Understanding NEC Article 668: Electrolytic Cells

NEC Article 668 provides standards for the installation and operation of electrical components in electrolytic cells used to produce various metals and chemicals. These systems require specialized safety considerations due to high currents and voltages in demanding industrial environments.

Scope of Article 668

This article applies to:

• Electrical installations for electrolytic cells, including auxiliary equipment and process power supplies.
• Facilities producing aluminum, chlorine, hydrogen peroxide, magnesium, sodium, zinc, and other chemicals.

Exclusions:

• Cells used for hydrogen production or electroplating.
• Cells that serve as energy sources.

Informational Note: Electrolytic cells and systems vary significantly based on raw materials, output capacity, and proprietary methods, necessitating adaptable code requirements.

Key Requirements for Electrolytic Cell Systems

1. Cell Line Working Zones
   • Defined as spaces near energized surfaces, extending:
     • 96 inches (2.5 m) above energized components.
     • 42 inches (1.0 m) horizontally from energized surfaces.
   • Working zones do not extend through walls, floors, or barriers.
2. Power Supply and Conductors
   • Direct-current (DC) power supplies must not be grounded.
   • Metal enclosures of power supply equipment operating above 50 volts must be grounded using protective relays or a 2/0 AWG copper grounding conductor.
3. Conductor Material and Connections
   • Cell line conductors can be bare, covered, or insulated, using materials like copper or aluminum.
   • Connections must use secure methods such as bolting, welding, or compression fittings.
4. Overcurrent Protection
   • Overcurrent protection is not required for cell line DC process power circuits, as specified in Article 240 exemptions.

Grounding and Portable Equipment

1. Grounding Exceptions
   • Portable equipment frames within the working zone do not require grounding, except when voltage does not exceed 200 volts DC or equipment is guarded.
2. Portable Equipment Requirements
   • Hand-held devices must use isolated ungrounded circuits powered by isolating transformers with ungrounded secondaries.
   • Equipment and receptacles must be clearly marked to prevent improper connections.

Auxiliary Equipment and Controls

1. Auxiliary Nonelectrical Connections
   • Air and water hoses connecting to electrolytic cells must be nonconductive and free of continuous conductive reinforcement.
2. Crane and Hoist Safety
   • Conductive crane and hoist surfaces in the working zone must be insulated from the ground.
   • Controls must use nonconductive materials or isolated circuits to avoid hazardous electrical conditions.

Applications

NEC Article 668 ensures safe operation in:

• Industrial Manufacturing: Aluminum, magnesium, and chlorine production.
• Chemical Processing: High-current systems producing sodium chlorate, fluorine, and hydrogen peroxide.

Conclusion

NEC Article 668 provides a comprehensive safety framework for electrolytic cells, addressing grounding, conductor design, and operational safety. These standards minimize risks and support efficient production in high-current, high-voltage industrial environments.