Nonincendive Wiring for Hazardous Areas: One Method, Two Approaches

Summary

Maintaining a safe environment while controlling installation and maintenance costs is one of the biggest concerns when applying wiring practices in Division 2 hazardous areas. To solve this challenge, companies are increasingly combining nonincendive with other wiring methods to achieve the results they want, from increased flexibility to reduced installation time and expense.

This article explains the two contrasting nonincendive approaches—energy limited and non-arcing, as referenced in the National Electrical Code (NEC). It also offers guidance about how combining nonincendive circuits, components and equipment with other Division 2 protection methods can benefit engineers and installers of equipment and devices.




The term “nonincendive” includes five definitions within two very different installation approaches for Division 2, as seen in the chart below:
The energy limited approach Nonincendive field wiring and nonincendive field wiring apparatus are energy limited during normal operation. This approach is similar to intrinsic safety (see page 7 for full NEC definitions). There are several advantages to this approach. It allows engineers to troubleshoot and work on issues live, connect and disconnect devices in the field, and use unclassified cable—rather than have contractors install rigid metal conduit and poured seals. In addition, because rigid metal conduit and poured seals are not used, devices can easily be relocated in the field if a physical change is needed. Because nonincendive field wiring and field wiring apparatuses are only applicable when energy is limited, similar to intrinsic safety, they are ideally suited for low-power devices.
 

The non-arcing approach

Nonincendive circuit, nonincendive component and nonincendive equipment are non-arcing during normal operation. They do not limit energy into the hazardous area. (See page 7 for full NEC definitions.) The main advantage of nonincendive circuits, components and equipment is that these devices can be used with higher-voltage applications. However, power must be turned off before making or breaking connections because they do not limit energy during normal operation.

A better approach: Combining nonincendive with other wiring methods

Typically, engineers only begin exploring Division 2 nonincendive wiring methods when they feel limited by their current wiring practices. Because limitations exist with every protection method, using a combination of methods often makes the most sense to achieve desired results. Specifying the proper wiring method for your application requires examining the equipment, power and energy requirements of the application. By clearly understanding how to combine nonincendive wiring practices with explosionproof, purge and pressurization and intrinsic safety in Division 2 areas, engineers can better specify the correct installation approach – or approaches–that works best for their applications.
 

Replacing explosion proof with nonincendive circuits, components and equipment

Explosion-proof (XP) enclosures, conduit and seals are the most common hazardous wiring approach in North America. However, there are often drawbacks, including the high cost of equipment, labor-intensive seals and conduit, and lack of flexibility with piped conduit. Using nonincendive equipment and components, with factory approved seals using TC or ITC cabling and connectors, instead of explosion-proof is one way to reduce installation costs and provide flexibility. When XP enclosures, conduit and seals are installed, whether it is in Division 1 or Division 2, they require an array of special equipment and procedures to ensure proper operational safety. Seals and rigid metal conduit must be installed by a licensed professional. Additionally, proper installation and maintenance of XP enclosures are required to ensure continued physical integrity and operational safety.

Nonincendive equipment must be labeled with a maximum surface temperature, known as a T rating. These ratings help engineers determine how much heat is being produced by a nonincendive device under normal operating conditions in an XP enclosure. Typical T ratings range from T6- T1 (85°C to 450°C). Engineers can apply these ratings to lower the risk of explosion within an enclosure by specifying only devices with a lower surface temperature than the minimum ignition temperature (MIG) of the gas inside an enclosure. For example, butane has an ignition temperature of 372°C. A nonincendive device with a T2 (300°C) maximum surface temperate rating would be acceptable to be installed in this XP enclosure.

Combining purge and pressurization with nonincendive circuits, components and equipment Like explosion-proof, purge and pressurization requires special equipment and procedures to ensure proper operational safety whether in Division 1 or Division 2 areas. In Division 2 areas, nonincendive circuits, components and equipment can be used to provide physical connections instead of rigid metal conduit, to save installation and maintenance costs.

There are three distinct purge and pressurization methods—pX, pY and pZ. Each method involves two steps. First, the area is purged with clean air (or inert gas) to remove the concentration of a hazardous gas or vapor to an acceptable level. Second, the area is pressurized by supplying clean air (or inert gas) to prevent the hazardous gas or vapor from entering the area. Depending on the method, power may or may not need to be disconnected if pressure falls below the minimum limit. Division 2 areas using the pZ method (shown on page 5) are a good example of how to incorporate nonincendive circuits, components and equipment with purge and pressurization.

If equipment is located in Division 2, it is not required to use explosion-proof enclosures, seals or rigid metal conduit; instead, nonincendive circuits, components and equipment can be used to provide physical connections to standard NEMA 4X enclosures. This makes it possible to decrease costs associated with installing rigid metal conduit and poured seals by instead using less expensive TC and ITC cables, and factory approved Class I Division 2 seals.

Combining intrinsic safety with nonincendive circuits, components and equipment Intrinsic safety is often a preferred wiring approach in hazardous areas because it is energy limited. Specific characteristics of intrinsically safe barriers – including their approval for use in Division 2 areas – allow engineers to also incorporate nonincendive circuits, components and equipment to further reduce risk of explosion. Intrinsic safety (IS) requires a variety of special equipment and procedures to ensure proper operational safety. Intrinsically safe circuits consists of three components:
 
• The apparatus or intrinsically safe field device: These are designed to limit the internal capacitance and inductance stored, when making or breaking connections to the field device.

• The associated apparatus or intrinsically safe barrier: These limit the current and voltage supplied to the intrinsically safe field device, when making or breaking connections, to the field device.
• The cables: The cables’ internal capacitance and inductance is added to the internal capacitance and inductance of the field device, in order to determine the total capacitance and inductance of the circuit.

Intrinsically safe barriers offer an intrinsically safe connection to the field apparatus by limiting the amount of energy that flows to the apparatus. Using nonincendive for the physical connections in Division 2 areas further limits explosion risk because the components and equipment are nonarcing under normal operation, eliminating the risk of a spark.
 

<sub>Reference of Standards NFPA 496: Standard for Purged and Pressurized Enclosures for Electrical Equipment, 2008 Edition. NFPA 70: National, Electrical Code, 2017 Edition

Disclaimer: This white paper is intended for informational purposes only. The information is subject to change. Please consult a hazardous area wiring industry professional about nonincendive wiring methods and best practices.</sub>