This resource does not replace the Occupational Health and Safety Act (OHSA) and its regulations and should not be used as, or considered, legal advice. Health and safety inspectors apply these laws based on the facts they find in the workplace.

Hazard summary

Diesel engine runaway occurs when a diesel engine begins to run uncontrollably and the engine cannot be shut down by conventional methods such as:

  • turning off the engine ignition switch or fuel system
  • shutting off the solenoid
  • disengaging the engine’s load

This can occur when a diesel engine takes in external flammable or combustible vapours, from the air, causing the diesel engine speed to exceed the maximum safe revolutions per minute (rpms) for the vehicle. When the engine begins taking in an alternative fuel source from the air it no longer requires diesel fuel to continue running.

Diesel engine runaway may occur in stationary, mobile and vehicular diesel engines. Depending on the richness of the vapor cloud, it can result in an explosion within seconds, leaving no opportunity to take corrective action once it begins.

Diesel engine runaway can cause:

  • serious worker injuries or death
  • significant damage to property, the environment and the diesel equipment itself

Examples of the hazard

Unloading hazardous liquid

Workers were unloading a hazardous liquid from the tank of a running vacuum truck into an open hopper at a hazardous waste processing facility.

During the unloading, flammable vapours were released into the atmosphere and entered the truck engine’s air intake system which initiated diesel engine runaway. It caused a flash fire resulting in serious burn injuries to one worker.

Vacuuming dry chemicals

A vacuum truck carrying flammable liquids in its debris tank was used to vacuum a dry chemical from an external intermediate bulk container.

The negative pressure of the vacuuming process drew flammable vapours from the truck’s debris tank, releasing the vapours out of the vacuum blower into the direction of a diesel truck idling nearby. The flammable vapours entered the air intake system of the idling diesel truck which caused the diesel engine to overspeed and catastrophically fail.

This engine failure caused by runaway provided the source of ignition for the flammable vapour cloud that had collected between the two trucks resulting in a flash fire and burn injuries to 3 workers.

Both examples involved the release of flammable vapours to a nearby running diesel engine.

Locations and sectors

This type of hazard may be found in all sectors including:

  • construction
  • farming
  • healthcare
  • industrial
  • mining

It can occur at any location where a diesel engine is operating and there is a potential for an external fuel source to be introduced into the diesel engine’s air intake system.

External fuel sources may include but not be limited to the following sources:

  • combustible vapours
  • combustible dusts
  • flammable vapours

Precaution measures

Measures for preventing diesel engine runaway due to external fuel sources include:

  • prohibiting the idling of diesel vehicles in areas where flammable vapours or combustible vapours or dust may be generated, unless adequate precautions are taken such as those described below
  • ensuring flammable liquids are stored in sealed containers and that vapours are re-directed to an area free of potential hazards or sources of ignition during dispensing or decanting
  • providing a ventilation system adequate to prevent hazardous concentrations of combustible or flammable vapours from collecting
  • installing automatic air intake shutdown device, a gas-detection shut off system, or an automatic diesel engine overspeed protection system installed in the engine’s intake system

Applicable legislation

Occupational Health and Safety Act

Sections 25 to 28 of the Occupational Health and Safety Act (OHSA) set out the roles and responsibilities of employers, supervisors, and workers to protect the health and safety of workers.

Ultimately, employers are responsible for taking every precaution reasonable in the circumstances for the protection of a worker.

These requirements apply to all activities in a workplace, including:

  • the safe operation of a diesel engine
  • the handling of combustible and flammable vapours and dusts
  • protection from the hazard of diesel engine runaway caused by external fuel sources

Part III: Duties of employers and other persons in the Guide to the Occupational Health and Safety Act provides additional details on the roles and responsibilities of the workplace parties set out in the OHSA.

The following sector-specific regulations under the OHSA outline requirements which may be applied to prevent diesel engine runaway from external fuel sources.

Workplace Hazardous Materials Information System Regulation (Regulation 860)

Section 7 of the regulation relates to instructions that must be given to workers regarding hazardous products.

Employers must ensure that every worker who works with a hazardous product or who may be exposed to one in the course of their work is instructed on:

  • the contents required on labels
  • the purpose and significance of the information contained on the labels
  • the contents required on a safety data sheet
  • the purpose and significance of the information on a safety data sheet
  • procedures for the safe use, storage, handling and disposal of a hazardous product
  • procedures for the safe use, storage, handling and disposal of a hazardous product when it is contained or transferred in a:
    • pipe
    • piping system including valves
    • process vessel
    • reaction vessel
    • tank car, a tank truck, an ore car, a conveyor belt or a similar conveyance
  • procedures to be followed when fugitive emissions are present
  • procedures to be followed in case of an emergency involving a hazardous product

Regulations for Industrial Establishments (Regulation 851)

Section 63 of the regulation addresses safety precautions to take in areas where certain types of processes may cause explosions.

A process that is likely to produce a gas, vapour, dust or fume, to such an extent as to be capable of forming an explosive mixture with air must be carried out in one of the following:

  • an area that has provision for safe disposal by burning under controlled conditions
  • an area that:
    • is isolated from other operations
    • has a system of ventilation adequate to ensure that the gas, vapour, dust or fume does not reach a hazardous concentration
    • has no potential sources of ignition
    • has provision for explosion venting
    • has, where applicable, baffles, chokes or dampers to reduce the effects of any explosion

Note: Mechanical ventilation systems

To comply with clause 25(2)(h) the ministry position is that mechanical ventilation must be provided at a rate capable of reducing the vapour concentration below 25% of the lower explosive limit of the solvent vapour mixture in the air. When practicable, the dispensing process must be interlocked with the ventilation system, so that if the ventilation system is stopped or fails, the dispensing process is also stopped. This is consistent with requirements in section 4.1.7.2 of the Ontario Fire Code.

Note: Potential sources of ignition

Potential sources of ignition include:

  • open flames
  • smoking
  • hot surfaces
  • electrical sparks
  • static electricity
  • cutting and welding
  • radiant head
  • frictional head and sparks

Note: Electrical installations used in dispensing areas must be inspected on site and approved by the Electrical Safety Authority as safe for use in hazardous areas. Additional details can be found at OESC Bulletin 18-1-20 -* Classification of hazardous locations.

Construction Projects Regulation O. Reg 213/91

Several sections of the Construction Projects Regulation may be applied to prevent diesel engine runaway from external fuel sources, including:

Material or equipment storage under Section 37

Material or equipment at a project must be stored and moved in a manner that does not endanger a worker.

Ventilation under Subsection 46(1)

  • A project must be adequately ventilated by natural or mechanical means if a gas, vapour, dust or fume may be capable of forming an explosive mixture with air.
  • The ventilation provided must prevent the gas, vapour, dust or fume from reaching a hazardous concentration.

Maintenance and use of vehicles, machinery and tools under Section 93

All vehicles, machinery, tools and equipment must be maintained in a condition that does not endanger a worker.

Vehicles, machines, tools and equipment must not be used:

  • while they are defective or hazardous
  • when the weather or other conditions are such that their use is likely to endanger a worker
  • while they are being repaired or serviced, unless the repair or servicing requires that they be operated

All vehicles, machines, tools and equipment must be used in accordance with any operating manuals issued by the manufacturers.

For vehicles, machines, tools and equipment rated at greater than 10 horsepower, copies of any operating manuals issued by the manufacturers must be kept readily available at the project.

Internal combustion engines requiring written consent by MLITSD Director before use in a tunnel under Section 316

Internal combustion engine must not be used in a tunnel on a project without the prior written consent of an MLITSD Director.

Mine and Mining Plants Regulation (Reg. 854)

Section 45 of the regulation addresses safety precautions to take in areas where certain types of processes may cause explosions.

A process that is likely to produce a gas, vapour, dust or fume to such an extent as to be capable of forming a flammable mixture with air must be carried out in an area which:

  • is isolated from other operations
  • has a system of ventilation which removes the gas, vapour, dust or fume
  • has no potential sources of ignition
  • has vents, baffles, chokes, dampers or other means to reduce the effects of any explosion, as may be required

Health Care and Residential Facilities Regulation (O. Reg. 67/93)

Section 95 of the regulation addresses safety precautions to take in areas where certain types of processes may cause explosions.

A process that is likely to produce dust, fume, gas or vapour to such an extent as to be capable of forming an explosive mixture when mixed with air must be carried out in one of the following:

  • an area that has provision for the disposal of the mixture by burning under controlled conditions
  • an area:
    • that is identified by a sign warning of the hazard
    • that is isolated from other operations
    • that has a system of ventilation adequate to ensure that the dust, fume, gas or vapour does not reach a hazardous concentration
    • that has no potential sources of ignition
    • where any heating or air vents leading to other areas have baffles, chokes or dampers to reduce the effects of any explosion
    • that has provision for explosion venting

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