- Understanding safety functions: Manual Reset
- Manual reset using an HMI
Question: Can a safety-related stop function, for example, be reset via a graphical object representing a reset button on an HMI?
The short answer: No, with an exception. Read on if you’d like to know more.
If you’re interested in knowing more about the manual reset function, see our previous post on the manual reset function.
Why can’t manual reset actuators appear on HMI screens? There are a few reasons for this.
First, ISO 13849 – 1  requires that manual reset actuators be separate control devices connected to the SRP/CS. The point could be made that an HMI is a separate device, however, as of this writing, there are no HMIs that are designed to connect to a Safety PLC.
Second,  requires that components used for the manual reset function not reduce the Performance Level (PL) of the safety function. Since there are no safety-rated HMIs, the only structural category that could be assigned to an HMI-PLC combination is Category B, a single-channel architecture using components rated for the circuit conditions. This structural category limitation means that the highest PL that could be assigned would be PL=b. Emergency stop functions must provide at least PL=c performance according to ISO 13850, so an HMI-based reset cannot be used with emergency stop functions. In addition, most industrial machines will require at least PL=c, d or e for their safety-related interlocks, so an HMI-based manual reset cannot be used to reset an interlock stop function.
Finally, the large controls component manufacturers, like Rockwell Allen-Bradley, Omron, Pilz, Schmersal, Siemens, Telemecanique, etc., do not recommend the practice for the reasons discussed.
PS – I mentioned in the video that rising edge signals are not used for the Manual Reset Function – in truth, rising edges are not used for safety-related signals. Rising edges can occur more readily due to electrical faults while falling edges are much less likely. For example, a falling edge generated by an electromechanical push button requires that the button is pushed and released, which helps avoid intentional defeat through a “tie-down” of the button.
 Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design, ISO 13849 – 1. International Organization for Standardization (ISO), Geneva. 2015.
 Safety of machinery — Emergency stop function — Principles for design, ISO 13850. International Organization for Standardization (ISO), Geneva. 2015.
 Realizing Reset Function in Safety Related Parts of Control Systems, 1st ed. Hoofddorp, Netherlands: OMRON Europe B.V., 2015.