In simple terms, there are two main types of barrier:
- Engineered barriers – physical equipment or technical systems designed to reduce or control risk.
- Human barriers – actions carried out by people to reduce, control or respond to risk.
Both types of barrier can be further categorised and the methods used to assess their effectiveness will depend on their function and how they perform their safety role.
Engineered Brarriers
Engineered barriers are physical or technical measures designed to prevent, control or mitigate hazardous events. They are generally divided into two categories:
Passive barriers
Passive barriers are physical features that continuously keep a hazard under control without requiring any action or activation. Examples include:
- Blast walls
- Fire walls
- Secondary containment
- Pressure-rated pipework
- Pressure vessels
Active barriers
Active barriers require a change in process conditions or another defined trigger before performing their intended safety function. They are designed to respond automatically to hazardous conditions to prevent an incident from occurring or to reduce its consequences if one does occur. Examples include:
- Pressure relief valves
- Trip systems
- Emergency shutdown systems
To remain effective, all engineered barriers should be properly specified, designed, installed, operated, inspected and maintained throughout their lifecycle. Appropriate hazard and safety studies should also be undertaken to define the Performance Standards for each safety critical barrier, ensuring each barrier performs its intended safety function when required.
Although engineered barriers are generally considered more reliable because they do not rely on immediate human intervention during operation, they are only effective when they are correctly managed and maintained.
It is also important to recognise the role people play in the lifecycle of engineered barriers. Human factors should be considered during their design, installation, operation and maintenance. Where people are responsible for inspecting, testing or maintaining an engineered barrier, these activities should be treated as safety critical and assessed using appropriate human factors techniques.
Human Barriers
Human barriers rely on people carrying out actions to prevent, control or respond to hazardous situations. These are generally grouped into two categories:
- Tasks
- Activities
While there is no absolute distinction between the two, a task will normally have the following characteristics:
- A clear start and finish.
- A series of defined steps.
- A specific outcome or change of status.
- Clearly defined circumstances in which it is carried out.
Human barriers that do not fit these characteristics are generally considered to be activities, although there can be some overlap. Common examples include:
- Process monitoring – an ongoing activity that relies on maintaining situational awareness rather than completing a defined sequence of steps.
- Responding to alarms – where effective performance depends on the overall alarm management system rather than a single alarm.
- Maintenance – applying general maintenance skills across a range of equipment.
- Emergency response – assessing the situation, setting priorities and adapting actions as circumstances change.
BowTie Pro is able to display, highlight and clearly indicate all four types of barriers
Example BowTie:
In these examples the First row is the type, the second row is the Accountable person is the and the third row is the responsible person.
Safety Critical Tasks & Decuments
Any barrier identified on a BowTie diagram should be regarded as safety critical.
A barrier may be supported by a single safety critical task or by several critical activities. Equally, a single critical activity may support multiple barriers across different major hazards. BowTie Pro supports all these relationships.
Supporting documents, such as maintenance procedures, operating instructions or inspection checklists, may also be linked to a barrier. These documents help ensure that safety critical activities are carried out consistently and effectively.
When defining a safety critical task, the following minimum information should be recorded:
- Who carries out the task?
Identify the role responsible for completing the task. Wherever possible, responsibilities should be assigned at a level that can be readily verified by a supervisor.
- What needs to be done?
Provide a brief description of the task and explain how the person knows what actions are required.
- Why is the task carried out?
Describe what triggers the task and how the person knows when it should be completed.
- How do we know it has been completed correctly?
Identify the evidence, checks or verification that demonstrate the task has been carried out correctly and the barrier remains effective.
Barrier Ownership
Every safety-critical barrier should have a clearly identified owner who is accountable for ensuring the barrier remains effective throughout its lifecycle. The owner is responsible for ensuring appropriate performance standards are defined, inspections and maintenance are completed, deficiencies are addressed, and supporting documentation remains current. Although individual safety-critical tasks may be delegated, accountability for the barrier’s ongoing performance remains with the designated owner.
Document Management in BowTies
Effective document management is essential to maintaining the integrity of safety-critical barriers throughout their lifecycle. Within a BowTie, documents provide the evidence that barriers have been correctly designed, implemented, operated and maintained in accordance with their defined Performance Standards. They also ensure that information is current, controlled and readily available to those responsible for managing or performing safety-critical activities. Well-managed documentation supports assurance, facilitates audits and demonstrates that barriers remain capable of performing their intended safety function.
Engineering Barrier Documents
Engineering barriers should be supported by controlled documentation that defines how they are designed, operated, inspected, tested and maintained. Typical documents include
- Performance Standards
- Design specifications
- Inspection and maintenance procedures
- Proof test schedules
- Engineering drawings
- Operating manuals and records of inspections
- Testing and repairs
These documents provide objective evidence that engineering barriers continue to meet their required performance and enable any deterioration or deficiencies to be identified and addressed before barrier integrity is compromised.
Human Factor Documents
Human dependent barriers rely on clear, accurate and accessible documentation to ensure safety-critical tasks are performed consistently and effectively. Typical documents include
- Operating procedures
- Work instructions
- Permit-to-work procedures
- Emergency response plans
- Competency requirements
- Training records
- Checklists and shift handover guidance
These documents support human performance by defining responsibilities, standardising working practices and reducing the potential for error. Regular review and revision ensure that documentation reflects changes to operations, lessons learned from incidents and improvements identified through audits or risk assessments, helping to maintain the effectiveness of human-dependent barriers over time.