Control panels that incorporate intrinsically safe circuitry that is extended into hazardous locations are required to be listed and labeled in accordance with UL Category NRBX, Industrial Control Panels Relating to Hazardous Locations, or the wiring extended from the control panel is required to meet the requirements of NEC Articles 501, 502, or 503. The provisions of Article 504, Intrinsically Safe Systems, may only be used where the circuitry originates in a control panel that meets the listing and labeling requirements of UL Category NRBX, Industrial Control Panels Relating to Hazardous Locations.
This information may be further referenced in the UL Directory (white book) under UL Category NITW, Industrial Control Panels, and UL Category NRBX, Industrial Control Panels Related to Hazardous Locations. The latter category requires control panels to be investigated for compliance with UL 913 in addition to UL 508. Control panels that incorporate intrinsically safe circuitry are required to be marked "Industrial Control Panel Relating to Hazardous Locations With Intrinsically Safe Circuit Extensions." If the control panel does not have this marking, all circuitry, including that extended from an intrinsic barrier, must be installed in accordance with NEC Articles 501, 502, or 503.