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12/17/18 Addressing Explosion Proof Classifications with Fans and Ventilation Systems

Article - December 18, 2018 By

Explosion Proof Electric Inline Axial Fan - Class I, Div. I - 15' Conductive Duct - 1/3 HP Engine

Addressing Explosion Proof Classifications with Fans and Ventilation Systems

Combustible work sites are classified (Class I, II or III and Division 1 & 2) based on the type of flammable substance, presence (frequent or occasional), accumulation levels and type of work conducted in the area. Because these factors are not constant, various sections of a flammable facility can take on different classifications.

In fact, the NEC suggests that adequate ventilation, under very specific circumstances, conditions and locations, can cause a section of a classified facility to be unclassified. Examples of such areas include parts of major repair garages, pits, repair facilities and coal preparation plants.

As criteria for leveraging ventilation as a workaround for classified locations is demanding, it is crucial to fully understand the NEC guidelines that underpin this practice.

What is Explosion Proof Ventilation?

Explosion proof ventilation is the use of class-rated fans, blowers, exhausts and supporting accessories to promote air flow in a combustible site. The primary role of this type of industrial equipment is to reduce the accumulation of flammable compounds in the area, such as confined spaces, drains, transformer vaults and silos. Unlike mainstream (non-classified) fans and blowers, explosion proof models are equipped with non-sparking parts and can feature controlled surface temperatures (mostly applicable for Class II rated locations) to deter potentially fatal ignitions.

Explosion proof air flow solutions can be permanent or portable, depending on the needs of the project or location. Generally speaking, the application of ventilation systems is recommended by the NFPA in the following types of combustible facilities: aircraft hangars (NFPA 409/91), plants that handle aluminum powder (NFPA 651/30), fuel cell power sites (NFPA 853) and more.

The most common sections of combustible buildings that require ventilation are floor and ceiling areas. This is because flammable compounds either sink or rise, based on their molecular weight. With this in mind, it makes sense to target those areas, ensuring minimal accumulation of combustible materials is taking place.

Ventilation and Classified/Unclassified Locations

According to NEC Article 511 (Commercial Garages, Repair and Storage), the presence of ventilation systems can affect the classification of various sections of major repair garages, minor repair garages and pits. Before going further, it is important to highlight that parking or storage garages are not considered to be classified areas, based on their respective definitions in the NEC manual (NEC 511.3).

The floor area of major repair garages and dispensing repair garages can be classified or unclassified, depending on provided ventilation (NEC 511-1). Specifically, the floor area of such locations is unclassified if provided ventilation adheres to a minimum of four air changes per hour for each square foot of floor area. If ventilation is not provided at the floor area, the entire section is classified as Class I, Division 2 – only up to 18” above the floor.

Next, the ceiling area of major repair garages and dispensing repair garages can be considered unclassified if (NEC 511-2): “ventilation is provided from not more than 18 in. from the highest point in the ceiling to exhaust the ceiling area at a rate of at least 1 cfm/sq ft at all times that the building is occupied, or when vehicles using lighter than-air gaseous fuels are parked below this area.” If ventilation is not provided, the ceiling area is a Class I, Division 2 location.

Pit areas of major repair garages and dispensing repair garages take on very stringent NEC recommendations – most likely due to its semi-confined nature. If ventilation is provided in the pit area, at a minimum of six air changes per hour, the location is Class I, Division 2. If ventilation is not provided, the area is Class I, Division 1 (NEC 511-4). Individuals should carefully note that if the floor area is unclassified (based on the guidelines above), the pit area is still considered to be a classified location.

NFPA and NEC Updates to Ventilation Practices

According to NFPA 120 (Standard for Coal Preparation Plants), ventilation systems can be applied to downgrade various location classifications within coal preparation plants to Class I, Division 2. However, safety equipment used to mitigate the presence of flammable substances in the environment, such as methane, must adhere to Class I, Division 1 installation standards.

A 2017 update to NEC 500.5(a) suggests that ammonia areas with constant ventilation or equipped with detection devices (concentrations not exceeding 150 parts per million) can be considered as unclassified.


There are numerous considerations that should be factored in, when setting up ventilation systems to circumvent classifications in flammable facilities. To ensure thorough removal or reduced accumulation of volatile substances in the area, individuals should use air-flow methods that cater to the unique properties of the compound present at the site.

For instance, explosive fibers and flyings (Class III) are not generally suspended in the air. These types of flammable materials mostly accumulate around large machines. By comparison, explosive gases and vapors (Class I) are notoriously known for circulating around facilities with ease; thus calling for ventilation systems that effectively address suspended (combustible) substances.

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