In industrial equipment, for some single-operator equipment, two-handed operation buttons are often used as safety protection devices to prevent operators from touching dangerous moving parts when driving the machine, causing safety accidents

The use of two-handed buttons “chaos”

On some devices, the two-hand operation button is treated as just a pair of start buttons, that is, when the two buttons are triggered at the same time, it allows the machine to start, in which case the two-hand function obviously does not play a protective role.

What is more, some operators, in order to simplify the operation process, use tape or hard blocks to keep one of the buttons triggered for a long time during the use of the equipment, and artificially “turn” the two-handed button into a one-handed operation button, in this case, the effective protection function of the two-handed button is “in name only”.

Two-handed buttons for security applications

When the two-handed operation button is used as the safety protection device of the equipment, how can we safely and reliably reduce the risk of the equipment and achieve real safety?

ISO 13851 has detailed instructions, two-hand operation button can be divided into TYPE I, II, III three types, different types of two-hand operation button have different safety requirements, applicable to different scenarios.

When the two-handed operation button is used as a safety protection device and the safety performance level above EN ISO 13849-1 PLc is required, TYPE III two-handed control devices should be used and their functions must meet the following requirements:

Use both hands to trigger the button at the same time, and the time difference should be less than 0.5 seconds, if more than this time, you need to release and press the button again to start the machine
Both hands need to be continuously operated, and the operation will stop when one or all of the buttons are released
Ask to release and re-press the two-hand twist before restarting the machine

Table 1 List of types of the THCD (Two-hand control device) and minimum safety requirements

In order to achieve the above security features, users also need to consider the following additional requirements:

1. To prevent one hand from triggering two buttons, the distance between two hands should be greater than 260mm

Wrong application of two-handed button: The distance between the buttons is too close, and two buttons can be operated with one hand

Of course, using a protective cover on the button or changing the installation position of the button can also prevent a single hand from triggering the operation button.

2. In order to prevent the fingers and elbow of one hand from triggering the two presses, the distance between the hands should be greater than 550mm, and the distance between the buttons should be less than 600mm from the point of view of ergonomics

The wrong application of the two-hand button: the distance between the operation buttons reaches 950mm, the distance is too far, the operation is inconvenient, and it does not meet the ergonomics

3.When the button is released, the machine must be able to stop in time, that is, the two-handed operation of the button installation must also consider the requirements of the safety distance, on this part of the requirements can refer to the standard ISO 13855

4. In order to prevent the risk of failure of the two-handed operation button control system, the design of the two-handed operation button control loop must refer to the requirements of ISO 13849-1. The two-handed operation button must be combined with the corresponding safety relay or use the safety certified two-handed control function block in the safety controller to achieve the safety function and meet the requirements of different performance levels.

To achieve these safety requirements, combined with the actual situation of risk assessment, two-handed operation of the button can truly reduce the risk of the equipment. However, it should be noted that the two-handed operation of the button is only reliable for a single operator, and for the situation that there may be multiple operations, it must be combined with other devices to ensure the safety of personnel.

Safety is the top priority of modern industrial production, in practical applications, in addition to the requirements of relevant safety standards, but also need to adapt to local conditions, take effective protective measures to ensure the safety of personnel, to avoid accidents.