FIXED ELECTRIC HEATING EQUIPMENT

FOR PIPELINES AND VESSELS A. General 427-1. Scope. The requirements of this article shall apply to electrically energized heating systems and the installation of these systems used with pipelines or vessels or both. 427-2. Definitions. For the purpose of this article: Pipeline: A length of pipe including pumps, valves, flanges, control devices, strainers and/or similar equipment for conveying fluids. Vessel: A container such as a barrel, drum, or tank for holding fluids or other material. Integrated Heating System: A complete system consisting of components such as pipelines, vessels, heating elements, heat transfer medium, 70-382 NATIONAL ELECTRICAL CODE thermal insulation, moisture barrier, nonheating leads, temperature controllers, safety signs, junction boxes, raceways, and fittings. Resistance Heating Element: A specific separate element to generate heat that is applied to the pipeline or vessel externally or internally. (FPN): Tubular heaters, strip heaters, heating cable, heating tape, heating blankets, and immersion heaters are examples of resistance heaters. Impedance Heating System: A system in which heat is generated in a pipeline or vessel wall by causing current to flow through the pipeline or vessel wall by direct connection to an ac voltage source from a dualwinding transformer. Induction Heating System: A system in which heat is generated in a pipeline or vessel wall by inducing current and hysteresis effect in the pipeline or vessel wall from an external isolated ac field source. Skin Effect Heating System: A system in which heat is generated on the inner surface of a ferromagnetic envelope attached to a pipeline and/or vessel. (FPN): Typically, an electrically insulated conductor is routed through and connected to the envelope at the other end. The envelope and the electrically insulated conductor are connected to an ac voltage source from a dual-winding transformer. 427-3. Application of Other Articles. All requirements of this Code shall apply except as specifically amended in this article. Cord-connected pipe heating assemblies intended for specific use and identified as suitable for this use shall be installed according to Article 422. Fixed electric pipeline and vessel heating equipment for use in hazardous (classified) locations shall comply with Articles 500 through 516. 427-4. Branch-Circuit Sizing. The ampacity of branch-circuit conductors and the rating or setting of overcurrent protective devices supplying fixed electric heating equipment for pipelines and vessels shall be not less than 125 percent of the total load of the heaters. The rating or setting of overcurrent protective devices shall be permitted in accordance with Section 240-3(b). B. Installation 427-10. General. Equipment for pipeline and vessel electrical heating shall be identified as being suitable for (1) the chemical, thermal, and physical environment; and (2) installation in accordance with the manufacturer's drawings and instructions. 427-11. Use. Electrical heating equipment shall be installed in such a manner as to be afforded protection from physical damage. 427-12. Thermal Protection. External surfaces of pipeline and vessel heating equipment that operate at temperatures exceeding 60°C (140°F) shall be physically guarded, isolated, or thermally insulated to protect against contact by personnel in the area. ARTICLE 427 - FIXED HEATING FOR PIPELINES 70-383 427-13. Identification. The presence of electrically heated pipelines or vessels, or both, shall be evident by the posting of appropriate caution signs or markings at frequent intervals along the pipeline or vessel. C. Resistance Heating Elements 427-14. Secured. Heating element assemblies shall be secured to the surface being heated by means other than the thermal insulation. 427-15. Not in Direct Contact. Where the heating element is not in direct contact with the pipeline or vessel being heated, means shall be provided to prevent overtemperature of the heating element unless the design of the heater assembly is such that its temperature limitations will not be exceeded. 427-16. Expansion and Contraction. Heating elements and assemblies shall not be installed where they bridge expansion joints unless provisions are made for expansion and contraction. 427-17. Flexural Capability. Where installed on flexible pipelines, the heating elements and assemblies shall have a flexural capability compatible with the pipeline. 427-18. Power Supply Leads. (a) Nonheating Leads. Power supply nonheating leads (cold leads) for resistance elements shall be suitable for the temperature encountered. Preassembled nonheating leads on approved heaters shall be permitted to be shortened if the markings specified in Section 427-20 are retained. Not less than 6 inches (152 mm) of nonheating leads shall be provided within the junction box. (b) Power Supply Leads Protection. Nonheating power supply leads shall be protected where they emerge from electrically heated pipeline or vessel heating units by rigid metal conduit, intermediate metal conduit, electrical metallic tubing, or other raceways identified as suitable for the application. (c) Interconnecting Leads. Interconnecting nonheating leads connecting portions of the heating system shall be permitted to be covered by thermal insulation in the same manner as the heaters. 427-19. Electrical Connections. (a) Nonheating Interconnections. Nonheating interconnections, where required under thermal insulation, shall be made with insulated connectors identified as suitable for this use. (b) Circuit Connections. Splices and terminations outside the thermal insulation shall be installed in a box or fitting in accordance with Sections 110-14 and 300-15. 70-384 NATIONAL ELECTRICAL CODE 427-20. Marking. Each factory-assembled heating unit shall be legibly marked within 3 inches (76 mm) of each end of the nonheating leads with the permanent identification symbol, catalog number, and ratings in volts and watts, or in volts and amperes. 427-21. Grounding. Exposed noncurrent-carrying metal parts of electric heating equipment that are likely to become energized shall be grounded as required in Article 250. 427-22. Equipment Protection. Ground-fault protection of equipment shall be provided for branch circuits supplying electric heating equipment not having a metal covering. 427-23. Nonmetallic Pipelines. Heating assemblies intended for heating covering. nonmetallic pipelines or vessels shall have an overall grounded metal D. Impedance Heating 427-25. Personnel Protection. All accessible external surfaces of the pipeline and/or vessel being heated shall be physically guarded, isolated, or thermally insulated (with weatherproof jacket for outside installations) to protect against contact by personnel in the area. 427-26. Voltage Limitations. The secondary winding of the isolation transformer (Section 427-27) connected to the pipeline or vessel being heated shall not have an output voltage greater than 30 volts ac. Exception: The voltage shall be permitted to be greater than 30 volts but not more than 80 volts if a ground-fault circuit-interrupter for personnel protection is provided. 427-27. Isolation Transformer. A dual-winding transformer with a grounded shield between the primary and secondary windings shall be used to isolate the distribution system from the heating system. 427-28. Induced Currents. All current-carrying components shall be installed in accordance with Section 300-20. 427-29. Grounding. The pipeline or vessel, or both, being heated that is operating at a voltage greater than 30 but not more than 80 shall be grounded at designated points. 427-30. Secondary Conductor Sizing. The ampacity of the conductors connected to the secondary of the transformer shall be at least 100 percent of the total load of the heater. E. Induction Heating 427-35. Scope. This part covers the installation of line frequency induction heating equipment and accessories for pipelines and vessels. (FPN): See Article 665 for other applications. ARTICLE 427 - FIXED HEATING FOR PIPELINES 70-385 427-36. Personnel Protection. Induction coils that operate or may operate at a voltage greater than 30 volts ac shall be enclosed in a nonmetallic or split metallic enclosure, isolated or made inaccessible by location to protect personnel in the area. 427-37. Induced Current. Induction coils shall be prevented from inducing circulating currents in surrounding metallic equipment, supports, or structures by shielding, isolation, or insulation of the current paths. Stray current paths shall be bonded to prevent arcing. F. Skin Effect Heating 427-45. Conductor Ampacity. The ampacity of the electrically insulated conductor inside the ferromagnetic envelope shall be permitted to exceed the values given in Article 310, provided it is identified as suitable for this use. 427-46. Pull Boxes. Pull boxes for pulling the electrically insulated conductor in the ferromagnetic envelope shall be permitted to be buried under the thermal insulation, provided their locations are indicated by permanent markings on the insulation jacket surface and on drawings. For outdoor installations, pull boxes shall be of watertight construction. 427-47. Single Conductor in Enclosure. The provisions of Section 300-20 shall not apply to the installation of a single conductor in a ferromagnetic envelope (metal enclosure). 427-48. Grounding. The ferromagnetic envelope shall be grounded at both ends and, in addition, it shall be permitted to be grounded at intermediate points as required by its design. The ferromagnetic envelope shall be bonded at all joints to assure electrical continuity. The provisions of Section 250-26 shall not apply to the installation of skin effect heating systems. (FPN): See Section 250-26(d) for grounding methods. G. Control and Protection 427-55. Disconnecting Means. (a) Switch or Circuit Breaker. Means shall be provided to disconnect all fixed electric pipeline or vessel heating equipment from all ungrounded conductors. The branch-circuit switch or circuit breaker, where readily accessible to the user of the equipment, shall be permitted to serve as the disconnecting means. The disconnecting means shall be of the indicating type, and shall be provided with a positive lockout in the "off" position. (b) Cord- and Plug-Connected Equipment. The factory-installed attachment plug of cord- and plug-connected equipment rated 20 amperes or less and 150 volts or less to ground shall be permitted to be the disconnecting means. 70-386 NATIONAL ELECTRICAL CODE 427-56. Controls. (a) Temperature Control with "Off" Position. Temperature controlled switching devices that indicate an "off" position and that interrupt line current shall open all ungrounded conductors when the control device is in this "off" position. These devices shall not be permitted to serve as the disconnecting means unless provided with a positive lockout in the "off" position. (b) Temperature Control Without "Off" Position. Temperature controlled switching devices that do not have an "off" position shall not be required to open all ungrounded conductors and shall not be permitted to serve as the disconnecting means. (c) Remote Temperature Controller. Remote controlled temperatureactuated devices shall not be required to meet the requirements of Sections 427-56(a) and (b). These devices shall not be permitted to serve as the disconnecting means. (d) Combined Switching Devices. Switching devices consisting of combined temperature-actuated devices and manually controlled switches that serve both as the controllers and the disconnecting means shall comply with all the following conditions: (1) Open all ungrounded conductors when manually placed in the "off" position; and (2) Be so designed that the circuit cannot be energized automatically if the device has been manually placed in the "off" position; and (3) Be provided with a positive lockout in the "off" position. 427-57. Overcurrent Protection. Heating equipment shall be considered as protected against overcurrent where supplied by a branch circuit as specified in Section 427-4.