Interlock (engineering)

An interlock is a feature that makes the state of two mechanisms or functions mutually dependent. It may be used to prevent undesired states in a finite-state machine, and may consist of any electrical, or mechanical devices or systems. In most applications, an interlock is used to help prevent any damage to the machine or to the operator handling the machine. Elevators are equipped with an interlock that prevents the moving elevator from opening its doors, and prevents the stationary elevator (with open doors) from moving. Interlocks may include sophisticated elements such as curtains of infrared beams, photodetectors, simple switches, and locks. It can also be a computer containing an interlocking computer program with digital or analogue electronics. Trapped-key interlocking is a method of ensuring safety in industrial environments by forcing the operator through a predetermined sequence using a defined selection of keys, locks and switches. It is called trapped key as it works by releasing and trapping keys in a predetermined sequence. After the control or power has been isolated, a key is released that can be used to grant access to individual or multiple doors. Below is an example of what a trapped key interlock transfer block would look like. This is a part of a trapped key interlocking system. In order to obtain the keys in this system, a key must be inserted and turned (like the key at the bottom of the system of the picture). Once the key is turned, the operator may retrieve the remaining keys that will be used to open other doors. Once all keys are returned, then the operator will be allowed to take out the original key from the beginning. The key will not turn unless the remaining keys are put back in its place. Another example is an electric kiln. To prevent access to the inside of an electric kiln, a trapped key system may be used to interlock a disconnecting switch and the kiln door. While the switch is turned on, the key is held by the interlock attached to the disconnecting switch.