(Eds. note: This article was originally written in 2011 and was updated in Nov. 2018.)
The “Hierarchy of Controls” is one approach to risk reduction that has become entrenched in the Occupational Health and Safety (OHS) sector. There are other approaches to risk reduction which are equally effective but are less rigidly structured. If you want to know more about those approaches, I recommend you visit Dr. Sidney Dekker’s site, “Safety Differently“, Dr. Robert Lng’s site, “Human Dymensions,” or Dr. Todd Conklin’s “Pre-Accident Investigations.” None of these approaches are wrong. Any approach that results in effectively reducing the risk for the people at the “sharp end of the stick” is a worthy approach. Onward.
The first step: Risk Assessment
Risk assessment is the first step in reducing the risk that your customers and users are exposed to when they use your products. The second step is Risk Reduction, sometimes called Risk Control or Risk Mitigation. This article looks at the ways that risk can be controlled using the Hierarchy of Controls.Figure 2 from ISO 12100-1 (shown below) illustrates this point.
The system is called a hierarchy because you must apply each level in the order that they fall in the list. In terms of effectiveness at reducing risk, the first level in the hierarchy, elimination, is the most effective, down to the last, PPE*, which has the least effectiveness.
It’s important to understand that questions must be asked after each step in the hierarchy is implemented, and that is “Is the risk reduced as much as possible? Is the residual risk a) in compliance with legal requirements, and b) acceptable to the user or worker?”. When you can answer ‘YES’ to all of these questions, the last step is to ensure that you have warned the user of the residual risks, have identified the required training needed and finally have made recommendations for any needed PPE.
*PPE – Personal Protective Equipment. e.g. Protective eye wear, safety boots, bump caps, hard hats, clothing, gloves, respirators, etc. CSA Z1002 includes ‘…anything designed to be worn, held, or carried by an individual for protection against one or more hazards.’ in this definition.
Introducing the Hierarchy of Controls
The Hierarchy of Controls was developed in a number of different standards over the last 20 years or so, with ISO 12100  coming to the forefront as the leading International standard. The idea was to provide a common structure that would provide guidance to designers when controlling risk.
Typically, the first three levels of the hierarchy may be considered to be ‘engineering controls’ because they are part of the design process for a product. This does not mean that they must be done by engineers!
We’ll look at each level in the hierarchy in detail. First, let’s take a look at what is included in the Hierarchy.
The Hierarchy of Controls includes:
1) Inherently Safe Design, including Hazard Elimination or Substitution (Design)
2) Engineering Controls (see [1, 2, 8, 9, 10, and 11])
b) Guards (Fixed, Adjustable, Movable w/interlocks)
c) Safeguarding Devices
d) Complementary Protective Measures
3) Information for Use (see [1, 2, 4, 7, 8, 12, and 13])
a) Hazard Warnings
c) HMI* & Awareness Devices (lights, horns)
4) Administrative Controls (see [1, 2, 4, 5, 7, and 8])
b) Standard Operating Procedures (SOPs),
c) Hazardous Energy Control Procedures (HECP) (see [5, 14])
d) Authorization/Permit to Work
5) Personal Protective Equipment
c) Training in use
*HMI – Human-Machine Interface. Also called the ‘console’ or ‘operator station’. The location on the machine where the operator controls are located. Often includes a programmable screen or operator display, but can be a simple array of buttons, switches and indicator lights.
The manufacturer, developer or integrator of the system can usually provide only the first three levels of the hierarchy, as they do not normally have control over the workplace where the equipment of the system is used. Where they have not been provided, the workplace or user should provide them.
The last two levels must be provided by the workplace or user.
Each layer in the hierarchy has a level of effectiveness that is related to the failure modes associated with the control measures and the relative effectiveness in reducing risk in that layer. As you go down the hierarchy, the reliability and effectiveness decrease as shown in Fig. 2 below.
There is no way to measure or specifically quantify the reliability or effectiveness of each layer of the hierarchy – that must wait until you make some selections from each level, and even then it can be hard to do. The important thing to understand is that Inherently Safe Design measures will be more effective than Guarding (engineering controls), which is more effective than Information for Use, etc.
1. Inherently Safe Design
The top level of the Hierarchy and the starting point in every effort to reduce risk is Inherently Safe Design. This level is more effective because the word “inherently” indicates that these control measures are baked into the design. Removing these control measures is therefore impossible without permanently damaging or destroying the product. For example, removing sharp corners by radiusing the corners during manufacturing is effectively irreversible. This level of the hierarchy includes:
- Consideration of geometrical factors and physical aspects (travelling and working areas of mobile machines, zones of movement, the contact area with the user, form and relative location of mechanical components, etc.);
- Taking into account general technical knowledge of machine design (mechanical stresses, material properties, emission values for noise, vibration, radiation, or toxic materials);
- Choice of appropriate technology;
- Applying the principle of positive mechanical action;
- Provisions for stability;
- Provisions for maintainability;
- Observation of ergonomic principles;
- Electrical Hazards;
- Fluidic (Hydraulic & Pneumatic) Hazards;
- Inherently safe design principles for control systems (includes the use of ISO 13849, IEC62061, IEC 61511, or IEC 61508 family standards);
- Switching on of internal or external power sources;
- Starting and stopping of mechanisms;
- The behaviour of the machinery when power sources are interrupted;
- Use of automatic monitoring;
- Safety functions implemented in programmable control systems (includes the use of ISO 13849, IEC62061, IEC 61511, or IEC 61508 family standards);
- Principles related to manual control;
- Control modes for setting, teaching, process changeover, fault-finding, cleaning or maintenance;
- Selection of control and operating modes;
- Applying measures to achieve electromagnetic compatibility (EMC);
- Provision of diagnostic systems to aid fault-finding;
- Minimizing the probability of failure of safety functions;
- Limiting exposure to hazards through reliability of equipment;
- Limiting exposure to hazards through mechanization or automation of loading (feeding)/ unloading (removal) operations;
- Limiting exposure to hazards through the location of setting and maintenance points outside danger zones.
The preceding list comes from the headings in ISO 12100 , chapter 6.2. [1, 6.2] includes much more detail on the types of measures that can be used to reduce risk using inherently safe design measures. I strongly recommend that all machinery designers, including mechanical and control systems designers, have a copy of ISO 12100 at hand while doing their design work.
The older definition of the first level of the Hierarchy only included hazard elimination and hazard substitution. These are still valid ways to reduce risk, but they have some specific failure modes that are worth discussing.
Hazard elimination is the most effective means of reducing risk from a particular hazard, for the simple reason that once the hazard has been eliminated there is no remaining risk. Remember that risk is a function of severity and probability. Since both severity and probability are affected by the existence of the hazard, eliminating the hazard reduces the risk from that particular hazard to zero. Some practitioners consider this to mean the elimination is 100% effective, however, it’s my opinion that this is not the case because even elimination has failure modes that can re-introduce the hazard.
Hazard elimination can fail if the hazard is reintroduced into the design. With machinery, this isn’t that likely to occur, but in processes, services and workplaces it can occur.
Substitution requires the designer to substitute a less hazardous material or process for the original material or process. For example, beryllium is a highly toxic metal that is used in some high tech applications. Inhalation or skin contact with beryllium dust can do serious harm to a person very quickly, causing acute beryllium disease. Long-term exposure can cause chronic beryllium disease. Substituting a less toxic material with similar properties in place of the beryllium in the process could reduce or eliminate the possibility of beryllium disease, depending on the exact content of the substitute material. If the substitute material includes any amount of beryllium, then the risk is only reduced. If it contains no beryllium, the risk is eliminated. Note that the risk can also be reduced by ensuring that the beryllium dust is not created by the process since beryllium is not toxic unless ingested.
Alternatively, using processes to handle the beryllium without creating dust or particles could reduce the exposure to the material in forms that are likely to cause beryllium disease. An example of this could be the substitution of water-jet cutting instead of mechanical sawing of the material.
Reintroduction of the substituted material into a process is the primary failure mode, however, there may be others that are specific to the hazard and the circumstances. In the above example, pre- and post-cutting handling of the material could still create dust or small particles, resulting in exposure to beryllium. A substituted material might introduce other, new hazards, or might create failure modes in the final product that would result in risks to the end user. Careful consideration is required!
If neither elimination or substitution is possible, we move to the next level in the hierarchy.
2. Engineering Controls
Engineering controls typically include various types of mechanical guards [16, 17, & 18], interlocking systems [9, 10, 11, & 15], and safeguarding devices like light curtains or fences, area scanners, safety mats and two-hand controls . These systems are proactive in nature, acting automatically to prevent access to a hazard and therefore preventing injury. These systems are designed to act before a person can reach the danger zone and be exposed to the hazard and therefore reduce risk by preventing access to the hazard(s).
Functional safety is sometimes called “control reliability.” Functional safety is the characteristic of a safety system that allows it to operate correctly in response to its inputs under the intended conditions of use. Barrier guards and fixed guards are not evaluated for reliability because they do not rely on a control system for their effectiveness. As long as they are located correctly in the first place, and are otherwise properly designed to contain the hazards they are protecting, then nothing more is required. On the other hand, safeguarding devices, like interlocked guards, light fences, light curtains, area scanners, safety mats, two-hand controls and safety edges, all rely on a control system for their effectiveness. Correct application of these devices requires correct placement based on the stopping performance of the hazard and correct integration of the safety device into the safety-related parts of the control system . The degree of reliability is based on the amount of risk reduction that is being required of the safeguarding device and the degree of risk present in the unguarded state [9, 10].
There are many detailed technical requirements for engineering controls that I can’t get into in this article, but you can learn more by checking out the references at the end of this article and other articles on this blog. If you are interested in learning more, I teach an online course on the topic called Functional Safety 101.
Failure modes for engineering controls are as many and as varied as the devices used and the methods of integration chosen. This discussion will have to wait for another article!
Of special note are “awareness devices.” This group includes warning lights, horns, buzzers, bells, etc. These devices have some aspects that are similar to engineering controls, in that they are usually part of the machine control system, but they are also sometimes classed as ‘information for use’, particularly when you consider indicator or warning lights and HMI screens. In addition to these ‘active’ types of devices, awareness devices may also include lines painted or taped on the floor or on the edge of a step or elevation change, warning chains, signage, etc. Signage may also be included in the class of ‘information for use’, along with HMI screens.
Failure modes for Awareness Devices include:
- Ignoring the warnings (Complacency or Failure to comprehend the meaning of the warning);
- Failure to maintain the device (warning lights burned out or removed);
- The defeat of the device (silencing an audible warning device by disconnection, stuffing foam into a horn, etc.);
- Inappropriate selection of the device (invisible or inaudible in the predominating conditions).
Complementary Protective Measures
Complementary Protective measures are a class of controls that are separate from the various types of safeguarding because they generally cannot prevent injury, but may reduce the severity of an injury or the probability of the injury occurring. Complementary protective measures are reactive in nature, meaning that they are not automatic. They must be manually activated by a user before anything will occur, e.g. pressing an emergency stop button. They can only complement the protection provided by automatic systems.
A good example of this is the Emergency Stop system that is designed into many machines. On its own, the emergency stop system will do nothing to prevent an injury. The system must be activated manually by pressing a button or pulling a cable. This relies on someone detecting a problem and realizing that the machine needs to be stopped to avoid or reduce the severity of an injury that is about to occur or is occurring. The emergency stop can only ever be a backup measure to the automatic interlocks and safeguarding devices used on the machine. In many cases, the next step in emergency response after pressing the emergency stop is to call 911. To learn more about emergency stop, see my series on this topic.
The failure modes for these kinds of controls are too numerous to list here, however, they range from simple failure to replace a fixed guard or barrier fence to the failure of electrical, pneumatic or hydraulic controls. These failure modes are enough of a concern that a new field of safety engineering called ‘Functional Safety Engineering’ has grown up around the need to be able to analyze the probability of failure of these systems and to use additional design elements to reduce the probability of failure to a level we can tolerate. For more on this, see [9, 10, 11].
Once you have exhausted all the possibilities in Engineering Controls, you can move to the next level down in the hierarchy.
3. Information for Use
This is a very broad topic, including manuals, instruction sheets, information labels on the product, hazard warning signs and labels, HMI screens, indicator and warning lights, training materials, video, photographs, drawings, bills of materials, etc. There are some excellent standards now available that can guide you in developing these materials [1, 12 and 13]. To learn more about hazard warning labels, see our series on this topic. To learn more about Information for Use, see this article.
The major failure modes in this level include:
- Poorly written or incomplete materials;
- Provision of the materials in a language that is not understood by the user;
- Failure by the user to read and understand the materials;
- Inability to access the materials when needed;
When all possibilities for informing the user have been covered, you can move to the next level down in the hierarchy. Note that this is the usual separation point between the manufacturer and the user of a product. This is nicely illustrated in Fig 2 from ISO 12100 above. It is important to understand at this point that the residual risk posed by the product to the user may not yet be tolerable. The user is responsible for implementing the next two levels in the hierarchy in most cases. The manufacturer can make recommendations that the user may want to follow, but typically that is the extent of influence that the manufacturer will have on the user.
4. Administrative Controls
This level in the hierarchy includes:
- Standard Operating Procedures (SOP’s);
- Safe working procedures e.g. Hazardous Energy Control Procedures (HECP), Lockout, Tagout (where permitted by law), etc.;
- Authorization; and
Training is the method used to get the information provided by the manufacturer to the worker or end user. This can be provided by the manufacturer, by a third party, or self-taught by the user or worker.
SOP’s can include any kind of procedure instituted by the workplace to reduce risk. For example, requiring workers who drive vehicles to do a walk-around inspection of the vehicle before use, and logging of any problems found during the inspection is an example of an SOP to reduce risk while driving.
Safe working procedures can be strongly influenced by the manufacturer through the information for use provided. Maintenance procedures for hazardous tasks provided in the maintenance manual are an example of this.
Authorization is the procedure that an employer uses to authorize a worker to carry out a particular task. For example, an employer might put a policy in place that only permits licensed electricians to access electrical enclosures and carry out work with the enclosure live. The employer might require that workers who may need to use ladders in their work take a ladder safety and a fall protection training course. Once the prerequisites for authorization are completed, the worker is ‘authorized’ by the employer to carry out the task.
Supervision is one of the most critical of the Administrative Controls. Sound supervision can make all of the above work. Failure to properly supervise work can cause all of these measures to fail.
Administrative controls have many failure modes. Here are some of the most common:
- Failure to train;
- Failure to inform workers regarding the hazards present and the related risks;
- Failure to create and implement SOP’s;
- Failure to provide and maintain the special equipment needed to implement SOP’s;
- No formal means of authorization – i.e. How do you KNOW that Joe has his lift truck license?;
- Failure to supervise adequately.
I’m sure you can think of MANY other ways that Administrative Controls can go wrong!
5. Personal Protective Equipment (PPE)
PPE includes everything from safety glasses, to hardhats and bump caps, to fire-retardant clothing, hearing defenders, and work boots. Some standards even include warning devices that are worn by the user, such as gas detectors and person-down detectors, in this group.
PPE is probably the single most over-used and least understood risk control measure. It falls at the bottom of the hierarchy for a number of reasons:
- It is a measure of last resort;
- It permits the hazard to come as close to the person as the PPE;
- It is often incorrectly specified;
- It is often poorly fitted;
- It is often poorly maintained; and
- It is often improperly used.
The problems with PPE are hard to deal with:
- You cannot glue or screw a set of safety glasses to a person’s face, so ensuring the protective equipment is used is a big problem that goes back to training.
- Many small and medium-sized enterprises do not have the expertise in the organization to properly specify, fit and maintain the equipment.
- User comfort is extremely important. Uncomfortable equipment won’t be used for long.
- Finally, by the time that properly specified, fitted and used equipment can do its job, the hazard is as close to the person as it can get. The probability of failure at this point is very high, which is what makes PPE a measure of last resort, complementary to the more effective measures that can be provided in the first three levels of the hierarchy.
- If workers are not properly trained and adequately informed about the hazards they face and the reasons behind the use of PPE, they are deprived of the opportunity to make safe choices, including the right to refuse the work.
Failure modes for PPE include:
- Incorrect specification (not suitable for the hazard);
- Incorrect fit (allows hazard to bypass PPE);
- Poor maintenance (prevents or restricts vision or movement, increasing the risk; causes PPE failure under stress or allows hazard to bypass PPE);
- Incorrect usage (failure to train and inform users, incorrect selection or specification of PPE).
Time to Apply the Hierarchy
So now you know something about the ‘hierarchy of controls’. Each layer has its own intricacies and nuances that can only be learned by training and experience. With a documented risk assessment in hand, you can begin to apply the hierarchy to control the risks. Don’t forget to iterate the assessment post-control to document the degree of risk reduction achieved. You may create new hazards when control measures are applied, and you may need to add additional control measures to achieve effective risk reduction.
The documents referenced below should give you a good start in understanding some of these challenges.
NOTE: , , and were combined by ISO and republished as ISO 12100:2010. This standard has no technical changes from the preceding standards but combines them in a single document. ISO/TR 14121-2 remains current and should be used with the current edition of ISO 12100.
Safety of machinery – Basic concepts, general principles for design – Part 1: Basic terminology and methodology, ISO 12100-1, 2003. (Withdrawn)
Safety of machinery – Basic concepts, general principles for design – Basic terminology and methodology, Part 2: Technical principles, ISO Standard 12100-2, 2003. (Withdrawn)
 Safety of Machinery – Risk Assessment – Part 1: Principles, ISO Standard 14121-1, 2012.
 Safety of machinery — Prevention of unexpected start-up, ISO 14118, 2017.
 Control of hazardous energy – Lockout and other methods, CSA Z460, 2013.
 Fluid power systems and components – Graphic symbols and circuit diagrams – Part 1: Graphic symbols for conventional use and data-processing applications, ISO Standard 1219-1, 2012.
 Pneumatic fluid power — General rules and safety requirements for systems and their components, ISO Standard 4414, 2010.
 American National Standard for Industrial Robots and Robot Systems — Safety Requirements, ANSI/RIA R15.06, 2012.
 Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design, ISO Standard 13849-1, 2015.
 Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems, IEC Standard 62061, 2005.
 Functional safety of electrical/electronic/programmable electronic safety-related systems, IEC Standard 61508-X, seven parts.
Preparation of Instructions — Structuring, Content and Presentation, IEC Standard 62079, 2001. Replaced by Preparation of instructions for use – Structuring, content and presentation – Part 1: General principles and detailed requirements, IEC 82079-1:2012.
 American National Standard For Product Safety Information in Product Manuals, Instructions, and Other Collateral Materials, ANSI Z535.6, 2010 (R2017).
 Control of Hazardous Energy Lockout/Tagout and Alternative Methods, ANSI Z244.1, 2016.
Safety of Machinery — Interlocking devices associated with guards — principles for design and selection, EN 1088+A1:2008. (Withdrawn) Replaced by Safety of machinery — Prevention of unexpected start-up, ISO 14118:2017.
Safety of Machinery — Guards – General requirements for the design and construction of fixed and movable guards, EN 953+A1:2009. (Withdrawn) Replaced by Safety of machinery — Guards — General requirements for the design and construction of fixed and movable guards, ISO 14120:2015.
 Safety of machinery — Guards — General requirements for the design and construction of fixed and movable guards, ISO 14120.
 Safety of machinery — Safety distances to prevent hazard zones being reached by upper and lower limbs, ISO 13857:2008.
 Safety of machinery — Positioning of safeguards with respect to the approach speeds of parts of the human body, ISO 13855:2010.