The signal control relay can pick and drive the trip to the clear position. A stick contact on the relay bypasses the check contact. Thus, the train stop must return to the tripping position before it can re-energize. In other words, it must cycle between the tripping and clear positions. Here, the cycle check is performed each time the signal clears, or it may be said that the cycle check is based on operation. Depending on the device, cycle checking can also be based on time, where the check might be daily, hourly, every second, or even more frequently. This approach is very common in electronic devices. Other methods may be required, particularly in electronic equipment, but are beyond the scope of this brief discussion.
Another example of a design consideration is the provision of slow-drop features on vital relays. It may be necessary to provide delays in circuit operation, and this may be accomplished in several ways. Two examples are a slow-drop relay, or a capacitor "snub" (frequently, a slow-drop relay is said to be snubbed or slugged). If a part of the coil of a relay is replaced by large copper washers or a solid cylinder of copper, the currents induced in the copper when the coil is de-energized will keep the armature in the energized position for a time (as much as a few seconds). Another way to make a relay slow drop is to add a capacitor in parallel with the coil. A resistor is usually placed in series with the capacitor, to limit the inrush when the circuit is energized, and to limit the current if the capacitor should short. It is not hard to obtain drop-away times from a few seconds to as much as several minutes by selecting the value of the capacitors. The designer must consider what happens if the connection to the capacitor should open, or if the capacitor should fail. If the time delay is critical to the safety of the operation, it probably would be better to use a relay with a slow-drop copper slug, because there is much less possibility of that device failing than of a capacitor circuit opening.
The design itself is documented in the drawings prepared for the signal system. The drawings are an important part of the safety of the signal system, because they not only describe the design but also show details that define some aspects of the construction. The term drawings in this discussion includes the software listings for microprocessor-based systems. Once the drawings are complete, the design must be checked to be sure that it complies with both the fundamental safety requirements and with the design safety requirements. A difficult part of the checking process is assuring that nothing was left out—it is relatively easy to see when a design is incorrect but harder to identify a missing design. Checking must include checks of the details, to identify termination points, for example.
Construction Safety
Once the system has been designed and checked, it has to be built. The safety of the system is further addressed in this step. The techniques used to wire equipment is different from ordinary electrical wiring, in an effort to provide a safe (and reliable) system. Wire insulation is more rugged than typical electrical installations. Ring terminals are commonly used so that wires do not accidentally become disconnected. Terminal posts are large and have large spacing, to minimize the possibility of accidental contact between wires. These and other construction techniques minimize the possibility of unintentional connections between wires.
Once the wiring is complete, the safety of the system is again verified by several layers of checking. At this point, the checking is performed by testing the system. Multiple testing steps are used, in sequence, to assure that the wiring complies with the design, and that the system operates as intended. The details of the steps may vary depending on the company standards, on the equipment involved, and on the issue.