Reader Question: Multiple E-Stops and Resets

This entry is part 7 of 11 in the series Emergency Stop

Control Panel with Emergency Stop Button.I had an interesting question come in from a reader today that is relevant to many situations:

“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:

  1. 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).
  2. 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:

7.2.2 Zones

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:

6.2.1.8.4

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.

Let’s take a case with a single e-stop button first. The same requirements apply for all e-stop devices. The requirements include:

  1. 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.
  2. E-stop devices cannot be located so that the operator must reach over or past a hazard to activate them.
  3. The button must latch in the operated position.
  4. 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.
  5. 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!

Understanding Risk Assessment

When people discuss ‘Risk’ there are a lot of different assumptions made about what that means. For me, the study of risk and risk assessment techniques started in 1995. As a technologist and controls designer, I had to somehow wrap my head around the whole concept in ways I’d never considered. If you’re trying to figure out risk and risk assessment this is a good place to get started!

What is risk?

From a machinery perspective, ISO 12100:2010 defines risk as:

“combination of the probability of occurrence of harm and the severity of that harm”

Risk can have positive or negative outcomes, but when considering safety, we only consider negative risk, or events that result in negative health effects for the people exposed.

The risk relationship is illustrated in ISO 12100:2010 Figure 3:


ISO 12100-2010 Figure 3
ISO 12100-2010 Figure 3


Where

R = Risk

S = Severity of Harm

P = Probability of Occurrence of Harm

The Probability of Occurrence of Harm factor is often further broken down into three sub-factors:

  • Probability of Exposure to the hazard
  • Probability of Occurrence of the Hazardous Event
  • Probability of Limiting or Avoiding the Harm

How is risk measured?

In order to estimate risk a scoring tool is needed. There is no one ‘correct’ scoring tool, and there are flaws in most scales that can result in blind-spots where risks may be over or under-estimated.

At the simplest level are ‘screening’ tools. These tools use very simple scales like ‘High, Medium, Low’, or ‘A, B, C’. These tools are often used when doing a shop-floor inspection and are intended to provide a quick method of capturing observations and giving a gut-feel assessment of the risk involved. These tools should be used as a way to identify risks that need additional, detailed assessment. To get an idea of what a good screening tool can look like, have a look at the SOBANE Déparis system.

Every scoring tool requires a scale for each risk parameter included in the tool. For instance, consider the CSA tool described in CSA Z434:

CSA Z434-03 Table 1As you can see, each parameter (Severity, Exposure and Avoidance) has a scale, with two possible selections for each parameter.

When considering selection of a scoring tool, it’s important to take some time to really examine the scales for each factor. The scale shown above has a glaring hole in one scale. See if you can spot it and I’ll tell you what I think a bit later in this post.

There are more than 350 different scales and methodologies available for assessing risk. You can find a good review of some of them in Bruce Main’s textbook “Risk Assessment: Basics and Benchmarks” available from DSE online.

A similar, although different, tool is found in Annex 1 of ISO 13849-1. Note that this tool is provided in an Informative Annex. This means that it is not part of the body of the standard and is NOT mandatory. In fact, this tool was provided as an example of how a user could link the output of a risk assessment tool to the Performance Levels described in the normative text (the mandatory part) of the standard.

Consider creating your own scales. There is nothing wrong with determining what characteristics (parameters) you want to include in your risk assessment, and then assigning each parameter a numeric scale that you think is suitable; 1-10, 0-5, etc. Some scales may be inverted to others, for example: If the Severity scale runs from 0-10, the Avoidability scale might run from 10-0 (Unavoidable to Entirely Avoidable).

Once the scales in your tool have been defined, document the definitions as part of your assessment.

Who should conduct risk assessments?

Lake YogaIn many organizations, I find that risk assessment has been delegated to one person. This is a major mistake for a number of reasons. Risk assessment is not a solo activity for a ‘guru’ in a lonely office somewhere!

Risk assessment is not a lot of fun to do, and since risk assessments can get to be quite involved, it represents a significant amount of work to put on one person. Also, leaving it to one person means that the assessment will necessarily be biased to what that person knows, and may miss significant hazards because the assessor doesn’t know enough about that hazard to spot it and assess it properly.

Risk assessment requires multiple viewpoints from participants with varied expertise. This includes users, designers, engineers, lawyers and those who may have specialized knowledge of a particular hazard, like a Laser Safety Officer or a Radiation Safety Officer. The varied expertise of the people involved will allow the committee to balance the opinion of each hazard, and develop a more reasoned assessment of the risk.

I recommend that risk assessment committees never be less than three members. Five is frequently a good number. Once you get beyond five, it becomes increasingly difficult to obtain consensus on each hazard. Also, consider the cost. As each committee member is added to the team, the cost of the assessment can escalate exponentially.

Training in risk assessment is crucial to success. Ensure that the individuals involved are trained, and that at least one has some previous experience in the practice so that they may guide the committee as needed.

When should a risk assessment be conducted?


Risk Assessment Lifetime Flow Chart
Risk Assessment in the Lifetime of a Product


Risk assessment should begin at the beginning of a project, whether it’s the design of a product, the development of a process or service, or the design of a new building. Understanding risk is critical to the design process. Cost for changes made at the beginning of a project are minimal compared to those that will be incurred to correct problems that might have been foreseen at the start. Risk assessment should start at the concept stage and be included at each subsequent stage in the development process. The accompanying graphic illustrates this idea.

Essentially, risk assessment is never finished until the product, process or service ceases to exist.

What tools are available?

As mentioned earlier in this post, the book ‘Risk Assessment: Basics and Benchmarks” provides an overview of roughly 350 different scoring tools. You can search the Internet and turn up quite a few as well. The key thing with all of these systems is that you will need to develop any software based tools yourself. Depending on your comfort with software, this might be a spreadsheet format, a word processing document a database, or some other format that works for your application.

There are a number of risk assessment software tools available as well, including ISI’s CIRSMA™ and DSE’s DesignSafe. As with the scoring tools, you need to be careful when evaluating tools. Some have significant blind spots that may trip you up if you are not aware of their limitations.

Remember too that the output from the software can only be as good as the input data. The old saw “Garbage In, Garbage Out” holds true with risk assessment.

Where can you get training?

There are a few places to get training. Compliance InSight Consulting provides training to corporate clients and will be launching a series of web-based training services in 2011 that will allow individual learners to get training too.

The IEEE PSES operates a Risk Assessment Technical Committee that is open to the public as well. See the RATC web site.

The Answer to the Scale Question

The Exposure Scale in the CSA tool has a gap between E1 and E2. Looking at the definitions for each choice, notice that E1 is less than once per day or shift, while E2 is more than once per hour. Exposures that occur once per hour or less, but more than once per day cannot be scored effectively using this scale.

Also, notice the Severity scale: S1 encompasses injuries requiring not more than basic first aid. One common question I get is “Does that include CPR*?”. This question comes up because most basic first aid courses taught in Canada include CPR as part of the course. There is no clear answer for this in the standard. The S2 factor extends from injuries requiring more than basic first aid, like a broken finger for instance, all the way to a fatality. Does it make sense to group this broad range of injuries together? This definition doesn’t quite match with the Province of Ontario’s definition of a Critical Injury found in Regulation 834 either.

All of this points to the need to carefully assess the scales that you choose before you start the process. Choosing the wrong tool can skew your results in ways that you may not be very happy about.

*Cardio-Pulmonary Resuscitation