What Is a Safety Limit Switch?
Safety Limit Switches are limit switches that have a direct open circuit operating mechanism and are used when reliability is required to ensure machine safety.
They are mainly used as sensors for interlocking devices in production equipment. Unlike ordinary limit switches, they do not lose their function as limit switches even if the contact points in the switch are welded due to a short circuit or high voltage application.
This makes it possible to realize safer interlock devices.
Applications of Safety Limit Switches
Safety Limit Switches are mainly used in the interlocking devices of machines that require a high level of reliability. Typical examples of interlocking devices are safety guards and doors that can be opened and closed. When the mechanism is closed, the switch is pressed to open the contacts, which are then verified by the control system to command the actuator to allow operation.
If the mechanism is open, for example during inspection, the switch is not depressed and the contacts are closed, so that the actuator will not start operation even if it is operated accidentally.
Principle of Safety Limit Switches
1. Direct Circuit Actuation Mechanism
Ordinary limit switches use springs to close and open the contacts when the switch is physically pressed, so that if the contacts are welded for some reason, the switch will not return and the hazardous condition cannot be communicated to the system.
Safety Limit Switches, on the other hand, use a direct opening mechanism that opens the contacts when the switch is depressed. This means that even if the contacts are welded, the welding can be released by the force of depressing the switch.
2. Fail-Safe Design
Even in the unlikely event that the welding is too strong to be released, the safety guard and door cannot be completely closed if the interlock device is designed with high accuracy. The device can be designed to be fail-safe such that it recognizes it as a dangerous condition and does not start operation.
This action of opening the contacts directly by an external actuator, such as opening or closing a door or rotating a cam, is called positive action. It is recommended that switches with this action be used as stand-alone interlocking devices.
Other Information on Safety Limit Switches
1. Mechanism to Prevent Contact Welding
A direct-circuit actuation mechanism is a switch mechanism that uses the force acting on the actuator to pull the contacts apart when the NC (normally closed) contacts of a safety switch are welded together. With direct circuit actuation, only NC contacts are available.
The contacts, also called NO (normally open) contacts, are represented by an arrow mark near the switch contacts in direct circuit actuation mechanisms. In contrast, safety relays use a technique called forced guiding.
This is a relay mechanism that prevents the NO and NC contacts from turning ON simultaneously when the safety relay contacts are welded together. The advantage is that by monitoring one contact, it is possible to diagnose whether the other contact is normal. However, the contacts cannot be pulled apart as in a direct circuit operating mechanism.
2. Interlock Device
Interlock is the concept of a safety device or safety mechanism to protect the safety of operators and users. It refers to a mechanism that prevents other operations unless certain conditions are met. It is applied to ensure worker safety in factories where robots, equipment, and various machines are in operation.
It is important to take measures to stop the equipment in an emergency if the worker approaches the equipment so closely that he/she unintentionally comes in contact with a part of the equipment. In addition, inspection work requires workers to be even closer to the equipment than normal work, so it must be performed after further ensuring the safety of the workers. Since equipment malfunctions can lead directly to accidents, it is necessary to ensure that interlocks are implemented as a preventive measure.