4.5: Prevention of unintended actuation


Last edit: 25/07/2023

Prevention of unintended actuation

4.5 The emergency stop device shall be designed to avoid unintended actuation. 

According to the standard, so far as practicable, unintended actuation shall be prevented by location rather than the use of other application design measures (e.g. protective shroud). It is crucial that the accessibility of the stop device is not compromised.

The following are some examples on the possible location of emergency stop devices in order to prevent unintended actuation:

  • locate the emergency stop device away from foreseeable heavily areas;
  • select the type of emergency stop device (choose button, cable, rope, bar, or pedal taking into account the aspect of preventing unintended actuation);
  • Select appropriate size or shape of the emergency stop device;
  • mount the emergency stop device within a recessed surface in the surrounding control panel.

Use of protective shroud

Regarding the use of this device for the prevention of unintended actuation: 

4.5 […] The use of a protective shroud around the emergency stop device should be avoided, except when necessary to prevent unintended actuation and other measures are not practicable. […]

The shroud around the pushbuttons is possible and necessary if the activation of the emergency can cause significant disturbances to the process. Consider, for example, the accidental actuation of the emergency during the operation of an Arc Furnance. The standard does not prohibit the use of these devices but should not abuse the shroud: for example, on a column drill, the use of a shroud is questionable.

It also describes the characteristics that a protective shroud must comply with (paragraph 4.5).

Incorrect use of protective shroud – Example 1

With regard to the use of the protective shroud for the prevention of unintended actuation of an emergency stop device, the standard indicates that:

For emergency stop devices intended to be actuated by hand the measures against unintended actuation shall not impede or hinder actuation with the palm of  the hand, from any foreseeable position of the machine operator and others who could need to actuate them.

In the example, the presence of protection makes it impossible to press the emergency button with the palm of the hand.

Incorrect use of protective shroud – Example 2

Again it is not possible to activate the emergency button with the palm of the hand. Therefore, even this realization does not comply with the standard