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UL Lists industrial control panels with intrinsically safe circuit extensions for hazardous locations

We've had questions from code W authorities regarding industrial control panels (ICPs) that include intrinsically safe circuits for extension into a hazardous location. Some of you have seen instances when an intrinsic safety barrier is used with an ICP for an ordinary location. This use may appear to meet requirements and codes, but you may want to take a closer look to be sure that the intrinsic safety barrier is being used in accordance with UL requirements. Usually, these requirements are contained in an installation drawing, commonly referred to as a control drawing. On a UL Listed ICP, the product label must refer to this control drawing number.

By examining this control drawing, you will see that ICPs with intrinsically safe circuit extensions for use in hazardous locations are quite different. But did you know that UL has a special category to cover them? The category is Industrial Control Panels Relating to Hazardous Locations (NRBX), and it appears in the Hazardous Locations Equipment Directory (red book).

This is a category completely separate from the one for ICPs for actual installation in hazardous locations -- Control Panels and Assemblies (NNNY) -- that also appears in the red book. It is also completely separate from the category for ICPs for use in ordinary locations -- Industrial Control Panels (NITW) -- that appears in the Electrical Construction Materials Directory (green book).

Currently, the Standards used to investigate products in the NRBX category are UL 508, the Standard for Safety of Industrial Control Equipment, and UL 913, the Standard for Safety of Intrinsically Safe Apparatus and Associated Apparatus for use in Class I, II and III, Division 1 Hazardous Locations. Article 504 of the NEC® specifies installation practices for intrinsically safe circuits. UL is now in the process of developing an outline of investigation that will consolidate requirements for panels for use in hazardous locations and panels for use in non-hazardous locations with intrinsically safe circuit extensions.

The specialized panels in category NRBX are intended for installation in non-hazardous locations, but they provide intrinsically safe (low energy) circuits that extend into a hazardous location. The low energy levels maintained by the intrinsically safe circuits prohibit ignition of the explosive atmosphere. However, it is important that these circuits and associated equipment be installed in accordance with the instructions provided. These instructions state that the wiring must be routed in a separate raceway or segregated from all power and other circuit wiring (including intrinsically safe wiring, in some cases) to prevent ignition-capable currents and voltages from combining with the intrinsically safe circuits. If wiring is not correctly routed or segregated, a fire or explosion could occur.

In addition, special attention must be given to the devices connected to the intrinsically safe circuits in the hazardous location to be sure that the ratings of the intrinsic safety barrier are not exceeded by the devices.

While panels with intrinsically safe circuit extensions (NRBX) are not intended for use in a hazardous location, the panels in the category NNNY are specially built for use in such areas.

These panels can consist of an explosion-proof enclosure or a general-purpose enclosure having purging/pressurization features. We have seen cases in which explosion-proof enclosures may have been used incorrectly, contrary to the requirements of the UL certification. Again, we suggest that code authorities take a close look at any of these panel installations to be sure that the explosion-proof characteristics are maintained.