The machinery and equipment of the early industrial age were extremely dangerous. It is not uncommon for both workers and farmers to lose fingers, hands and feet, or even their lives due to being involved in rotating machinery. As a result, this has prompted the development of protection systems and other safety equipment.
Interlocking refers to the interdependence of the status of two or more equipment functions, and it is the core of today's safety system functions. The interlock can prevent the equipment from hurting the operator or damaging its components. For example, when the protective device is opened, the interlock can prevent the device from starting; if the protective device is opened during operation, the interlock can stop the device.
Many simple interlocking systems are purely mechanical. For example, in the design of some equipment, an interlocking cam is installed on the shaft of the guard. When the guard is opened, the cam is mated with the cam on the drive shaft of the equipment to prevent the shaft from rotating. This means that the equipment can only operate when the guard is closed.
Today's equipment mostly uses electronic safety circuits, and even uses microprocessor control to implement interlocking safety systems. Regardless of the flexibility of arranging protective devices or the complexity of safety procedures, electronic safety circuits have advantages over mechanical solutions.
A typical electronic safety circuit only allows the device to operate when the circuit is closed-this configuration is called normally closed (NC) operation. In addition, safety components are connected in series in the circuit to maximize efficiency, reduce complexity and reduce costs.
Imagine a typical safety device, when the corresponding part of the protective device is closed, the device's multiple limit switches remain normally closed. These limit switches are connected in series with the safety device, so if any part of the protection device is not properly closed, the entire circuit is disconnected and the equipment will not operate. In fact, the control components in the safety circuit also need to be connected in series to ensure that safety can still be guaranteed in the event of any poor connection or sudden interruption (such as cut off) of the safety components.
Regarding the series connection of safety circuits, there is one thing to note: if the circuit contains more than four safety switches or frequently opened and closed doors or switches, it will reduce the performance level of the design (PLr-will be explained in detail in the next article), And increase the risk of fault concealment. The latter refers to a state in which one switch is disconnected or a fault occurs, causing another switch to disconnect or the fault cannot be recognized. Fault masking is most likely to occur in devices that contain passive contacts, such as relays, where there is no power connection other than the switch connection. If this risk is unacceptable, more complicated wiring and methods may be required.
