Emergency Stop and Emergency Switching Off Functions for Machine Tools

Emergency functions are an essential part of machine safety. They are designed to quickly stop hazardous movements and processes in an emergency without creating additional risks. In this article, we explain the key differences between Emergency Stop and Emergency Switching Off, outline the applicable standards and highlight what operators of machine tools should consider.

Requirements and Basic Principle

The purpose of an emergency function is to stop hazardous processes without creating a new hazardous situation. The required number of emergency devices and their installation locations should be determined as part of the risk assessment for the specific workplace.

It is important that emergency devices are readily accessible to both the machine operator and anyone nearby. At the same time, they must be positioned in a way that prevents deliberate misuse or accidental operation.

Sequence in an Emergency

1. Emergency Stop or Emergency Switching Off is activated – the machine stops.

2. The emergency function must be manually reset before restart is possible.

3. The start command is given – the machine resumes operation.

The Difference between Emergency Stop and Emergency Switching Off

Which Machines Require Which Function?
Whether a machine requires an Emergency Stop, Emergency Switching Off, or both depends on the results of the risk assessment.

• For mechanical hazards, an Emergency Stop is generally sufficient.

• Emergency Switching Off is typically used where protection against electrical hazards is required.

• Where both types of hazards are present, both safety functions must be provided.

The Following Applicable Standards Provide Guidance:

• EN ISO 13850 defines the requirements for the Emergency Stop function of machinery to ensure the safety of people and equipment.

• EN 60204-1 specifies the safety requirements for the electrical equipment of machines. It forms part of the European framework for machinery safety.

• Machinery Directive 2006/42/EC defines the essential health and safety requirements for machinery placed on the EU market.

Key Requirements

Emergency functions must be permanently available, easily identifiable (red/yellow color coding) and easy to actuate. When activated, they establish a safe condition that must not be cancelled automatically:

• The Emergency Stop function always has the highest priority and overrides all other operating modes.

• The command remains active until it is manually reset.

• After resetting, the machine must not restart automatically (restart prevention).

• A separate start command is required to restart the machine (e.g. via a key-operated switch).

This sequence prevents unintended restarting and ensures that the operator retains full control over the restart process.

Priority over All Other Functions

All other control commands are overridden as soon as the Emergency Stop is activated, regardless of the machine’s operating mode (e.g. automatic mode, manual mode or setup mode). No function may prevent or delay the Emergency Stop, including safety interlocks or software locks.

Immediate Action

The stop command is executed immediately without waiting for other processes to be completed.

Restart Prevention

Restart prevention is a mandatory safety principle in accordance with EN ISO 12100. This means that a machine must not restart automatically when the power supply is restored after a shutdown caused by a power failure, maintenance work or another interruption. DGUV Regulation 3 requires the restart prevention function of electrically operated machines to be tested.

Key-Operated Switch

A key-operated switch is a switching device that can only be operated using the correct key. It is used to enable or disable specific functions, for example restarting a machine after an Emergency Stop. It therefore acts as an access control measure, ensuring that only authorized personnel can return the machine to operation.

Comparison with a Standard Start Pushbutton

• Start push-button: Anyone can start the machine once the safety conditions have been met.

• Key-operated switch: The machine can only be started when the key is inserted and turned.

→ Typical in industrial workshops: Key-operated switches are often used for operating mode selection (automatic mode, manual mode) or for resetting an Emergency Stop.

The Motor Brake on Machine Tools

The motor brake is a safety-related brake mounted directly on the motor. It ensures that the drive shaft or spindle stops as soon as the motor is switched off or an Emergency Stop or Emergency Switching Off is activated. A motor brake reduces the overrun time of rotating tools (e.g. drills, milling cutters and saw blades) and prevents hazardous residual movement. It is typically designed as a spring-applied brake. This means that it is engaged by spring force when power is removed but requires electrical power to be released.

On older drilling machines, lathes and milling machines, retrofitting a motor brake is often the decisive factor in making the machine compliant with current safety requirements. The brake ensures that the tool comes to an immediate stop after an Emergency Stop instead of continuing to rotate for several seconds, which would present a significant accident risk.

Relationship to the Emergency Stop / Emergency Switching Off

The motor brake is controlled via the safety circuit, including the emergency functions. The sequence is as follows:

• The Emergency Stop is activated → the safety circuit opens.

• The motor is immediately disconnected from the power supply.

• The motor brake engages automatically by spring force, making it fail-safe.

• The shaft/spindle comes to a stop very quickly.

Important Safety Aspects

The motor brake itself is not an Emergency Stop device. Instead, it is activated by the safety circuit. It must act immediately – the machine must not be allowed to coast to a stop.

Relevant standards: EN ISO 13850 (Emergency Stop), EN 60204-1 (Electrical equipment of machines), EN ISO 16090-1 (Safety of machine tools) and EN ISO 13849 (Safety-related parts of control systems).

Motor brakes are considered safety-related components and must be functionally tested as part of inspections (e.g. DGUV Regulation 3 and periodic machine inspections).

Motor Brake vs. Spindle Brake

• Motor brake: Mounted on the motor and acts on the motor shaft or drive shaft. Typical for workshop machines.

• Spindle brake: Acts directly on the spindle or axis. Commonly used on vertical or CNC Z-axes. Of limited relevance for conventional workshop machines.

Safety Starts with the Right Assessment

A thorough risk assessment is always the essential prerequisite for compliance with applicable safety standards. During customer visits, we regularly encounter older machines where emergency functions are either completely absent or installed in locations that are difficult to reach. If your workshop also contains inadequately protected machines, please get in touch with us. We will help you retrofit your machinery to ensure compliance and improve workplace safety.

Important: The emergency functions presented here (Emergency Stop and Emergency Switching Off) provide an additional layer of protection and are not a substitute for other safety measures.

If you need support in assessing risks or selecting suitable safety devices (for example for spindles or chucks) or  Emergency Stop switches we will be happy to advise you. In many cases, a photograph sent by email is all we need for an initial assessment.

Author: Manuel Müller (Head of Sales, MOWOTAS GmbH)
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