“When you have multiple E-Stop buttons I have often gotten into an argument that says you can have a reset beside each one. I was taught that you were required to have a single point of reset. Who is correct?”
— Michael Barb, Sr. Electrical Engineer
The Short Answer
There is nothing in the EU, US or Canadian regulations that would forbid having multiple reset buttons. However, you must understand the overlapping requirements for emergency stop and prevention of unexpected start-up.
The Long Answer:
First I need to define two different types of reset for clarity:
- Emergency Stop Device Reset: Each e-stop device, i.e. button, pull cord, foot switch, etc., is required to latch in the activated state and must be individually reset. Resetting the e-stop device is NOT permitted to re-start the machinery, only to permit restarting. (NFPA 79, CSA Z432, ISO 14118).
- Restarting the machine is a separate deliberate action from resetting the emergency stop device(s).
ANSI B11-2008 provides some direct guidance on this topic:
A machine or an assembly of machines may be divided into several control zones (e.g., for emergency stopping, stopping as a result of safeguarding devices, start-up, isolation or energy dissipation). The machine and controls in different zones shall be defined and identified. Controls for machines in zones can be local for each machine, across several machines in a zone, or globally for machines across zones. The control requirements shall be based on the operational requirements and on the risk assessment.The interfaces between zones, including synchronization and independent operation, shall be designed such that no function in one zone creates a hazard(s) / hazardous situation in another zone.
CSA Z432-04 has similar wording:
When zones can be determined, their delimitations shall be evident (including the effect of the associated emergency stop device). This shall also apply to the effect of isolation and energy dissipation.
- Button must be in ‘easy-reach’ of the normal operator position. I consider ‘easy-reach’ to be the range I can touch while sitting or standing at the normal operator position. This position is not necessarily in front of the control panel. This is the position where the operator is expected to be while carrying out the tasks expected of them when the machine is operating. This is the requirement that drives having multiple buttons in most cases.
- E-stop devices cannot be located so that the operator must reach over or past a hazard to activate them.
- The button must latch in the operated position.
- The button must be robust enough to handle the mechanical and electrical stresses that will be placed on it when used. i.e. rugged buttons are required.
- When the e-stop device is reset – i.e returned to the ‘RUN’ position – the machine is NOT permitted to restart. It is only PERMITTED to restart. It must be restarted through another deliberate action, like pressing a ‘Power On’ button.
So what do you do with the ‘POWER ON’ or safety circuit reset button? The first question to ask is: ‘What happens when I reset this circuit, applying power to the control circuits?”
Case A: If it is impossible to see the entire machine from the location of the reset button, then I would recommend a single reset button located at the HMI or main console. The operator must check to make sure the machine is clear before re-applying power. Where the machine is too big to be completely visible from the main operator console, then I would also recommend:
- warning horn,
- warning lights, and
- a start-up delay that is long enough to allow a person to get clear of the machine before it starts moving.
Case B: If the machine is simply ‘enabled’ at this point, but no motion occurs, then multiple ‘reset’ or ‘power on’ buttons may be acceptable, depending on the outcome of the risk assessment and start/stop analysis. Having said that, the operator will likely have to return to a main console to reset the machine and restart operation, and chances are there is only one HMI screen on the machine, so there may not be any advantage to having multiple reset buttons.
I would recommend doing two things to get a good handle on this: Conduct a detailed risk assessment and include all normal operations and all maintenance operations. Then conduct a start/stop analysis to look at all of the starting and stopping conditions that you can reasonably foresee. Combine the results of these two analyses to find the starting and stopping conditions with the highest risk, and then determine if having multiple reset buttons will contribute to the risk or not. You may also want to look at the control reliability requirements for the emergency stop system based on the outcome of the risk assessment and the start/stop analysis.
In a case where there are multiple emergency stop devices, locations are important. There must be one at each normal workstation to meet the regulatory requirements in most jurisdictions, and within ‘easy reach’. You may also want some inside the machine if it is possible to gain full body access inside the machinery. i.e. inside a robot work cell. Make sure that the buttons or other devices are located so that a person exposed to the hazard(s) inside the machine is not required to reach over or past the hazard to get to the button.
Michael, I hope that settles the argument!