocking machine or frame. Where applicable, we will show modifications or extensions necessary to operate the plant remotely, e.g., via CTC. <> Let us get some assumptions and conventions out on the table. * Power Supply We describe DC relay circuits, although some plants use AC. This does not fundamentally affect the logical design. DC is generally supplied by batteries, connected to provide both polarities (see the comment below on three-state circuits). The common return is denoted by "C", and the two supply polarities by "B" and "N". Bob Braden [Page 2] Interlocking Signal Control Circuits June 1991 FIGURE 1: Battery Conventions B C N | Battery | Battery | | | | +---i|i|i|i|---+---i|i|i|i|---+ * Relay Terminology A standard (non-polar) relay is a two-state device. When its coil is energized, its contacts are transferred: "back contacts" are opened and "front contacts" are closed. Following common practice for signalling schematics, we represent a relay coil by a box, and we use the convention that energizing the coil raises the contact springs. Thus, front contacts are always shown above and back contacts below the spring. See Figure 2 for an example. FIGURE 2: Example Relay Circuit Schematic (Back contact) BB (Front contact) ____o----------+ +-----> C AA |^ |___| B>-----+ +------------+ | | V____o |___|ZZ Battery current (B) flows through a front contact of relay AA (its coil is not shown) and a back contact of relay BB, to energize relay ZZ. Thus, relay ZZ will be energized if AA is energized and BB is not. We note that the order of a set of contacts in series may be changed without affecting the result. Practical engineering considerations like minimizing wire lengths may cause relay and lever contacts to appear in different orders. Trade-offs between cost and redundancy (safety) lead to other variations in the arrangements of circuits. * Three-State Circuits In signalling, some variables naturally have three states. Common examples are (1) the position of a switch (locked normal, Bob Braden [Page 3] Interlocking Signal Control Circuits June 1991 locked reversed, or somewhere in the middle), and (2) the indications of a three-color searchlight signal. To economize on wiring, a polarized (B, N, or off) DC voltage is often used to transmit such three-state variables, and polar[ized] relays, which have corresponding "left", "right", and "off" positions, are used to receive them. A polar relay has two kinds of contacts: polar and neutral. A polar contact has two positions, which we may designate "left" and right", and is shown diagramatically by a vertical spring (see Figure 3). A neutral contact also has two positions, energized with either polarity, and de-energized. When the coil is de-energized, the back neutral contacts are closed, but the position of the polar contacts is undefined. <> In Figure 3, the polar relay ZZ repeats the state of polar relay YY. A polar and a neutral contact on YY are connected in series, and the polar contact selects one of the two battery polarities. This series connection of a neutral front contact and a polar contact on the same polarized relay is frequently used. FIGURE 3: Repeating a Polar Relay <--- YY ---> ZZ +----+ +--------------------------------+ +----> C | V____o <- neutral contact |___| o | | / <- - - polar contact |___| +-->/ <--+ | | ^ ^ N B The same principle is used with polar AC relays. The two energized states are represented by AC voltage either in phase with a reference voltage or 180 degrees out of phase. A polar AC relay has two windings, one of which is supplied by the constant reference voltage. * Fail-Safe Circuits All interlocking circuits are designed to be fail-safe. If a circuit fails, the system must default to the safe condition: signals at their most restrictive aspect, track circuits Bob Braden [Page 4] Interlocking Signal Control Circuits June 1991 occupied, or switches locked. For example, a circuit must be completed to clear a signal or to unlock a switch lever. * Switch and Signal Designations Signals and switches are designated by numbers. In some plants, signals are given even numbers and switches odd numbers. An even/odd numbering plan typically indicates the use of an electro-mechanical interlocking machine with two banks of levers, one above the other. The upper (odd-numbered) levers control switches, while the lower (even numbered) levers control signals. The positions of a switch (turnout) are called "Normal" (N) and "Reverse" (R). Two switch machines are sometimes wired to operate together from the same lever, and therefore have the same number. Common examples are the two ends of a crossover, or a siding switch and a matching derailer on the siding lead. There are three different styles for numbering signal levers; see Figure 4. In early mechanical plants, each of the signal heads (high, medium, and restricted speed) on a single standard was controlled by its own lever with a distinct number, as shown in Figure 4A. With the introduction of electrical signal controls, a single lever came to be used for all the heads on a single standard, with head (aspect) selection performed by the circuits to be described below. For circuit purposes, multiple heads are generally distinguished by letters, with "A" for the top (high-speed) head. In either Figure 4A or 4B, the controlling lever has two positions, "Reversed" (R) to clear the signal and "Normal" (N) to place it at its most restrictive aspect (Stop). Electro-mechanical interlocking machines introduced three- position ("Left" (L), "Normal" (N), and "Right" (R)) signal control levers to clear either of the opposing signals on the same track. The signals were then designated L and R; see Figure 4C. This approach is retained in modern CTC practice. Bob Braden [Page 5] Interlocking Signal Control Circuits June 1991 FIGURE 4: Signal Designations A. Lever per head 8 2 4 6 |--O O-O-O--| ----- ------interlocking limits---------- ------- B. 2-Position Lever a b c |--O 8 2 O-O-O--| ----- ------interlocking limits---------- ------- C. 3-Position Lever a b c |--O 2L 2R O-O-O--| ----- --------interlocking limits-------- ------- Most of the examples in this document assume two-position signal levers in numbering signals (Figure 4B). This choice was made for simplicity and clarity, not because it represents the most common current practice. Sections 3.2 and 3.3 below present examples of the circuit differences that result from having a lever per head (Figure 4A) or 3-position levers (Figure 4C). Each lever operates a set of electrical contacts through a mechanical linkage. These contact are sometimes called "bands" when they are realized by fixed metal fingers bearing against a cylinder that is geared to rotate with the lever position; metal contact strips or "bands" are wrapped around the cylinder at the appropriate circular locations to bridge the contact fingers. In a circuit schematic, a lever contact is denoted by a circle containing the letter(s) designating the positions in which the contact is closed, e.g., "N" for "Normal", or "LN" for Left-to- Normal. In this document, we use parentheses instead of a circle; thus "(N)" represents a lever contact closed in the Normal position. The number of the lever appears above the letter(s). * Relay Naming Conventions Finally, there are some widely-used conventions for designating relays. Here are some examples; more will be introduced later. Bob Braden [Page 6] Interlocking Signal Control Circuits June 1991 Home Relay: H[R] Energized to clear designated signal to (at least) an Approach aspect. For example, here are plausible designations of the H relays for the signals in Figure 4: Figure 4A: 8HR, 2HR, 4HR, 6HR Figure 4B: 8HR, 2AHR, 2BHR, 2CHR Figure 4C: 2HR, 2LAHR, 2LBHR, 2LCHR The suffix "R" means "relay"; it may be <> omitted when the context is clear. Circuits for energizing the H relays are described in sections 2 and 3 below. Proceed Relay: J[R] In some circuits, a J relay is energized in conjunction with the corresponding H relay to clear a signal to the Proceed aspect. All the forms shown for the H relay apply to the J relay also. Circuits for energizing the J relays are described in Section 5.2 below. The circuits are sometimes known as "90 degree circuits", referring to the Proceed aspect for an upper-quadrant semaphore signal. Figure 5 shows the function of the H and J relays in selecting the aspect of one head of a color-light signal designated "2A". Figure 22 shows the corresponding signal control circuit for the two heads of a position-light signal. Note that each head of a position-light signal may require a second H relay to control the 135 degree aspect that is unique to position light and color-position light signals. The two H relays are then designated HA and HB. Track Relay: TR or TPR A track circuit, used to detect block occupancy, energizes a track relay TR. Its coil is connected directly to the two rails at one end of the block, and the track circuit battery at the other end of the block normally energizes the relay. The wheels of a train occupying the block shunt the relay and deenergize the track relay. Bob Braden [Page 7] Interlocking Signal Control Circuits June 1991 The "P" in the second form designates "rePeater", a relay that is controlled by a front contact of a track relay to repeats its state. Another convention uses "M" instead of "P" to indicate "repeater"; for example, the Pennsylvania RR used track circuit repeater relays denoted by "TM". Switch Repeater Relay: SS, NW, RW These relays repeat the physical position of a track switch (or pair of switches). An SS relay is polar; it will be energized and poled to the left (right) only when the corresponding switch is (switches are) locked in the normal (reverse) position, and otherwise it will be de-energized. The circuit to energize the SS relay includes a circuit breaker that is mechanically linked to the switch points and also a second circuit breaker that is mechanically linked to the lock mechanism in the switch machine. As a result, the SS relay state truly represents the physical state of the switch points and locking mechanism. A pair of non-polar relays NW and RW is sometimes used instead of a polar SS relay to repeat the position of each switch or crossover. If the switch is locked in the N (R) position, its NW (RW, respectively) relay will be energized. See Section 3.5 for more details. FIGURE 5: Typical Signal Lighting: Color-Light Signal Head 2AJ +------+ . . 2AH V | . . +------------o_____ +-----[G]----+ V |^ . . | + >---------o_____ +------------[Y]----+ |^ . . | +------------------------------[R]----+----> - . . . . We are now ready to describe the general principles in the design of signal control circuits, i.e., the circuits necesssary to energize the H and J relays. Section 2 gives an overview of the primary signal control circuits that energize the H relays. Section 3 presents an example and a number of important and interesting variations. Section 4 discusses the subsidiary circuits used for Bob Braden [Page 8] Interlocking Signal Control Circuits June 1991 block occupancy checking, and finally Section 5 discusses the J relay circuits. 2. BASIC SIGNAL CONTROL CIRCUIT PRINCIPLES We begin with the primary signal control circuits that energize the H relays. Depending upon the aspect of the next signal in advance, energizing an H relay may also cause the J relay to be energized. The H relay circuits depend upon input from: o Track circuit occupancy, as indicated by TR or TPR relays. o Switch position, as indicated by switch position repeater relays. o Switch lever contacts o Signal lever contacts The H relay circuits (also known as "45 degree circuits", in analogy with semaphore operation) have the following functions to perform [Phillips]: [a] Switch position check [b] Opposing signal check [c] Track occupancy check [d] Direct control and aspect selection [e] Time Locking/Approach Locking check Figure 6 (which is split into two parts to fit on the page) shows in simplified form the circuits that are used to perform these functions. The letters in brackets label the elements performing each of the functions. Bob Braden [Page 9] Interlocking Signal Control Circuits June 1991 FIGURE 6: Simplified cheme of Signal Control Circuit 6A. General Circuit [b] [b] 2 8 +---(N)-------(N)----+ | | _______________ | | ( ) | <------+-----( SS Protection )----+-----// (Fig 6B) To Signal ( ) 8 control ---( Network )--- ( ) to other signal control ---( [a] )--- circuits (_______________) 6B. Signal 2 Control Circuit [d] [d] ____ 2 [e] [c] / : --(R)-+ +---->C / : _|__| 2S 2A /Aspect: | | 2AH From SS //----+ +---+ +-----+ Select: |___| Protection V__|_o V____o \Netwrk: 2 Network |__ \ : --(R)-+ +---->C (Fig 6A) | \____: _|__| | | 2BH |___| We now explain each of the functions performed in Figure 6. [a] Switch Position Check The switch position check allows a signal to be cleared only if track switches are set for a valid route through the interlocking. It is based upon a network ("SS Protection Network" in Figure 6A) of switch lever and switch position repeater contacts. This network is an analog of the actual track topology. Thus, there will be an electrical path through this contact network if and only if there is a corresponding path through the tracks and switches of the interlocking. Suppose that the switch positions are such that a train passing signal 2 would exit the interlocking plant past opposing signal 8. Assume that signal lever 8 is Normal but lever 2 is Reversed. In Figure 6A, battery flows through the (N) lever 8 Bob Braden [Page 10] Interlocking Signal Control Circuits June 1991 contact, across the SS protection network, and into the control circuit for signal 2 (shown in Figure 6B). There it passes through an (R) contact on lever 2 to energize one of the home relays for signal 2. If there is a conflicting switch position then there will be no circuit through the SS network, so signal 2 cannot be cleared. The SS Protection Network is often assembled using polar SS relays that repeat the physical position of the switches. The complete network is then assembled using the basic circuit elements shown in Figure 7. In this figure, the elements of track topology are shown on the left, and the corresponding circuit network elements are shown on the right. Instances of these circuit elements are interconnected to model the particular track and switch layout. FIGURE 7: Elementary SS Protection Circuits A. Turnout 3SS 3 _____________ ---(N)--+ +----+ +--------- /3 | o V____o / | / ____/ +-->/ <--+ 3 | ---(R)--------------+ B. Crossover 5 5SS -------(N)--+ +----+ +----- | o V____o | / ____________ +-->/ <--+ /5 5 | / +--(R)-------------+ ___5__/______ | | 5 5SS -----+--(N)--+ +----+ +------ | o V____o | / +-->/ The circuit also includes corresponding contacts of the switch level on the interlocking machine: (N) for normal, (R) for Bob Braden [Page 11] Interlocking Signal Control Circuits June 1991 reverse. Thus, the SS Protection Network verifies correspondence between the position requested by the tower operator and the actual switch position. [b] Opposing Signal Check The same circuit also checks that the opposing signal lever (whose identity depends upon the route) is at normal. In Figure 6A, assume again that the switches are lined up for a route between signals 2 and 8. If levers 2 and 8 were reversed simultaneously [which may be prevented by mechanical interlocking], there would be no battery source to operate the corresponding H relays, so neither signal could be cleared. [c] Track Occupancy Check In Figure 6B, we assume that all relevant track occupancy checks for a given signal are summarized in a single relay, known as the "track indicating relay" and designated by "A". As we describe in Section 4, there are several conditions included in energizing this relay. Most importantly, if any track circuits are occupied in the route that has been set up, the A relay will be de-energized. [d] Direct Control and Aspect Selection The aspect to be displayed -- e.g., high, medium, or restricted speed -- is determined by which H relay is energized. This in turn is controlled by a fan-out network of switch lever contacts shown in Figure 6B as the "Aspect Selection Network". This network is generally constructed in analogy to the track plan, reflecting the tree of possible routes rooted at the signal in question. Figure 8 shows a typical aspect selection network (in dotted box), assuming that the high-speed route requires crossover 5 Normal, the medium speed route requires 5 Reversed and 3 Reversed, and the restricted route assumes 5 Reversed and 3 Normal. This example matches the track diagram shown in Figure 9 below. Bob Braden [Page 12] Interlocking Signal Control Circuits June 1991 FIGURE 8. Aspect Selection Network . . . . . . . . . . . . . . . 5 . 2 2 ---+---(N)------------------------(R)---+ +------+--(R)-- . | . |___| | | High Speed | | 2AH | . | . |___| | | 5 3 2 | . +---(R)-----+------(R)---------(R)---+ +------+ | . |___| | . | Medium Speed | | 2BH | | . |___| | . | 3 2 | +------(N)---------(R)---+ +------+ . . . . . . . . . . . . . . |___| Restricted | | 2CH |___| Figure 8 shows another feature of all H relay circuits. The H (and J) relays are located at trackside near the signals they control, which means that the relays must be energized via cables that may run some distance from the tower. There is concern that an inter-wire short within the cable duct could cause an H relay to be spuriously energized, resulting in a signal being cleared erroneously. Therefore, the current to the H relay coil is returned to battery common (C) at the tower via an R contact on the controlling lever 2, closed only when lever 2 is Reversed. With this arrangement, the H relay could be spuriously energized despite lever 2 being Normal only if there were shorts to BOTH leads of the relay and only if these shorts had OPPOSITE polarity, an extremely unlikely event. This principle, which we refer to here as "double-tailed" control of the H relay, is often extended with additional checks in the return lead. The extent of such checks varies considerably among different plants, depending upon the engineering trade-off of safety vs. cost. For example, it is common to include a front contact of the track occupancy relay (2A in Figure 6B) in the H relay return lead(s); see for example Figure 11. Some designs repeat the aspect selection tree (Figure 8) in reverse; others include switch lever contacts to check the position of opposing switches. Bob Braden [Page 13] Interlocking Signal Control Circuits June 1991 [e] Time Locking/Approach Locking check Interlocking signal levers are generally subject to Approach Locking or Time Locking, to prevent the signalman from changing the position of switches during the time a train is approaching an interlocking home signal. When the train has already accepted a Proceed aspect and is coming at high speed, an accident could result from restoring the signal lever to Normal (Stop) and immediately throwing a switch to change the route. To prevent such accidents, a signal lever lock, which prevents the restoration of the signal lever to the full Normal position, can be released only after a time interval (e.g., 2 minutes) has expired. With Time Locking, such a time interval is always necessary to restore a signal lever to Normal. With Approach Locking, the interval is necessary only if a train is occupying the approach block to the home signal. The timer (if it is mechanical) is normally wound up. The signalman moves the signal lever as far as possible towards Normal; this opens the "R" signal control contacts and the signal returns to Stop. The signalman then releases the timer, which immediately opens a front contact to electrically ensure the signal is at Stop. Figure 6 shows this contact on the timer 2S; the "handle" symbol through the contact indicates a mechanical timer. When the timer winds down, it closes a back contact (not shown) that enables the signal lock, to allow the lever to be moved to full Normal. This unlocks the mechanical and electrical locking on the switches. The signalman must wind up the timer in order to close the 2S front contact and clear the signal again. These features will all be present in some form for control of a home signal. The control circuits for dwarf signals, which govern movements against traffic and strictly within interlocking limits, typically are much simpler. Since their most permissive indication is Restricted -- proceed at restricted speed and watch for obstructions -- track occupancy need not be checked. Indeed, since dwarf signals are used to control "shunting movements" with standing cars, it is important that track occupancy not be checked. In addition, there is no aspect selection, since there is only one clear aspect. See the example in Figure 12 below. Bob Braden [Page 14] Interlocking Signal Control Circuits June 1991 3. AN EXAMPLE, WITH VARIATIONS Most of the examples in this paper are based upon the example track layout shown in Figure 9. This diagram shows the signals, switches, and track circuits in a relatively simple interlocking plant. The gaps in the track indicate the boundaries between track circuits. FIGURE 9. Example Track Layout 2 O-O-O--| ___8T___ ______________1_T_________ ___________5_T_________ ___1_2_0_T_____ 1\ /5 |-O 8 \ / \ / 10 O-| __1_1_2_T__ _____A_1_T_____\_1____ ________A_5_T_____/_5__________ _______1_0_T__ /3 |--O-O-O 12 / / ___1_4_T______ ________A_3_T_____/ |--O-O-O 14 3.1 Example Circuits Figure 10 shows the SS Protection Network for the layout of Figure 9. Figure 11 shows a representative signal control circuit for signal 2; this circuit is attached to the upper righthand side of the SS protection Network of Figure 10. Bob Braden [Page 15] Interlocking Signal Control Circuits June 1991 FIGURE 10. SS Protection Network for Figure 9. 1SS 5 5SS 8<---+ +--+ +-------------------(N)--+ +--+ +-->2 V____o o | | o V____o (Fig 11) / 1 | | / +->/ <--(R)-+ | +->/ <--+ | 1 | | 5 | +---------(N)-----+ +-(R)------------+ | | | | 1SS 1 | 3 3SS | 5 5SS 12<---+ +--+ +(N)-+--(N)-+ +--+ +-+-(N)-+ +--+ +--->10 V____o o | | o V____o | o V____o (Fig 12) / | | / | / +->/ | +-->/ <--+ +->/ | | | +--------+ | 3 | 14<----------------------(R)------------+ FIGURE 11: Signal 2 Control Circuit for Figure 9. 2 +---(N)--- C V__|_o | V____o | V____o _|__| | V____o __| | | 2AH | | | | | | |___| | | | 5 3 14TR 2 | | +-(R)-+-(R)--+ +--(R)----+ +---+ | | V____o _|__| | | | 2BH | | | | | |___| | | | 3 2 | +---+ +-(N)---------(R)----+ +---+ | _|__| | 2PB 2CH | | | --*-- 3 |___| +--o o--+ +-(R)-+ | 2A | | 5 | 2AH 2BH 2 2 +--+ +--+-+-(N)-+-o____ ____o-(R)----+ +--(R)---->C _V___o ^| ^| _|__| +-+ 2N | | |___| Bob Braden [Page 16] Interlocking Signal Control Circuits June 1991 The circuit of Figure 11 peforms the track occupancy checking, aspect selection, and time locking provisions as discussed earlier. Two additional features are shown. First, the track circuits beyond the interlocking limits (8TR and 14TR), which not included in the 2A controls, are included explicitly in the H relay leads. Second, a control circuit has been added for a Call-On indication control relay 2N. In the energizing path for 2N, a front contact of the track indicating relay 2A is in parallel with a manual pushbutton 2PB. The pushbutton allows the Call-On indication to be given even if the route through the interlocking is occupied. On an electro- mechanical interlocking machine, there is a pushbutton associated with each signal lever. Once it is depressed, a pushbutton is typically held closed by a mechanical latch until the corresponding signal lever is restored to Normal. The 2N energizing path is further conditioned upon switch lever contact 5 Normal or 3 Reversed; this ensures that the Call-On applies only to the high or medium speed route, not the restricted-speed route. Back contacts on the high and medium speed H relays in series ensure that a less restrictive indication is not already being given. Including the 2A front contact in the 2N circuit can cause the Call-On indication to be displayed automatically, even without the operator pushing the button. Suppose a train passes signal 2 and the operator restores lever 2 to Normal and then reverses it again for a following train. Once the first train clears the interlocking, relay 2A will be energized. However, as long as the train is in the block in advance of the interlocking, the track relay 8TR (or 14TR) will be deenergized, so that neither the high-speed nor the medium-speed H relay can be energized. In this case (2A front contact closed, 2AH and 2BH back contacts closed), the Call-On relay 2N will be energized. Thus, the following train will receive the Call-On (Sstop and Proceed") indication automatically as soon as the earlier train clears the interlocking; this will be replaced by Approach when the train moves beyond the next automatic block signal. Analogous circuits for the other two high signals 12 and 14 are obvious variations of Figure 11. For dwarf signals, the circuits are considerably simplified. Figure 12 is the circuit for signal 10. Bob Braden [Page 17] Interlocking Signal Control Circuits June 1991 FIGURE 12: Dwarf Signal Control Circuit 10 +---(N)--- C P V__|_o _|__| N __| | | 10HR | |___| 3.2 Varation: Separate Signal Levers per Head Suppose the signals are controlled by separate levers per head as shown in Figure 4A. There are trivial changes in the signal control circuit, as illustrated in Figure 13. FIGURE 13: Signal 2 Control Circuit -- Version with Separate Lever per Head 2 4 6 +---(N)--(N)--(N)---- C P V__|_o |V____o | V____o _|__| | V____o N __| | | | | 2HR | | | | |___| | | | 5 3 14TR 4 4 | | +-(R)-+-(R)--+ +--(R)----+ +----(R)--+ | | V____o _|__| | | | | | 4HR | V | |___| | (call-on) | 3 6 6 | +-(N)---------(R)----+ +----(R)--+ _|__| | | 6HR |___| 3.3 Variation: 3-Position Signal Levers Using 3-position signal levers (as in Figure 4C) leads to signal control circuits that are essentially identical but with some subtle differences. Figure 14 shows the track diagram of Figure Bob Braden [Page 18] Interlocking Signal Control Circuits June 1991 9, altered to renumber the signals appropriately. Note that now only two signal levers are involved. FIGURE 14. Example Track Layout, with 3-position Signal Levers a b 2L O-O--| ___8_T__ ______________1_T_________ _______5_T_______________ ____1_2_0_T___ 1\ /5 |-O 2R \ / \ 1 5 / 12L O-| __1_1_2_T_ _____A_1_T_____\_____ _________A_5_T____/___________ _______1_0_T__ /3 |--O-O 12R / b a / ___1_4_T______ _________A_3_T____/ |--O-O 12R d c Figure 15 shows the control for dwarf signal 12L, corresponding to Figure 12. Note that the battery feed B is controlled by a lever contact (NR); this is closed if the lever is either in the N or the R position (or anywhere in between). This contact can be considered as logically 'not L' (!L in "C" programming notation). FIGURE 15: Dwarf Signal Control Circuit Corresponding to Figure 14. 12 +---(NR)--- C V__|_o _|__| __| | | 12LH | |___| 3.4 Variation: Individual Track Relays Our examples so far have assumed that all track occupancy checks for a given signal are summarized in one A relay. Another alternative is to include the individual track relays directly in Bob Braden [Page 19] Interlocking Signal Control Circuits June 1991 the signal control circuit. An obvious approach would seem to be to include the track relays in the SS Protection Network, in positions corresponding to their locations in the real track layout. Unfortunately, this obvious approach usually will not work, because it must be possible to clear som signals (e.g., call-on and restricted-speed aspects of home signals, and dwarf signals) even if their governed route includes occupied blocks. The usual approach that is used is to combine the occupancy check function with aspect selection. This is economical because the Aspect Select Network is typically a tree of routes, rooted at the signal in question. The individual track relays can be easily placed in this tree at the places corresponding to their position in the track layout. Figure 16 illustrates this for our example signal 2 in the track layout of Figure 9. The 2TPS "track repeater stick" relay contains the occupancy check for the block immediately in advance of the signal, as explained below in Section 4. In this figure, we omitted the 2SR time locking contact, to fit on the page. FIGURE 16. Signal Control Using Aspect Selection and Occupancy Check Network 2 +--(N)-- C o____V | o____V o____V _|__| | o____V | 2AH| | | | |___| | | | | 5 A5TR 3 A3TR 14TR 2 | +-(R)-+ +-+-(R)-+ +-+ +--(R)--+ +--+ o____V | o____V o____V _|__| | | 2BH| | | | |___| | | | | 3 A1TR 2 | +-(N)--+ +-------(R)--+ +--+ o____V _|__| 2CH| | |___| Bob Braden [Page 20] Interlocking Signal Control Circuits June 1991 3.5 Variation: Unpolarized Switch Position Repeater Relays We earlier described the SS Protection Network using polar SS relays (see Figures 6, 7, and 10). Figure 17 shows the elementary circuits appearing in the SS Protection Network if non-polar switch repeating relays are used instead. FIGURE 17: Non-Polar SS Protection Elements A. Turnout _____________ --------------------------+ +--------- /3 3RW 3NW | | / +----+ | | ____/ | V +--o___V_ | | ____o--+ | | ^ ^ | --------+ +-------------+--------+ B. Crossover 5RW 5NW ---------o____ +-----+ ^| | V____o---------- +----+ ____________ ^ /5 | / +----------+ ___5__/______ 5RW | V ---------o____ +---------- ^| 5NW | +-----------o___V_ 3.6 Variation: Aspect Selection using SS relays We have shown the common case of aspect selection based upon a network of switch lever contacts. Another possibility is to use the SS or NW/RW relays, which indicate actual switch position rather than theoretical position. The difference seems to be a matter of taste, since the SS Protection Network checks that switches and switch levers are in correspondence. Bob Braden [Page 21] Interlocking Signal Control Circuits June 1991 3.7 Variation: Interior Signals The circuits given so far have assumed that every signal governs an entry point to the interlocking plant. There is a variation on the general scheme of Figure 6 for a signal that is interior to the plant. To illustrate this, assume that there is a dwarf signal numbered 20 in advance of dwarf 8, between crossovers 1 and 5 in Figure 9. Figure 18 illustrates this interior signal. We must note that this example is somewhat artificial, as there would probably be no operational reason for dwarf signal 20 at that location; however, it will illustrate the circuit principles. FIGURE 18. Interior Signal Example 2 O-O-O--| ___8_T___ ______________1_T_________ _______5_T_________________ ____1_2_0_T___ 1\ /5 |-O 8 \ |-O 20 / \ 1 5 / 10 O-| __1_1_2_T__ _____A_1_T_____\_1____ ________A_5_T_____/_5__________ _______1_0_T__ /3 |--O-O-O 12 / / ___1_4_T______ ________A_3_T_____/ |--O-O-O 14 We must insert the appropriate signal control circuit for signal 20 at the appropriate place WITHIN the SS Protection Network. As illustrated schematically in Figure 19, the SS network is "broken open" at the appropriate place to insert signal 20 control. Bob Braden [Page 22] Interlocking Signal Control Circuits June 1991 FIGURE 19: Control Circuit for Interior Signal (Figure 18) 2 +-----(N)-----(N)-+ ______ | | ______ +-(N)--(N)-+ ______ | . Circuit . | ______ +-(N)- 90 degrees o AHA V o +------------o_____ V |^ o + >---------o_____ +------> 45 degrees o |^ o | | AHB +------> 135 degrees o | V o +------------o_____ o |^ +------> 0 degrees o o o /\/ | | | / B |_|/ BOTTOM HEAD CONTROL | \ o | \ * BJ +------> 90 degrees o BHA V +------------o_____ o V |^ * + >---------o_____ +------> 45 degrees o ^ | BHB +------> 135 degrees o | V * +------------o_____ o +---------+---------+----> Center light (* above) BHA V BHB V BN V +-o____ +-o____ +-o____ | | | +>---+---------+---------+ Bob Braden [Page 26] Interlocking Signal Control Circuits June 1991 The following table shows the aspects and corresponding relay com- binations. Proceed: AHA & AJ Approach: AHA Approach Medium: AHA & BHA & BJ Stop: (none) Medium Clear: BHA & BJ Medium Approach: BHA Restricting: BHB Permissive (Manual Block) (?): AHB Call-on: BN 4. TRACK OCCUPANCY CHECK 4.1 Track Repeater Stick Relay Interlocking home signals are often semi-automatic in operation. This means that an explicit operator action is needed to clear the signal for each train, while block occupancy automatically forces the signal to Stop. This is accomplished with a "stick circuit", i.e., a relay whose control circuit includes its own contact. This is illustrated in Figure 23, which shows the "track repeater stick" relay for signal 2 of our example (Figure 9). This relay repeats the track relay 5TPR for the first block in advance of signal 2. Suppose that lever 2 has been reversed, clearing signal 2. When a train passes the signal and enters track circuit 5T, the front contact on track repeater relay 5TPR opens and deenergizes 2TPS. This deenergizes the H relay (see Figure 16) and sets the signal automatically to Stop. However, when 2TPS is deenergized, its own front contact is opened, preventing its being energized again UNTIL THE OPERATOR RESTORES THE SIGNAL LEVER TO NORMAL. Hence, to clear signal 2 for a following train, the operator must return the lever to Normal and then reverse it again after the previous train has exited the block 5T. Bob Braden [Page 27] Interlocking Signal Control Circuits June 1991 FIGURE 23. Track Repeater Stick Relay 5TPR 2TPS B>-----+ +-----+---+ +---+------+ +-----> C V____o | V____o | |___| | | | | | | |___| 2TPS | | | 2 | +----(N)----+ 4.2 Track Indicating Relay As we discussed earlier, some home signal control circuits introduce a single "Track Indicating" relay that summarizes block occupancy within the interlocking. The Track Indicating relay also serves as a track repeater stick relay, to obtain semi- automatic operation of the home signal. This is illustrated schematically in Figure 24. FIGURE 24. Schematic Track Indicating Relay Circuit . . . . . . . . . . Route- . 5TPR . dependent . 2A B>----+ +--- Track Relay ----+---+ +---+------+ +-----> C _V___o . Network . | _V___o | _|__| . . . . . . . . . | | | | | | |___| 2A | 2 | +----(N)----+ The "Route-dependent Track Relay Network" (R-dTRN) selects all track relays which must be checked for safety. It may include all track circuits whose occupancy could threaten the safety of a train entering the interlocking at signal 2, on any high-, medium-, or slow-speed route. For a restricted-speed or call-on route, the R-dTRN is always a closed circuit; only the track circuit immediately in advance of the home signal, 5T, is included for such routes. We could synthesize an R-dTRN by starting from a Aspect Selection and Occupancy Check Network (described in Section 3.4 and Figure 16), and connect in parallel the high-speed and the medium-speed outputs. We would then shunt this tree-like circuit with a Bob Braden [Page 28] Interlocking Signal Control Circuits June 1991 combination of switch lever contacts that would be closed for any restricted speed route. This tree-like circuit would have the general form of a set of parallel paths, each of which included one or more track relay contacts in series with a combination of switch contacts (s1, ... sn in diagram below) to select the appropriate path. +---(s1)-----+T1 +---...----+ | _V___o | | | +---(s2)-----+T2 +---...----+ ------+ _V___o +------- . . . . | | +---(sn)-----+Tn +---...----+ | _V___o | | | +-----(restricting)---------+ However, in practice circuit designers use the "dual" form of this circuit. ("Dual" is a term defined in Boolean algebra and the theory of relay switching circuits). The dual is a series connection of all of the track relay contacts, each shunted by a combination of switch lever contact(s) when it is not relevant. The shunting combinations are shown in the following diagram as t1, ... tn: T1 T2 Tn ----+--+ +--+--+ +--+-...-+ +---+----- | _V___o | _V___o | _V___o | | | | | | | | | +---(t1)--+---(t2)--+-...--(tn)---+ | | | | +------(restricting)--------------+ This transformation is illustrated in Figure 25. Figure 25A shows the tree-like form of R-DTRN resulting from the network of Figure 16, after some minor simplification, while Figure 25B shows the dual circuit form that is actually used. Bob Braden [Page 29] Interlocking Signal Control Circuits June 1991 FIGURE 25. Track Indication Relay Circuit Synthesis A: Direct Synthesis from Figure 16 5 1TR ------+---(N)----+ +-----------+----- | _V___o | | | | 5 A5TR A3TR | +---(R)-+--+ +--+ +----+ | _V___o _V___o | | | | 3 | +----(N)-----------+ B: Dual Circuit 1TR A3TR A5TR ----+--+ +--+---+ +----+ +---+----- | _V___o | _V___o _V___o | | | | | 5 | 5 | +---(R)--++---(N)--------------+ | | | 3 | +----(N)--------------+ Figure 26 shows complete and typical Track Indicating Relay circuits for the home signals in the Figure 9 example. "Threaten the safety" may include the possibility of a collision with another train that has overrun a Stop signal, and this may lead to the inclusion of track circuits that are not obvious. For example, refer to the track layout of Figure 9. Suppose that the route past home signal 2 over crossover 5 reversed and 3 normal is a restricted-speed route; in that case, one might not expect track circuit A1T to enter into the circuit for Track Indicating Relay 2A. However, should a train overrun home signal 12, it could collide with a train passing signal 2, unless crossover 5 is normal. To help prevent such a collision, track relay A1TR is included in the 2A relay circuit in Figure 25. Similarly, the circuit for relay 12A includes the 1TR contact when crossover 1 is reversed, because overrunning signal 8 in that case could interfere with a train passing signal 12. Bob Braden [Page 30] Interlocking Signal Control Circuits June 1991 FIGURE 26. Track Indicating Relay Circuits 5TR 1TR A1TR A5TR A3TR 2A B>--+ +-+-+ +-+-+ +--+ +--+ +--+-+--+ +--+---+ +--> C _V___o | _V___o | _V___o _V___o _V___o | | _V___o | _|__| | | 5 | | | | | | +-------------(N)------+ | | |___| 2A | 3 5 | | 2 | +-------(N)---(R)--------------+ +---(N)---+ A1TR 1TR A5TR A3TR 12A B>--+ +-+-+ +-+--------+ +--+ +--+-+--+ +--+---+ +--> C _V___o | _V___o | _V___o _V___o | | _V___o | _|__| | 1 | | | | | | +--(N)--+ | | | |___| 12A | 5 | | 12 | +-----------(R)----------------+ +---(N)---+ A3TR 1TR A1TR A5TR 14A B>--+ +-+-+ +-+--------+ +--+ +--+-+--+ +--+---+ +--> C _V___o | _V___o | _V___o _V___o | | _V___o | _|__| | 1 | | | | | | +--(N)--+ | | | |___| 14A | 5 | | 14 | +-----------(R)----------------+ +---(N)---+ Considering signal overrun conflicts may also change the conditions under which already-included track relay contacts are considered. For example, in the circuit for relay 2A, track circuit 1 would seem to be relevant only if crossover 5 is normal. However, a train overrunning signal 8 may threaten a train passing signal 2 even if crossover 5 is reversed. Hence, 1TR is included unconditionally in the circuit for relay 2A, and similarly A3TR is included unconditionally in the circuit for relay 12A. 5. PROCEED (90 degree) CONTROLS The H relay clears a signal from its most restrictive aspect (normally Stop); a further selection must be made among Proceed, Approach, and perhaps Approach Medium, depending upon the aspect displayed by the next signal in advance. A typical interlocking plant is surrounded by automatic block signals Bob Braden [Page 31] Interlocking Signal Control Circuits June 1991 (ABS's), so the signal in advance that determines the clear aspect of an interlocking signal is often an ABS. The circuit arrangement used for the 90 degree controls in the interlocking will be consistent with the ABS signalling practice. <> To give examples of the 90 degree controls, we need to expand our track layout example to include the surrounding ABS's; see Figure 27. FIGURE 27. Example Track Layout, with ABS's 110 2 120 O-*--| O-O-O--| O-*--| ___1_1_0_T___ ________8_T___ ________ __________ ______1_2_0_T___ ____ \ / 8 |-O \ / \ / O-|10 ___1_0_2_T___ _______1_1_2_T__ ______\__ __/______ ______1_0_T____ ____ /3 112|--*-O 12|--O-O-O / 122|--*-O / O-*--|114 / ___1_1_4_T___ ____1_4_T___ _______/ 14|--O-O-O We describe two techniques used for controlling the Proceed aspect. 5.1 Polarized HR One practice we have seen [Reading Railroad] is to use a polarized H relay, with the two energized conditions corresponding to Approach and Proceed aspects. Figure 28 shows the neutral and polar contacts of the H relay controlling a color-light signal. Bob Braden [Page 32] Interlocking Signal Control Circuits June 1991 FIGURE 28: Polarized H Relay for Aspect Selection of Color-Light Signal +-----------------+ | | | AH o . . AH | / . . | +-->/ <---------[G]----+ V | . . | + >---------o_____ +------------------[Y]----+ |^ . . | +------------------------------[R]----+----> - . . . . We then modify the general scheme shown in Figure 6A to feed current of the appropriate polarization into the SS Protection Network to energize the H relays of opposing signals. For example, assume the layout of Figure 27. To energize polarized H relays for Eastbound signals 12 and 14, we insert polarized current at the point where the opposing dwarf signal 10 is attached. The resulting circuit for the dwarf signal 10 is shown in Figure 29. FIGURE 29: Dwarf Signal Control Circuit for polarized H Relays. 14 +--+-- C V____o _|__| | | 10HR |___| The circuit breaker shown at the upper right of Figure 29 is on signal 122, the first ABS signal in advance of home signals 12 and Bob Braden [Page 33] Interlocking Signal Control Circuits June 1991 14. Assume that this signal is a semaphore. The circuit breaker selects N battery if the semaphore arm is at 45 degrees (Approach) or 90 degrees (Proceed), and B battery otherwise (Stop). If the signal uses lights rather than mechanical movement, this polarity selection will be performed by contacts on the H relay for the ABS signal or on the track relay for the block governed by the ABS signal (122T). This polarized current is broken over front contact(s) of track circuit relays(s) between the interlocking plant and the signal (e.g., 10TR), so that no current will flow if the block is occupied. Finally, this polarized current is fed into the SS Protection Network if signal lever 10 is at Normal and if either 12 or 14 is Reversed. If both lever 12 and lever 14 are Normal, then B battery is fed in; this will be used to energize the opposing H relay for dwarf signal 8. <> 5.2 J Relay Circuit -- 3 Aspect Signalling Figure 30 shows the generic form of a circuit to energize the J relays for signal number n. Energizing the nAJ (nBJ) relay provides the Proceed aspect on the high-speed signal head (medium-speed) signal head, respectively. Figure 30 would be a fragment of the circuit for the J relays of all signals in the same direction. B battery enters this circuit at the left of the figure, from circuit breakers (or the equivalent relay contacts) on ABS signals in advance of the interlocking; two such signals are shown, labeled X and Y. The circuit breakers are closed if the signal is at Approach or Proceed; this function could also be performed by a front contact on the ABS track relay. The high signal head is normally cleared for only a single route through the interlocking, so the choice of which advance ABS should control the AJ relay is unique. As shown in Figure 30, if that ABS is not at Stop, the AJ relay is energized through a front contact of the AH relay, which must be energized before AJ can be energized. There may be multiple medium-speed routes from a given home signal, so the "Medium-Speed Route Network" selects the advance Bob Braden [Page 34] Interlocking Signal Control Circuits June 1991 ABS signal appropriate to the current route. This network may be constructed of switch lever contacts or of SS relays. If the selected ABS is not at Stop, battery flows through the Medium- Speed Route Network, through a front contact on the signal's BH relay (which must therefore be energized), through a back contact of the AJ relay (to make sure there is not a conflict with the high signal), and into the BJ relay coil. FIGURE 30: General Schematic for J Relays nAH +--------------------+ +----------+ +----> C | _V___o |___| | | | nAJ X ABS | |___| B>--+--+ | . . . . . . . nAJ 45o o90 | . . ____o---+ +---->C |^ | . . nBH |^ |___| 0o o---//--+----. Medium- .---+ +--+ | | nBJ . . _V___o |___| . Speed . . . Y ABS . Route . B>--+--+ (mAJ) . . 45o o90 ^ . Network .-----> (mBJ) |^ | . . 0o o---//---+---. . . . . . . . . The actual circuit has an additional feature, shown in Figure 31. The J relays, like the H relays, may be located at trackside some distance from the tower. Therefore, the J relay control circuit needs the same protection against accidental shorts in the cable bus that was described in Section 2[d] for the H relays. That is, both leads of a J relay coil need to be controlled by the same contacts, as shown in Figure 31. Instead of returning to C battery, the current from the J relay passes through another front contact on the controlling H relay, through an inverse MSRN network, and back to the advance ABS signal indication control. To avoid two lines from the interlocking to the advance ABS, we introduce an ABS repeater relay labelled yJW. Bob Braden [Page 35] Interlocking Signal Control Circuits June 1991 FIGURE 31: Two-Tailed Control of J Relays y ABS B>--+--+ 45o o90 |^ 0o o----------//-----------------+ +----->C _|__| | | yJW |___| nAH nAH +-----------+ +--------+ +-----+ +------------+ | _V___o |___| _V___o | | | |nAJ | | |___| | | . . . . nAJ . . . . | | . . ____o--+ +-----+ . . | yJW | . . nBH |^ |___| |nBH . . | B>-+ +--//-+--. MSRN.--+ +-+ | |nBJ | +---.Inv- .--+ _V___o . . _V___o |___| _V___o .erse . | . . .MSRN . | yJW . . . . | B>-+ +--//-+-------------------------------------------------+ _V___o For simplicity in the rest of this section, we will not show the double-tail circuitry; however, it is present in the real (PRR) circuits. The Medium-Speed Route Network (MSRN) is necessary only in a complex track layout. The example of Figure 27 is too simple to require an MSN; there is a unique advance ABS signal for each of the high signals 2, 12, and 14, as shown in the following table. ABS Signal: Controls: 110 2AJ 114 2BJ (medium speed route) 122 12AJ 122 14BJ (medium speed route) Another point to observe about the MSRN is that it does not in general map the track layout, since the medium-speed routes are a subset of possbible routes. It only selects connectivity between the advance ABS indication lines and the appropriate BJ relays. Bob Braden [Page 36] Interlocking Signal Control Circuits June 1991 Figure 32 shows a more complex track layout and two possible MSRN's, corresponding to different choices on medium-speed routes. Figure 32A shows the track layout, with two home signals "m" and "n", and three advance ABS signals "x", "y", and "z". Figure 32B and 32C show possible circuits for the home signal J relays, for two slightly different assumptions on the medium-speed routes; the MSRN's are shown within the dotted boxes. The circuit breakers for the advance ABS signals are now shown explicitly, but are understood to feed into the left side of Figures 32B and 32C. Both 32B and 32C assume that signal "m" has a medium-speed route across 7 reversed and 9 and 11 normal, to signal "y", and that "n" has two medium speed routes, one across 11 reversed to "x" and the other across 9 reversed to signal "z". Figure 32B handles the case of signal "m" having a second medium speed route, across both 7 and 9 reversed to ABS signal "z". Note that the MSRN in Figure 32B is a replica of the track plan. <> However, the possible medium- speed routes may be a subset of those allowed by the MSRN. For example, consider the (unlikely) route from home signal "m" over crossovers 7 and 11 both reversed, and 9 normal. The MSRN in Figure 32B would seem to allow relay mBJ to be energized from advance signal "x". However, the aspect selection circuit for home signal "m" will probably not allow this as a medium-speed route, in which case relay mBHA will not be energized, preventing mBJ from being energized. As another example, the aspect selection circuitry may disallow the route across 7 reversed and 9 reversed as a medium-speed route. In this case, Figure 32B could still be used; however, it would also be possible to simplify the circuit in this case to that shown in Figure 32C. Bob Braden [Page 37] Interlocking Signal Control Circuits June 1991 FIGURE 32: More Complex Example of Medium-Speed Routes A. Track Plan O-o--| x O-O-O-| m _________________________________________________ 11\ /7 O-o--| y \ / O-O-O-| n ____________________\_1_1______7/__________________ /9 O-o--| z / O-| ______________________9_/________________________ B. Assuming 7 (R) -> 9 (R) is Medium Speed Route for Signal m nAH +-----------------------------------+ +---------+ +----> C | _V___o _|__| | | | mAJ | |___| | . . . . . . . . . . . . . . . mAJ | . . ____o---+ +---->C | . . mBH |^ _|__| x //--+--.--+ +--------+ +-+ | | mBJ . | MSRN | . _V___o |___| . | | . . | | . nAH +-----|--------------------|--------+ +---------+ +----> | . | | . _V___o _|__| | . | | . | | nAJ | . | 11 7 | . |___| | . +-(R)-+ +--(R)-+ . nAJ | . | | . ____o--+ +---->C | . 11 | 9 | 7 . nBH |^ _|__| y //--+-------(N)-+-(N)-+-+--(N)----------+ +--+ | | nBJ . | . _V___o |___| . | . . | . . 9 | . z //---------------(R)--+ . . . . . . . . . . . . . . . . Bob Braden [Page 38] Interlocking Signal Control Circuits June 1991 C. Alternative when 7 (R) -> 9 (R) is not Medium Speed Route . . . . . . . . . . . . . . . mAJ mAJ . . ____o---+ +---->C | . 7 . mBH |^ _|__| x //--+--.--+ +------------(R)----------+ +-+ | | mBJ . | | . _V___o |___| . | | 11 . . +-------(R)-+ . nAJ nAJ . | | . ____o--+ +---->C | . | | 9 . nBH |^ _|__| y //--+---------+ +-(N)-+-----------+ +--+ | | nBJ . | . _V___o |___| . | . . | . . 9 | . z //----------------------(R)-+ . . . . . . . . . . . . . . . . 5.3 J Relay Circuit -- 4-Aspect Signalling Finally, we discuss four-aspect signalling, in whigh Approach Medium follows Approach. We describe the method that is used for position-light signals by the Pennsylvania RR. <>. With four-aspect signalling, the advance ABS controls which one of the three "clear" indications -- Approach, Approach Medium, or Proceed -- is to be displayed by a home signal. To economize on line wires between blocks, this three-state condition may be transmitted using a polarized voltage to energize a polarized relay, as illustrated in Figure 33. This figure assumes the layout of Figure 27. Note that ABS signal 110 is reached via a high-speed route using 4-aspect signalling, while ABS signal 114 is reached only over a medium-speed route with so 3-aspect signalling. Bob Braden [Page 39] Interlocking Signal Control Circuits June 1991 FIGURE 33: Control of Clear Indication from Advance Signal N>--+ V 110J 110TR+-----------//--------+ ____o-----o____V | ^| | B>--+ +-----------//------+ | +----+ +---->C V 110J 110TR| _|__| | _|__| ____o-----o____V | P |1JW | | |1JWSA ^| |___| | |_X_X_X| N>--+ | B>---+ | o1JW | / | +-->/ <-----+ | | +--------------+ 114TR +--------------------+ +---->C B>--o____V _|__| | |3JW |___| The following table shows the three states of 1JW: Auto Block Signal 110 1JW Home Signal Indication _______________________ _____ _________________________ Stop (110TR de-ener.) off Approach Approach right Approach Medium Appr.Med. or Approach left Proceed This three-state technique illustrated in Figure 33 is used not only between an interlocking and the first ABS signal, but also between adjacent ABS signals. In the latter case, the 1JW relay is replaced in the circuit by the J relay for the following ABS signal. Thus, the J relay for an ABS signal is polarized, with the two energized polarities corresponding to Proceed and Approach Medium. Figure 33 illustrates another feature of the use of polarized relays for three-state control. When the applied voltage makes a transition from + to -, causing the relay to poll from one side to the other, it must go through the de-energized state briefly. This transitory state could cause strange results, e.g., a spurious signal indication flashing briefly. To avoid this, the Bob Braden [Page 40] Interlocking Signal Control Circuits June 1991 neutral contact on the polar relay is not used; instead, the two polled positions energize a slow-acting ("SA") relay, whose contacts act as neutral contacts for the polar relay. <> Finally, Figure 34 shows the four-aspect J relay control circuit used on the PRR, except that the real J relay control circuit is double-tailed. Bob Braden [Page 41] Interlocking Signal Control Circuits June 1991 FIGURE 34: Four-Aspect J Relay Example from PRR Practice A. Signal Control Circuit ___ 2 2 2LA / :----(L)---------+-----+ +------+---(L)---+ +-->C / : | _|__| | _V___o / : 2LBJ V | |2LAH | / : +----o_____ |___| | /Aspect : | | ----+ Select : V 2LAH | \Network: ____o-----------+ +------+ \ : 2 |^ _|__| \ :--(L)-+ | |2LBHA \ : |___| \___: B. J Relay Control Circuit 1JW 2LAH +-----------------+----+ o-----+ +-----+ +--> C | | | / _V___o _|__| | | +->/ | | 2LAJ | | |___| | . . . . . . . +---------+ 2LAJ | . . | ____o---+ +-->C 1JWSA | . . V 2LAH ^| _|__| B>--+ +--+ . +----------+ ____o---+ | | 2LBJ o____V . | . V 2LBHA ^| |___| . | . ____o---+ . | . . | . . | . . | . 3JW . | . B>--+ +-----------+ . o____V . . . . . . . Looking first at the signal control circuit, Figure 34A, we see that if both the high-speed relay 2LAH and the medium-speed J relay 2LBJ are energized, then 2LBHA is also energized. Referring to Figure 22 and the following table, we see that the combination of AH, BH, and BJ will cause a position-light signal to display the Approach Medium indication. Referring now to Figure 34B, suppose that 2LAH is energized. o If 1JW and 1JWSA are energized with 1JW poled to the left, Bob Braden [Page 42] Interlocking Signal Control Circuits June 1991 then 2LAJ will be energized, displaying Proceed on the top signal. A back contact of 2LAJ will then prevent 2LBJ from being energized. o However, if 1JWSA is closed but 1JW is poled to the right, then 2LAJ will not be energized but 2LBJ will be. This will cause 2LBHA to be energized as explained in the previous paragraph, so Approach Medium will be displayed. o Finally, if 1JW and 1JWSA are deenergized, then neither the AJ nor the BJ relay will be energized, so the Approach aspect will be displayed on the high-speed signal head. Bob Braden [Page 43]