Emergency stop switches, frequently also referred to as Emergency Stop pushbuttons, are essential in averting risks within an industrial setting. However, there are many regulations to be followed to ensure their proper use. But which of these guidelines are particularly important? And which criteria must be strictly adhered to when selecting a suitable emergency stop switch? You will find the answers here.
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Emergency stop installations are an integral part of functional safety.
In accordance with the Supply of Machinery (Safety) Regulations 2008, no machine may pose a risk to people or the environment. To ensure compliance in this regard, almost every machine must be equipped with at least one emergency stop control device. In this way, the risks of unforeseen hazard situations can be minimised. Adherence to the Machinery Directive is a prerequisite the granting of a UKCA or CE marking. Without the fulfilment of this criterion, a machine can neither be sold nor operated within the UK.
Emergency stop systems are to be considered as supplementary protective measures against unforeseen hazards. In no way do they replace the need for a risk assessment. If work processes require it, the additional use of safety technology may therefore be necessary.
Emergency stop and emergency shutdown systems are often confused. However, there is one significant difference.
Emergency stop control devices protect against hazardous movements, without switching the system off altogether.
Emergency shutdown systems, on the other hand, refer primarily to emergency OFF main switches. These completely disconnect the machines from the power supply, in order to protect against dangerous voltages.
The two systems cannot function simultaneously, as the emergency stop function requires a power supply in order to operate, by means of a control signal
A brief summary: In the event of an emergency, an emergency stop system triggers a shutdown of the machine, whereas an emergency shutdown system switches off all systems entirely.
Depending on the risk assessment, and in production operations in particular, protection against hazardous movements is prioritised. As such, this is primarily where emergency stop switches are used.
Fig. 1: Difference between emergency shutdown and emergency stop switches
To ensure the proper use of emergency stop control installations, a number of regulations must be considered. The most important directives are discussed below.
The BS EN ISO 13850:2015 standard defines how emergency stop switches should be implemented. As such, they standard must be displayed at every control stand, as a minimum requirement. They must be mounted in a clearly visible and a quickly and safely accessible location.
As soon as an emergency stop switch is activated, the triggered function takes precedence over all other processes. This means that the system shutdown must take place immediately. The activation of the emergency stop device must result in the foolproof locking of the system. To this end, the BS EN 60947-5-5 standard prescribes that this process must be accompanied by an emergency stop signal.
If all processes are to be started up again, the system must be unlocked at the actuation location. However, this alone should not result in an automatic system restart. The processes must also be manually re-started.
Fig. 2: Emergency stop switches must be clearly visible
In addition, in accordance with the BS EN 60204 standard, the clear visibility of the emergency stop control unit must be guaranteed. To this end, the emergency stop switch must have a red button on a yellow background. The same standard prescribes that emergency stop pushbuttons must have positivelyopening contacts, whereby reliable contact separation is guaranteed, with a view to ensuring machine safety. A wire breakage poses no risk to achieving system shutdown, as this will not result in contact being established.
BS EN ISO 13849-1 also stipulates that emergency stop systems must be implemented in accordance with Performance Level c (PLc) as a minimum requirement.
Therefore, particular caution is required in the case of the series connection of emergency stop installations. Although the switches can, theoretically, be connected in series, in case of simultaneously activation this will result in a risk of possible error masking, which can in turn reduce the performance level. For this reason, series connection is not recommended.
However, if emergency stop systems are connected in series, adherence to the required performance level is absolutely essential. This applies, in particular, in case of the additional use of safety sensors, as simultaneous activation is very probable in this case. For a more precise implementation, the ISO/TR24119 standard must be considered here.
In order to ensure the compliant implementation of the emergency stop switches and to avoid cross-connections, emergency stop systems should undergo regular inspections with regard to their functionality. Here, the connection of reliable evaluation units such as safety relays or safety controllers is required, which generate and evaluate test pulses.
Furthermore, the regular physical testing of the triggering of the emergency stop system is recommended. This test process is not stipulated in any standard, but is definitely recommended.
Fig. 3 - Connection example for short-circuit detection using test pulses
Emergency stop switches that are suitable for your application must first fulfil the aforementioned basic requirements. Furthermore, you can draw on up to four selection criteria when making a decision regarding an emergency stop pushbutton.
1) Number of channels
You can choose between 1 or 2-channel versions, whereby only the NC contacts count. The NO contacts only perform a reporting function with emergency stop switches.
A hazard and risk assessment is an essential part when it comes to making a correct choice.
Emergency stop switches must at least fulfil the requirements of Performance Level c (PLc). This can be achieved with 1-channel versions. From Performance Level d (PLd), 2-channel versions are recommended.
While some emergency stop pushbuttons can be purchased as complete units, including the contacts, with other versions the contacts must be ordered separately based on the number of channels.
2) Type of reset
It is important that the switches can be activated easily and without incident. This also applies for resetting. Otherwise, depending on your personal preference, you can choose between switches with a pull, turn, or key-based unlocking. With the latter option, you must make sure that the key can always be removed, to enable safe actuation.
3) Protective collars
The use of protective collars can make the activation of emergency stop switches difficult in case of an emergency and should, if possible, be avoided. However, they are essential if unintentional actuation is to be avoided and if the emergency stop control devices cannot be positioned in an alternative sensible location.
4) Special features
Emergency stop switches with an illuminated ring ensure improved visibility. As an option, these can also display the respective switching status.
Emergency stop installations are frequently connected with emergency stop pushbuttons in particular. However, there are many other versions, such as pull-rope emergency switches, which, as their name suggests, are triggered by pulling on a rope, and are particularly well-suited for use on long conveyor lines.
However, emergency stop switches can, for example, also be integrated into other control units, such as SIMATIC HMI Mobile Panels, enabling switches, and two-hand control panels. These models are characterised by their flexibility and also their proximity to the hands of the operator.
If the operator is unable to use his hands, foot switch versions represent a practical alternative. However, designs without protective caps should only be considered if no other solutions are possible. Otherwise, you are advised against their use.
What is the point in using emergency stop input devices if they are not properly evaluated?
There are three possible solutions to choose from when it comes to evaluation: safety relays, small safety controllers, or failsafe memory-programmable control units.
Safety relays are a possible option if you prefer to have fewer safety functions and a hard-wired installation. Networking is not possible when using safety relays.
Choose safety small controllers if you want a flexible and configurable solution with two to eight safety functions. Parameterisation requires no prior knowledge and can be easily graphically implemented by means of drag & drop. Networking may be possible as an option, depending on the system.
With failsafe safety controllers, you have everything covered: a wide range of safety functions, a high level of changeability, and networkability. However, to implement this solution you should have programming knowledge to realise the full potential of your installation.
Fig. 4: Selection criteria for emergency stop switch evaluation units
You are now aware of the most important selection criteria for emergency stop switches. In the Automation24 range you will find various high-quality models from Eaton and Siemens at an excellent price-performance ratio. Make your contribution to the safety of your application today and choose the right emergency stop switch from the range! We will also be happy to provide you with further expert advice.
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