Purpose

To determine the need for conduit seals in hazardous locations, one must understand how, why, and where seals are required in Class I locations. There are two principle reasons for installing conduit seals:

·         To prevent explosions (flames) within the conduit and equipment from being communicated from one portion of the system to another. (Generally, conduit seals are required within 18 inches of the point of entry to explosionproof enclosures. The reason for this is to contain explosions and flames within the enclosure and prevent them from being rapidly transmitted through the conduit systems.)

·         To minimize the likelihood of gases or vapors migrating from a hazardous location to other, unclassified locations.

Sealing fittings can only be considered complete if the seal is dammed and poured as specified in the manufacturer’s installation instructions. The manufacturer’s instructions will identify the specific sealing compound(s) that can be used with a particular sealing fitting. An incomplete seal is a Code violation and, more importantly, poses a significant risk because it cannot be relied upon to perform the two critical safety functions described earlier. Some have claimed they could not complete the seal because they need to be able to draw wires in or out at a later date. Others have used inferior sealing methods such as silicone or electrical sealing putty, methods that, while creative, are noncompliant.

Achieving Code Compliance

The requirements covering the actual conduit seal and seal fitting in Class I, Divisions 1 and 2 locations are located in 501.15(C). One of the most important rules in achieving the intended safety performance is that sealing fittings are required to be listed. This ensures the fitting meets established product safety standards. Listed and labeled equipment must be installed in accordance with installation instructions per 110.3(B). Sealing fittings cannot contain splices and must be located in accessible locations, without exception.

Completing an effective conduit seal requires installing a damming fiber and a sealing compound that is specific for the particular fitting. This compound must be durable for the surrounding atmosphere or liquids. The melting point cannot be less than 93°C to withstand flames and heat from explosions. This compound has to be at least ⅝ inch thick and not less than the trade size of the conduit run in which it is installed, to ensure adequate performance. For example, if the seal is installed in a 2-inch conduit, the thickness of the compound in a completed seal cannot be less than 2 inches thick.

Conductor fill is restricted in conduit runs where sealing fittings are installed. The Code, in Chapter 9, Table 1 generally permits up to a 40 percent fill for conduits containing more than two conductors, but 501.15 limits the fill to 25 percent of the conduit cross-sectional area of the raceway. This provides adequate room in the fitting for separating the conductors and achieving an effective seal around each conductor individually as they pass through the fitting (as shown in the photo accompanying this article). If the fill is in excess of 25 percent, the conductors can bunch up in the middle of the seal, creating spaces between them through which gases or vapors could migrate. Some conduit seal manufacturers produce sealing fittings that are oversized and can accommodate the larger 40 percent fill requirements. These fittings are required to be identified for the higher fill percentages.

Multiconductor Cables

Cable sealing requirements are provided in 501.15(D). Conduits containing multiconductor cable assemblies used for voice, data, or control in hazardous locations must also be sealed. Where multiconductor cables pass through conduit seals, there are two methods of achieving effective and compliant seals. The method used depends on whether or not the cable jacket can transmit gases or vapors through the core of the cable. If the cable jacket or sheath allows passage of gases or vapors, the jacket has to be removed so the conductors in the assembly can be separated when installing the seal damming compound. For cables that do not transmit gases or vapors through the sheath to the cable core, the cable can be dammed and sealed as an individual conductor as it passes through the seal fitting. Multiconductor cable manufacturers should be able to provide evidence of this suitability. 

Seal the Deal

Sealing is required for conduits and cables installed in Class I, Division 1 and 2 locations. Conduit seals must be listed and must meet specific requirements to achieve an effective seal that performs as anticipated. Conduit seals prevent passage of explosion and flames beyond explosionproof equipment and minimize the migration of gases or vapors from classified locations to unclassified locations.

A seal fitting installation is not complete if the seal is not dammed and poured. Leaving the sealing fittings without installing the damming fiber and sealing compound specified in the installation instructions invites the possibilities of explosion and fire. For effective performance and safety, seals are not only required in various locations of the conduit systems, but they must be packed and poured to perform their intended functions. This article explored the general requirements for conduit seals in Class I, Divisions 1 and 2 locations only. Code users are encouraged to verify the applicable sealing requirements related to the specific classified location(s) involved. Where seals are required by NEC, seal the deal.

  Michael Johnston is NECA’s Executive Director of Standards and Safety.  He served as a principal representative on NEC CMP-5 for the 2002, 2005, and 2008 cycles and is currently the chair of CMP-5 representing NECA for the 2011 NEC cycle. Mike is a member of IBEW and has experience as an electrical journeyman, wireman, foreman, and project superintendent. He is a member of the NFPA Electrical Section and Education Section as well as a member of the UL Electrical Council, and is an active member of the National Safety Council.