When you look at a machine you will see a big red button with a mushroom style. This red button with a yellow label will be near the operator console. Do you know what is it? It is an emergency stop button! This safety device has lots of application areas and can be used in machines, electrical cabinets, and robots in the industry. It is available in many shapes and sizes based on its functionality and application. Both IEC and NEMA standards dictate some of the requirements for it. Before describing the operating principle, let’s look at its definition:
An emergency stop button is a fail-safe control switch. As soon as it is actuated, it will stop the operation of all equipment. The function of an emergency stop button is to open a circuit to prevent hazards. It can be operated manually and must remain in its open position until the normal operation can be restored.
E-stop is not only important to the safety of the machinery and people in an industrial environment, but it is also quite often required to be present, usually within the line of sight of workers.
How does an emergency stop button work?
When an emergency occurs, an operator will press the actuator to begin the e-stop sequence. The actuator will mechanically actuate a normally closed contact, which will open the circuit. As a result, power supplied to the equipment will be disconnected, ensuring the equipment stays off until intentionally reactivated.
Resetting the emergency stop can only be possible as the result of a manual action at the location where the emergency stop was initiated. The resetting of the device can be by turning a key, rotating the button in the designated direction, or by a pulling motion.
There are three main components of an E-Stop: Actuator, contact block, and holder.
The actuator is the interface to the human operator. Once pressed, it will mechanically change the state of the contact block. Actuators of emergency stop devices shall be colored RED. The direction of unlatching shall be clearly identified when resetting is achieved by rotation of the button.
Wiring terminals are located in the contact block. Contact blocks have two contact configurations: Normally open (NO) and normally closed (NC). The different configurations refer to the initial position of the contact before it is actuated. Commonly NC contacts are used in e-stop applications. E-Stops typically offer a range of contact options including gold over silver or silver over palladium and silver-plated screw
terminals with available quick-connect terminals.
Contact block holder
The holder is the part that connects the actuator to the contact blocks. It is like a bridge between the actuator and blocks.
To activate and reset an emergency button, you need to use an actuator. There are three types of actuators available:
The operator is pushed in and locks into stop; released by pulling back. Indicated by the circle symbol on the head.
The operator is pushed in and locks into the stop, released by twisting. Indicated by the circular arrows on the head.
The operator is pushed in and locked into position to stop; released only with a key.
Most standards and specifications require that an emergency stop should have a red actuator and should be located on a yellow background. These colors indicate that the device is an emergency control.
Most suppliers of pushbuttons accomplish the yellow background in one of three ways.
- Use of a yellow enclosure.
- Use of a large Emergency Stop legend that is yellow.
- Coloring the “stem” of the push button yellow.
Emergency stop buttons should be accessible. So, you can’t put a cover over it. But you can prevent accidental operation by using a yellow protective shroud. A protective shroud is a collar that surrounds an emergency button to prevent inadvertent activation.
The difference between a stop push button and an emergency stop button
The stop button is usually located right next to the start button, and the two are the same size. If you press the stop button the circuit will stop and it can be restarted by pressing the start button. When the emergency stop button is pressed, no device will operate until the operator pulls the button back.
The emergency stop button is larger than normal buttons and quite distinct. It has a mushroom-shaped button head. It is usually located where the operator can easily see and push.
Emergency stop buttons are only required to be tested to approx. 6,050 operations. This implies that emergency stop buttons are not meant for everyday use. But a normal pushbutton has a longer mechanical lifetime compared to an emergency button.