Is manual reset a safety function?

Last edit: 10/05/2023

Should the Reset button in a machinery be handled by a safety module, or can it be one of several inputs to the Automation PLC?

The question can be rewritten in a more correct language by asking if the Reset is a Safety Function.

CONSIDERATIONS: The answer is that, in several applications, the reset is a Safety Function and therefore it cannot be routed through the Automation PLC. Where is that written? It is stated in EN ISO 13849-1: 2015.
Chapter 5.2 lists some typical Safety Functions, the most important of which is the Safety-Related Stop (5.2.1).

The next example is about:

5.2.2 Manual reset Function

The following applies in addition to the requirements of table 8.

After a stop command has been initiated by a safeguard, the stop condition shall be maintained until safe conditions for restarting exists.

The re-establishment of the safety function by resetting of the safeguard cancels the stop command.

If indicated by the risk assessment, this cancellation of the stop command shall be confirmed by a manual separate and deiberate action (manual reset).

A situation whereby the risk assessment may indicate that the Reset is a Safety Function is when there is a safeguarded area with a risk of Whole Body Access. 
A safeguarded robot area is a typical case.

In order to reduce the risk, the reset button must meet a few conditions, among which:
–    From its position, outside the safeguarded area, there is full visibility that nobody is inside
–    It is implemented as a Safety Function

The risk of a person being trapped inside the area can be reduced with other methods: for example, with the use of personal keys the operator has to keep with himself.

The reset of the working area of a machine tool, since there is no risk of being trapped inside the safeguarded area, is not required. If the manufacturer decides to install it anyway, it can be handled as a normal control function, and therefore the signal can be routed to the Automation PLC. In this case it is not a safety function!

Since you now understand the Reset can be a safety function, you wonder what should be its safety level. The new edition of ISO 13849-1:2021 clarifies that aspect in a note:

NOTE 1: It is not always necessary that the manual reset function has the same PLr as the associated safety function.

When you build your control panel, always ask the colleague who did the risk assessment if the reset buttons have to be routed to a Safety PLC and never give for granted that it is not necessary

Finally, we point out that in IEC 62046 in paragraph 5.6 there is the following note:

Resetting a restart interlock of an ESPE application is always a safety-related function. Measures shall be provided to reduce the probability of the restart interlock being reset by a transient or steady-state fault condition. Such measures can in clude, for example, requiring both a rising and falling edge signal within a defined time (e.g. between 150 ms and 4 s) from a manually actuated reset device.

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