When to Use Seal Offs and Barriers for Explosion Proof Equipment

Combustible liquids, vapors and dust are capable of entering equipment via very small openings. Because of this, seal fittings or seal offs must be applied at various sections of conduit systems. The NEC provides numerous recommendations on types of seal fittings, proper application for seal offs, as well as when and where to use such protective barriers along the raceway.

The effects of an exposed or lacking seal in combustible sites can be catastrophic. In 2009, Electrochem Solutions, a battery manufacturing plant, was fined by OSHA for an explosion related to a bad seal, which resulted in two injured workers. Inspectors highlighted that the plant required a waterless environment and linked the explosion to volatile materials leaking into a manual filling station.

As the guidelines set forth by the NEC are open to interpretation, standards surrounding seal fittings can be confusing to implement. This article aims to simplify seal off applications for equipment and conduit systems installed in flammable, Class-rated environments.

Why Use Seal Offs? (Explosive Pressure)

Conduits are not vapor tight and require additional components to create a compact, non-explosive environment for systems. As a solution, seal offs are used to create a protective barrier along the raceway. Seal fittings minimize the passage of combustible substances and flames, so that such elements do not spread around the system or cause ignitions, e.g. from a flammable environment to a non-combustible location in the building. Moreover, the units help reduce volatile materials from accumulating, which prevents an unwanted phenomenon called pressure piling.

Where to Use Seal Offs

Generally speaking, seal offs must be applied to explosion proof systems every 18 inches. According to NEC 501.15a[3], seal offs must be applied in each nipple or conduit connection to an explosion proof enclosure. A drain seal is required where moisture could potentially accumulate, so that a proper exit path can be established, while maintaining its sealed features. For group B locations, such as facilities that handle hydrogen, butadiene, propylene oxide and ethylene oxide, sealed connections are needed every 6” of the flameproof system.

In the event conduit is not pressurized, seal offs are required for conduit entries into a pressurized enclosure (NEC 501.15a[2]). This application is not relevant for Division 2 rated locations. Conduits exiting from a Class I, Division 1 environment to any location, as well as conduits exiting from a Class I, Division 2 environment to an unclassified area requires seal offs (NEC 501.15a[3], b[2]).

It is important to point out that seal offs serve as criteria for determining the explosion proof characteristics of a product or system (NEC 501.15c). To ensure sealing standards are met, the compound must be capable of sealing vapors, environmental hazards and feature a melting point of 200F (minimum). It is best practice to make sure splices and taps are not included in sealing fittings. Conductor fills should be kept at 25% (some installations may require higher fills).

According to NEC 501.15d, sealing at all terminations for cables is required, if the wiring component is known to transmit gases or vapors via the cable core. For a proper seal, the compound should not be smaller than the trade size of the conduit (minimum 5/8” thick). With this in mind, if the conduit is 1.5” the sealing compound should not be less than 1.5” thick.

Exceptions and Factory Sealed Equipment

There are several exceptions to applying seal offs, including factory sealed equipment and units housed in a hermetically sealed chamber. Additionally, Class III rated systems do not require seal offs.

In most cases, the seals should be explosion proof (as noted above) – though there are also some exceptions to this practice. The seal does not have to be explosion proof when it is applied between a Class I, Division 2 area and an unclassified location.