Understanding NEC Article 665: Induction and Dielectric Heating Equipment

NEC Article 665 provides safety standards for the construction and installation of induction and dielectric heating equipment. These systems are primarily used in industrial and scientific applications, with a focus on processes like heating, melting, and welding for metals and nonmetals.

Scope of Article 665

This article applies to:

• Induction heating, melting, and welding equipment.
• Dielectric heating equipment used for industrial processes such as material drying and welding plastics.

Exclusions:

• Medical or dental applications.
• Line frequency pipeline and vessel heating (refer to Article 427, Part V).

Referenced Standards:

• FCC Rules (47 CFR 18) for spurious radiation control.
• NFPA 86, addressing electric heating systems using induction and dielectric methods.

Key Definitions

1. Induction Heating: Generates heat in electrically conductive materials through a varying magnetic field.
2. Dielectric Heating: Heats nonmetallic materials by creating molecular vibration in an alternating electric field.

Key Requirements for Induction and Dielectric Heating Systems

1. Output Circuit Design
   • Must limit current flow to ground to ensure less than 50 volts appear on accessible parts under fault conditions.
   • Output circuits may be isolated from the ground to protect against faults and damage.
2. Wiring and Ampacity
   • Conductors must support the simultaneous operation of connected equipment.
   • Ampacity calculations should include the largest equipment group’s ratings and standby currents.
3. Disconnecting Means
   • Equipment must have a readily accessible disconnect located within sight of the controller or lockable in the open position.
4. Control Features
   • Systems with multiple control points require interlocks to ensure equipment is energized from one point only.
   • Foot switches must have shields to prevent accidental activation.

Guarding and Grounding

1. Enclosures and Shields
   • Heating components must be housed in noncombustible enclosures with access controls and interlocks to prevent operation when open.
2. Grounding and Bonding
   • Grounding minimizes radio frequency voltages between equipment and earth ground, ensuring safety and functionality.
   • Special bonding techniques, such as using copper or aluminum sheets, reduce stray currents and radio interference.
3. Hazard Markings
   • Signs must display “DANGER — HIGH VOLTAGE — KEEP OUT” at areas containing over 150 volts.

Special Considerations

1. Dielectric Heating Shielding
   • Applicators require shielding or protective cages with interlocked doors to cut power when accessed.
2. Capacitor Safety
   • Capacitors used at high frequencies require robust fault detection to prevent case rupture and potential hazards.
3. Frequency and Power Levels
   • Induction systems: 50 Hz to 500 kHz for heating; 100 to 800 kHz for welding.
   • Dielectric systems: Operate at assigned radio frequencies (13.56 MHz, 27.12 MHz, etc.) and microwave frequencies.

Applications

Article 665 supports the safe operation of:

• Industrial Manufacturing: Heat treating, welding, and material drying.
• Scientific Research: Controlled heating and material testing.
• Specialized Processes: Preheating wood fibers, welding plastics, and food processing.

Conclusion

NEC Article 665 ensures the safe and efficient use of induction and dielectric heating equipment by addressing wiring, grounding, shielding, and operation. These standards protect personnel, equipment, and the surrounding environment in industrial and scientific settings.