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According to EN ISO 13850, each machine (with the exception of hand-held and hand-operated machines) must have an emergency stop function which can be initiated by a single human action. This e-stop function must be initiated by an appropriate controlled stop device: By actuating an emergency stop switch or an emergency stop switch, the existing hazard is switched off via safety switchgear.
The emergency stop button is often incorrectly referred to as an emergency switching-off button. To understand the the difference read our blog "What's the difference between emergency stop and emergency switching off?"
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Which emergency switching off or emergency stop switchgear should be used depends on the results of the risk assessment and the intended application.
For simple drives, the supply-disconnecting device (main switch) can be used as an emergency stop switch under certain circumstances. However, switching off the energy supply must not cause hazardous situations (stop category 0 in accordance with EN ISO 13850). This can be achieved using a circuit breaker with a door coupling rotary handle, for example.
Actuating the emergency stop switch at Q1 switches off the energy supply to the entire system. The under-voltage release at Q3 prevents the dangerous drive from restarting. This enables you to attain a safety integrity level of SIL 1, or Performance Level "c."
The control circuit can also be used to implement an emergency stop circuit or an emergency shutdown for stopping machines and systems in the event of a hazard. This circuit is used for simple drives in which the motor contactor is engaged during operation, for example.
The safety contactor/contactor Q1 is de-energised by pressing the emergency stop button S1. Q1 switches off the energy supply (closed-circuit principle). This enables you to attain a safety integrity level of SIL 1, or Performance Level "c."
If several electrical circuits need to be interrupted, safety relays are used. These are special relays which comply with the EN 60947-5-1, EN 60204-1 and VDE 0113-1 standards.
With "normal" relays, when the load in a system is switched on and off again toward the end of the electrical service life, or in the event of a fault, the metal contacts can sometimes fuse together. In this situation, even if an emergency stop button or an emergency stop switchgear is actuated, the machine would continue to run.
Special relays are available in single or dual-channel types to configure safety-related applications. The internal logic of the safety relays monitors the safety circuits (emergency stop, protective door) and activates the enabling-current paths if no faults are detected. The enabling-current paths will be switched off if the safety device has been actuated or in the event of a fault.
For extensive control systems in which several electrical circuits must be interrupted, and where hazards to the operator or machine can arise, you can use a safety relay to achieve an emergency shutdown. For example, Eaton offers the safety-related easySafety relay for this purpose, which is a combination of a safety and control relay in one device. The required emergency stop circuit can be quickly implemented using pre-designed and verified safety function blocks.
Features:
A higher safety integrity level can be attained by expanding to a redundant emergency shutdown. The control circuit device, input wiring and command processing for this shutdown are redundant and self-monitoring. The following circuit uses the Eaton safety relay ESR5 and can be used if an immediate shutdown of the energy supply does not lead to hazardous situations.
Using this kind of circuit allows a safety integrity level of SIL 3, or Performance Level "e" to be attained.
There are many more ways to implement emergency stop circuits. For anyone wanting to study "machine and system safety" in more depth, the Eaton Safety Manual provides detailed information. Not only does it explain all relevant standards, it also offers you the opportunity to get to grips with the extensive subject of functional safety technology, one step at a time. Numerous example circuits show how functional safety can be implemented in safety applications by means of electrical, electronic and programmable components and systems.
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