Article 427

Fixed Electric Heating Equipment for Pipelines and Vessels

Part I. General

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.Informational Note: For further information, see ANSI/IEEE 515-2011, Standard for the Testing, Design, Installation and Mainte‐ nance of Electrical Resistance Trace Heating for Industrial Applications; ANSI/IEEE 844-2000, Recommended Practice for Electrical Impe‐ dance, Induction, and Skin Effect Heating of Pipelines and Vessels; and ANSI/NECA 202-2013, Standard for Installing and Maintaining Industrial Heat Tracing Systems.
2. Definitions.Impedance Heating System. A system in which heat is gener‐ ated 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 dual-winding 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.Pipeline. A length of pipe including pumps, valves, flanges, control devices, strainers, and/or similar equipment for conveying fluids.Resistance Heating Element. A specific separate element to generate heat that is applied to the pipeline or vessel externally or internally.Informational Note: Tubular heaters, strip heaters, heating cable, heating tape, heating blankets, and immersion heaters are examples of resistance heaters.Skin-Effect Heating System. A system in which heat is gener‐ ated on the inner surface of a ferromagnetic envelope attached to a pipeline or vessel, or both.Informational Note: Typically, an electrically insulated conduc‐ tor is routed through and connected to the envelope at the other end. The envelope and the electrically insulated conduc‐ tor are connected to an ac voltage source from a dual-winding transformer.Vessel. A container such as a barrel, drum, or tank for holding fluids or other material.
3. Application of Other Articles. Cord-connected pipe heating assemblies intended for specific use and identified as suitable for this use shall be installed according to Article 422.
4. Continuous Load. Fixed electric heating equipment for pipelines and vessels shall be considered continuous load.

ARTICLE 427 — FIXED ELECTRIC HEATING EQUIPMENT FOR PIPELINES AND VESSELS 427.27

Part II. Installation

1. General. Equipment for pipeline and vessel electric heating shall be identified as being suitable for (1) the chemi‐ cal, thermal, and physical environment and (2) installation in accordance with the manufacturer’s drawings and instructions.
2. Use. Electric heating equipment shall be installed in such a manner as to be afforded protection from physical damage.
3. Thermal Protection. External surfaces of pipeline and vessel heating equipment that operate at temperatures exceed‐ ing 60°C (140°F) shall be physically guarded, isolated, or ther‐ mally insulated to protect against contact by personnel in the area.
4. Identification. The presence of electrically heated pipelines, vessels, or both, shall be evident by the posting of appropriate caution signs or markings at intervals not exceed‐ ing 6 m (20 ft) along the pipeline or vessel and on or adjacent to equipment in the piping system that requires periodic servic‐ ing.
   Part III. Resistance Heating Elements
5. Secured. Heating element assemblies shall be secured to the surface being heated by means other than the thermal insulation.
6. 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.
7. Expansion and Contraction. Heating elements and assemblies shall not be installed where they bridge expansion joints unless provisions are made for expansion and contrac‐ tion.
8. Flexural Capability. Where installed on flexible pipe‐ lines, the heating elements and assemblies shall have a flexural capability that is compatible with the pipeline.
9. Power Supply Leads.
   1. Nonheating Leads. Power supply nonheating leads (cold leads) for resistance elements shall be suitable for the tempera‐ ture encountered. Not less than 150 mm (6 in.) of nonheating leads shall be provided within the junction box. Preassembled factory-supplied and field-assembled nonheating leads on approved heaters shall be permitted to be shortened if the markings specified in 427.20 are retained.
   2. Power Supply Leads Protection. Nonheating power supply leads shall be protected where they emerge from electri‐ cally 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.
   3. 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.
10. Electrical Connections.
    1. Nonheating Interconnections. Nonheating interconnec‐ tions, where required under thermal insulation, shall be made with insulated connectors identified as suitable for this use.
    2. Circuit Connections. Splices and terminations outside the thermal insulation shall be installed in a box or fitting in accordance with 110.14 and 300.15.
11. Marking. Each factory-assembled heating unit shall be legibly marked within 75 mm (3 in.) of an end of the nonheat‐ ing leads with the permanent identification symbol, catalog number, and ratings in volts and watts or in volts and amperes.

1. Ground-Fault Protection of Equipment. Ground-fault protection of equipment shall be provided for electric heat tracing and heating panels. This requirement shall not apply in industrial establishments where there is alarm indication of ground faults and the following conditions apply:
   1. Conditions of maintenance and supervision ensure that only qualified persons service the installed systems.
   2. Continued circuit operation is necessary for safe opera‐ tion of equipment or processes.
2. Grounded Conductive Covering. Electric heating equipment shall be listed and have a grounded conductive covering in accordance with 427.23(A) or (B). The conductive covering shall provide an effective ground path for equipment protection.

1. Heating Wires or Cables. Heating wires or cables shall have a grounded conductive covering that surrounds the heat‐ ing element and bus wires, if any, and their electrical insula‐ tion.
2. Heating Panels. Heating panels shall have a grounded conductive covering over the heating element and its electrical insulation on the side opposite the side attached to the surface to be heated.

Part IV. Impedance Heating

1. Personnel Protection. All accessible external surfaces of the pipeline, vessel, or both, being heated shall be physically guarded, isolated, or thermally insulated (with a weatherproof jacket for outside installations) to protect against contact by personnel in the area.
2. 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.Exception No. 1: In industrial establishments, the isolation transformerconnected to the pipeline or vessel being heated shall be permitted to have an output voltage greater than 30 but not more than 80 volts ac to ground where all of the following conditions apply:
   1. Conditions of guarding, maintenance, and supervision ensure that only qualified persons have access to the installed systems.
   2. Ground-fault protection of equipment is provided.
3. Voltage Limitations. The secondary winding of the isolation transformer connected to the pipeline or vessel being heated shall not have an output voltage greater than 30 volts ac.
   Exception No. 2: In industrial establishments, the isolation transformer connected to the pipeline or vessel being heated shall be permitted to have an output voltage not greater than 132 volts ac to ground where all of the following conditions apply:
   1. Conditions of guarding, maintenance, and supervision ensure that only qualified persons service the installed systems.
   2. Ground-fault protection of equipment is provided.
   3. The pipeline or vessel being heated is completely enclosed in a grounded metal enclosure.
   4. The transformer secondary connections to the pipeline or vessel being heated are completely enclosed in a grounded metal mesh or metal enclosure.
4. Induced Currents. All current-carrying components shall be installed in accordance with 300.20.
5. Grounding. The pipeline, vessel, or both, that is being heated and operating at a voltage greater than 30 but not more than 80 shall be grounded at designated points.
6. 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.

Part V. Induction Heating

1. Scope. This part covers the installation of line frequency induction heating equipment and accessories for pipelines and vessels.Informational Note: See Article 665 for other applications.
2. 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.
3. 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.

Part VI. Skin-Effect Heating

1. 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.
2. Pull Boxes. Pull boxes for pulling the electrically insu‐ lated conductor in the ferromagnetic envelope shall be permit‐ ted to be buried under the thermal insulation, provided their locations are indicated by permanent markings on the insula‐ tion jacket surface and on drawings. For outdoor installations, pull boxes shall be of watertight construction.
3. Single Conductor in Enclosure. The provisions of

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 ensure electrical continuity.

The provisions of 250.30 shall not apply to the installation of skin-effect heating systems.

Informational Note: See Article 250 for grounding methods.

Part VII. Control and Protection

1. Disconnecting Means.
   1. Switch or Circuit Breaker. Means shall be provided to simultaneously disconnect all fixed electric pipeline or vessel heating equipment from all ungrounded conductors. The branch-circuit switch or circuit breaker, where readily accessi‐ ble 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 capable of being locked in the open (off) position. The disconnecting means shall be installed in accordance with 110.25.
   2. Cord-and-Plug-Connected Equipment. The factory- installed attachment plug of cord-and-plug-connected equip‐ ment rated 20 amperes or less and 150 volts or less to ground shall be permitted to be the disconnecting means.
2. Controls.
   1. Temperature Control with “Off” Position. Temperature- controlled switching devices that indicate an “off” position and that interrupt line current shall open all ungrounded conduc‐ tors when the control device is in this “off” position. These devices shall not be permitted to serve as the disconnecting means unless capable of being locked in the open position.
   2. Temperature Control Without “Off” Position. Tempera‐ ture controlled switching devices that do not have an “off” posi‐ tion shall not be required to open all ungrounded conductors and shall not be permitted to serve as the disconnecting means.
   3. Remote Temperature Controller. Remote controlled temperature-actuated devices shall not be required to meet the requirements of 427.56(A) and (B). These devices shall not be permitted to serve as the disconnecting means.
   4. Combined Switching Devices. Switching devices consist‐ ing 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 condi‐ tions:
   1. Open all ungrounded conductors when manually placed in the “off” position
   2. Be designed so that the circuit cannot be energized auto‐ matically if the device has been manually placed in the “off” position
   3. Be capable of being locked in the open position
3. Overcurrent Protection. Heating equipment shall be considered protected against overcurrent where supplied by a branch circuit as specified in 210.18 and 210.23.