What You Should Know About Emergency Stop Push Buttons

An emergency stop is not a safeguarding method in itself and it does not replace other protective measures. It is a last-resort control, used only in an emergency when other safety measures are no longer sufficient. It is not intended for normal machine stopping, process control, or maintenance shutdowns.

According to ISO 13850, an emergency stop push button must be red and mounted on a yellow background. It must be easily accessible from all relevant operating positions and must not be obstructed. The button must latch mechanically in the activated position and require a manual reset before a restart is permitted. In addition, the emergency stop function must stop hazardous movement as quickly as possible without creating additional risks.

There are several common misconceptions about emergency stop functions. One of the most frequent is the idea that a single emergency stop push button is sufficient for an entire production line. In reality, the button must be reachable from every workstation, and the number and placement of devices are always determined on the basis of a risk assessment. Another misconception is that an emergency stop is simply a push button that can be wired into any control circuit. In fact, the emergency stop is part of the machine’s safety-related control system and must be connected via a safety relay or safety controller so that redundancy and fault monitoring are achieved in accordance with the relevant requirements.

It is also often assumed that once the machine stops, the situation is automatically safe. This is not always the case. Stored energy, such as hydraulics, pneumatics, or the kinetic energy of heavy moving parts, may still present a hazard even after the machine has been stopped. For this reason, an emergency stop alone does not guarantee a safe working condition.

Using an emergency stop push button for routine machine stopping is an incorrect and unsafe practice. Emergency stops are intended only for emergency situations. Frequent use wears the mechanism and can reduce operators’ sensitivity and reaction to genuine emergencies. The emergency stop is also not suitable for ensuring safety during service or maintenance tasks. It will stop the machine, but it does not isolate the power supply. A safe maintenance condition requires the power to the machine to be physically isolated using a lockable main switch or equivalent device, so that the machine cannot start or move unintentionally.

The method used to implement the emergency stop function depends on the size and complexity of the machine. In simple machines, a safety relay provides a reliable way of monitoring the emergency stop function and performing basic diagnostics. In larger and more complex systems, a safety controller or safety PLC allows coordinated management of multiple emergency stop devices, safety zones, and restart logic.

The emergency stop function must always meet the safety level defined in the risk assessment. This safety level is expressed either as a Performance Level (PL) in accordance with ISO 13849-1 or as a Safety Integrity Level (SIL) in accordance with IEC 62061. The required level is determined by the severity of possible injury, the frequency and duration of exposure, and the possibility of avoiding the hazard, and it cannot be chosen arbitrarily. In addition, ISO 13850 specifies that the emergency stop function must achieve at least Performance Level c.

To achieve the required safety level, the emergency stop circuit must include two independent channels so that a single fault cannot lead to the loss of the safety function, as well as continuous fault monitoring such as contact monitoring and short-circuit detection. The system must use certified components, such as safety-rated emergency stop push buttons, safety relays, and safety controllers. Regular testing and validation are also essential to ensure that the system performs as intended throughout its service life.

Simply interrupting the power circuit with an emergency stop push button is not sufficient. The entire emergency stop function must be designed, validated, and documented as part of the overall functional safety of the machine.

Summary

The emergency stop push button is the last line of defence in a production environment, not the first. When implemented correctly, tested regularly, and used only in emergency situations, it protects both personnel and equipment. At the same time, it is important to remember that the emergency stop is only one element in a complete and comprehensive safety system.