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15: Engineering Safety
15.8: The Hierarchy of Controls in Engineering Design

15.8: The Hierarchy of Controls in Engineering Design

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The Hierarchy of Controls in Engineering Design

Understanding the Control Strategy

In engineering safety, hazard mitigation follows a strict prioritization framework known as the Hierarchy of Controls. Developed by safety engineers and industrial hygienists, this model arranges risk mitigation strategies from most effective to least effective. The core engineering principle is straightforward: design out the risk at the source whenever possible, rather than relying solely on human behavior or protective equipment.

Most Effective to Lease Effective

ELIMINATION - Physically remove the hazard

SUBSTITUTION - Replace the hazard with a safer alternative

ENGINEERING CONTROLS - Isolate people from the hazard

ADMINISTRATIVE CONTROLS - Change the way people work

PPE - Protect the individual with gear

Higher-level controls (Elimination, Substitution, Engineering) alter the physical design or operating environment. They offer long-term protection independent of human behavior. Lower-level controls (Administrative, PPE) depend heavily on user compliance, training, and continuous oversight, making them inherently more susceptible to human error.

The Five Levels of Hazard Control

1. Elimination

Physically removes the hazard entirely from the process, system, or workflow.

Engineering Impact: Eliminates $100\%$ of the risk associated with that specific hazard.
Design Strategy: Re-evaluate process requirements to determine if the dangerous step, material, or component is strictly necessary.

2. Substitution

Replaces a hazardous substance, energy source, or piece of equipment with a safer alternative.

Engineering Impact: Reduces the severity of potential harm while preserving process utility.
Design Strategy: Select materials with lower toxicity, lower flammability, lower operating pressures, or reduced voltage.

3. Engineering Controls

Implements physical, mechanical, or electronic systems to isolate personnel from the hazard.

Engineering Impact: Controls the hazard at or near the source without requiring action from the worker.
Design Strategy: Incorporate physical guards, local exhaust ventilation (fume hoods), automatic emergency shutoffs, physical interlocks, and sound-insulating enclosures.

4. Administrative Controls

Establishes policies, training, signage, and standardized operational rules to govern human behavior near hazards.

Engineering Impact: Minimizes hazard exposure duration or frequency through procedural discipline.
Design Strategy: Develop Standard Operating Procedures (SOPs), enforce shift rotation, mandate pre-lab safety check-ins, and post hazard warnings.

5. Personal Protective Equipment (PPE)

Provides specialized clothing or equipment worn by individuals to create a barrier between themselves and potential hazards.

Engineering Impact: Serves as the final line of defense; does not eliminate or reduce the hazard itself, only protects the wearer if containment fails.
Design Strategy: Match specific gear (e.g., chemical resistance ratings, impact goggles, thermal gloves, hearing protection) precisely to residual risks.

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