Relay Nomenclature
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| Description (Prefix) | Apparatus (Last letter) | ||
|---|---|---|---|
| A | Approach; Automatic; Relay Contact - Arm | A | Axle Counter |
| B | Block; Bolt; Relay Contact - Back | B | Block Instrument |
| C | Checking or Proving; Coding | C | Contact |
| D | Clear (green) [distant]; Decoding; Relay Contact - De-energised | D | - |
| E | Light; Lamp; Heat (externally applied); Emergency; Earth | E | Light; Lamp; Earth |
| F | Fog; Flashing; Feed; Relay Contact - Front | F | Detonator Placers |
| f | Frequency | f | Fuse |
| G | Signal | G | Signal apparatus |
| g | - | g | Lightning Arrestor |
| H | Caution (yellow) [Home] | H | Capacitor |
| HH | Preliminary Caution (double yellow) | - | |
| I | AWS | I | Inductor |
| J | Time (delaying action) | J | Rectifier; Diode |
| K | Indicating or detecting | K | Indicator electro-mechanical |
| L | Locking; Left | L | Lock |
| M | Marker; Magnetic | M | Motor |
| N | Normal | N | Release; Hand-operated switch; Push-button or key |
| O | Retarding | O | Resistor; Heater |
| P | Repeating | P | Lever Latch or Trigger contact |
| Q | Treadle or Bar | Q | Local coil of a double-element relay; Treadle; Bar |
| R | Reverse; Right; Danger (red) | R | Relay or contactor |
| Rx | Receiving | Rx | Receiver |
| S | Stick | S | - |
| T | Track Circuit | T | Transformer |
| Tx | Transmitting | Tx | Transmitter |
| U | Route [roUte] | U | Unit |
| V | Trainstop | V | Trainstop apparatus |
| W | Points [sWitch] | W | Points operating apparatus |
| X | Audible annunciator; Level crossing; Wrong direction | X | Audible annunciator (bell, buzzer, horn, etc) |
| Y | Slotting or disengaging | Y | Slotting or disengaging apparatus |
| Z | Special (to be explained on plan) | Z | Special unit (to be explained on plan) |
Source: http://static.scbist.com/scb/uploaded/331_1354714587.pdf