Before I waffle on with some technical details, I need to explain that AWS can be installed in a number of configurations. For plain line on a unidirectional line, that's the simplest arrangement. Mounted in the four foot is a permanent magnet and immediately following it is an electro-magnet. For bidirectional lines or reversible lines the arrangements depend on where the signals are and on a number of other requirements. If there is no opposite direction signal nearby, then the arrangement is normally a permanent magnet with a suppression coil (normally called a "suppressor") immediately followed by an electro-magnet.
So, for code 2 faults, that's the train equipment being unable to detect the magnetic field from the AWS electro-magnet. It's the magnetic field from the permanent magnet with no further magnetic field being detected (from the electro-magnet) that causes the horn to sound in the cab.
There are lots of possible reasons why this may be the case.
Some of the most common are:
- Problem with the train equipment (not applicable if multiple trains report the same signal or AWS),
- Damaged cable to the electro-magnet in the four foot (fairly common fault),
- Corroded wire or terminal in the terminal box for the electro-magnet (the terminal boxes sometimes fill up with water if the deal is not perfect), or it being deliberately disconnected because it's got water in it),
- Defective electro-magnet (not a very common fault, unless affected by water, can last for twenty or more years),
- The disconnection link left disconnected (either by mistake or intentional for some reason),
- Defective main lineside cable,
- Blown fuse (sometimes caused by a defective diode),
- Fault in the control system, such as a faulty relay contact.
The vast majority of these problems are not difficult to fix. some only require as on d one to two hours of work.
Code 8 faults are typically a failure of the suppression coil of a permanent AWS magnet. If the AWS applies to an opposite direction signal, then the suppression coil should cancel out the magnetic field from the permanent magnet so that nothing is received in the cab. There are some other circumstances where suppressor are used, but I won't go into the details here.
The list of problems is similar to those I listed for code 2 faults. However, unlike electro-magnets, some makes/designs of suppressors are not very reliable. Some lasting less than three years.
Changing any of the magnets in the four foot is a bit of a ball ache, because they are heavy and awkward to handle. So the reduction of S&T teams from three people to just two does hinder the work.
But as others have said, the S&T being able to get access may delay fault investigation and therefore rectification. The testing required to fault find a code 8 fault may require a long length of the line to be clear of traffic for example.
And alas, managers don't consider these faults to be urgent or important, as they don't bring trains to a halt or significantly delay trains. Other work therefore may take priority.
It's important that drivers keep reporting any defective equipment or any other defects. If you are concerned that something is not being fixed within a reasonable time, bring it to the attention of your manager. Your manager can make enquiries. All defects and faults should be reported by the signaller to fault control, where they will be logged in a computer system. The signaller is then given a fault number. Your company can ask Network Rail for details of a reported fault.
