High-Voltage Installation and Section 36 of the Canadian Electrical Code (CEC)

Rationale and Objectives of High-Voltage Installation

• Rationale: Certain commercial and industrial sites possess consumer-owned electrical installations operating in excess of $750\,V$. These require specialized consideration due to high-voltage characteristics and potential hazards to personnel and equipment. The Canadian Electrical Code (CEC) addresses these via Section 36: High-Voltage Installation.

• Main Outcome: Learners will be able to determine specific requirements for high-voltage (HV) installations.

• Key Objectives:

  • Describe hazards related to HV installations.

  • Identify components of HV cable and their purposes.

  • Describe the theory of electrical stress control for HV cables.

  • Describe the procedures for splicing and terminating HV cables.

  • Identify CEC requirements for HV installations regarding wiring, protection, and grounding.

• Primary Resources: Canadian Electrical Code, Part I (23rd Edition) and Alberta Electrical STANDATA.

Objective One: High-Voltage Hazards

• Definition of High-Voltage Installation: Any installation where equipment operates at greater than $750\,V$. This includes utility substations and industrial distribution systems.

• Key Hazards:

  • Inadvertent Contact: Awareness is vital because HV bus and lines are often uninsulated or lack sufficient insulation to prevent electrocution. Limits of approach must be strictly maintained; higher voltages require greater safety distances for workers and tools.

  • Environmental Factors: In outdoor substations, snow accumulation under the bus can reduce the distance to overhead conductors, making safe summer routes impassable in winter.

  • Equipment Similarity: Substations often feature repeating patterns (breakers, transformers, bus arrangements). Fatalities have occurred because workers mistakenly climbed energized equipment thinking it was the de-energized unit they were previously working on.

  • Touch Potential: The potential difference between any point on the ground where a person stands and any point that can be touched simultaneously by either hand during a fault.

  • Step Potential: The potential difference between any two points on the ground surface that can be touched simultaneously by a person's feet during a fault.

  • Back Feed: Energizing a circuit from the opposite direction of normal flow (e.g., emergency generators or failure to isolate parallel feeds). Precautions include isolating all energy sources, potential transformers, and applying protective grounds.

  • Mutual Induction: Generation of voltage in a conductor due to nearby changing magnetic fields. High values can build up in long, de-energized lines near energized lines. Precautions include applying grounds to dissipate energy.

  • Capacitive Charge: Charge buildup in long lines, transformers, and motors. Even after removal from service, grounding must be applied to bleed the charge. Voltage can rise again after grounding removal due to dielectric absorption (polarization of the insulation).

  • Arc Flash and Arc Blast: Similar to low-voltage environments but with higher energy levels due to proximity to the source. Mitigation requires Personal Protective Equipment (PPE) and hot line tools (hot sticks).

  • High Voltage Transients: Occur during lightning storms or switching operations. Lightning can cause insulation breakdown; switching can raise system voltage well above the insulation rating.

Preparations and Protective Equipment

• Rubber Gloves: Used for work on energized equipment. Workers must be fully trained as gloves are the primary barrier.

  • Glove Classes and Voltage Ratings:

    • Class 00: $0.5\,kV$

    • Class 0: $1.0\,kV$

    • Class 1: $7.5\,kV$

    • Class 2: $17\,kV$

    • Class 3: $26.5\,kV$

    • Class 4: $36\,kV$

  • Construction: Often dual-colored (inside vs. outside) to help identify if the glove is inside out and to reveal nicks or cracks.

  • Leather Protectors: Must always cover rubber gloves to prevent mechanical damage. Protectors must be shorter than the rubber glove to maintain an insulated guard zone at the top. Coat sleeves must not cover the rubber gloves.

  • Maintenance: Inspected before each use and tested by an authorized facility every six months.

Objective Two: High-Voltage Cable Components

• Conductor Materials: Typically copper (Cu) or aluminum (Al), usually stranded for flexibility.

• Copper Specifics: Smaller for the same conductivity as aluminum, but must be coated with tin or lead alloy to prevent chemical reactions with rubber insulation.

• Stranding Types:

  • Solid: Typically used for smaller conductors up to $No.\,2\,AWG$.

  • Concentric Round: 6 strands helically wrapped around 1 center strand; subsequent layers contain 12, 18, and 24 strands.

  • Compact Round: Strands laid in one direction and rolled to eliminate spaces. Results in a smaller diameter than concentric round but is less flexible.

  • Compact Sector: Used for smaller diameter multi-conductor cables. Stiffer and harder to splice, but cheaper and lighter.

  • Annular: Strands wound around a core (conducting or non-conducting). Used for sizes above $1000\,kcmil$ to mitigate the skin effect, where AC flows more densely near the surface.

  • Segmental: Three or four stranded sectors lightly insulated from each other. Used for single-conductor cables requiring high capacity, minimum diameter, and reduced skin effect.

• Major Cable Components (Sequential Order):

  1. Conductor: Carries the current.

  2. Strand Shield (Conductor Shielding): Conducting/semi-conducting tape or extruded plastic. It fills air gaps, evens surface irregularities, and prevents water penetration.

  3. Insulation: Various types depending on installation needs.

  4. Insulation Shielding (Non-Metallic and Metallic): Confines the electrostatic field to the insulation, creating symmetrical radial voltage stress. Prevents stress concentrations that cause insulation breakdown.

  5. Jacket (Sheath): Outer protective layer against moisture, heat, physical damage, and corrosion. Materials include lead, metallic, rubber, or thermoplastic.

• Insulation Types and Properties:

  • Thermoplastic Polyvinyl Chloride (PVC): High dielectric loss factor; requires careful handling below $0^\circ C$.

  • Thermoplastic Polyethylene (PE): Low power factor/dielectric loss; needs carbon black if exposed to sunlight.

  • Thermoset Cross-Linked Polyethylene (XLPE): Very high dielectric strength, corona resistance, and moisture resistance.

  • Ethylene Propylene Rubber (EPR): Highly resistant to corona discharge and ozone.

Objective Three: Electrical Stress Control and Tracking

• Creepage (Flashover Distance): The distance along the insulation surface between the insulation shield and the bare conductor.

• Tracking Process:

  • Thermal expansion/contraction or excessive bending creates voids (air pockets) in the insulation.

  • Voids ionize under electrostatic force, causing corona discharge.

  • Corona discharge scorches/carbonizes adjacent insulation, creating a conductive path or "track."

  • Tracking leads to eventual cable failure.

• Stress Points: Concentrations of stress occur where the insulation shield ends. If not controlled, this leads to rapid deterioration.

• Stress Cone: The most common device to reduce stress concentrations. It builds a cone-shaped profile of insulation. The insulation shield is extended via semi-conducting tape to the thickest part of the cone, allowing a gradual reduction in voltage stress.

Objective Four: Splicing and Terminating High-Voltage Cables

• General Precautions: Must be performed only by trained personnel. Cleanliness is critical; tools and hands must be free of condensation, perspiration, and dirt.

• General Terminating Procedure:

  1. Roughing Jacket: Use approved abrasive to ensure a moisture-tight seal.

  2. Removing Jacket: Avoid nicking the metallic shield.

  3. Terminating Metallic Shield: Maintain bonding-to-ground (CEC Rule 36-104) using bonding straps (solid, braided, or mesh).

  4. Terminating Non-Metallic Shield: Cut perpendicular to the cable to prevent air gaps; do not nick the insulation.

  5. Removing Insulation: Use a tapering tool to smooth edges and protect against stress/air gaps. Sand and clean with approved fluid to remove semiconductor residue.

  6. Attaching Terminal Lug: Avoid under-crimping (loose connection/heat) or over-crimping (damaged strands). Use anti-corrosion joint compounds.

  7. Building Stress Cone: Follow specific kit instructions based on voltage and environment.

• Termination Classes:

  • Class 1: Track-resistant, moisture-proof, and stress-resistant. Suitable for indoor/outdoor use where contaminants/moisture are present. Uses silicone tape.

  • Class 2: Track and stress resistant.

  • Class 3: Provides stress relief only. Suitable only for clean, dry areas.

Objective Five: CEC Requirements for High-Voltage Installations

• General Requirements:

  • Consulting with the supply authority and inspection department is mandatory (Rule 36-000).

  • Warning Notices (Rule 36-006): Extensive use of signs and diagrams is required. Signs on cable trays must be spaced at maximum $10\,m$ intervals.

• Wiring Methods (Rule 36-100):

  • Indoor ungrounded conductors must be in vaults, electrical rooms, or tray cables in trays.

  • HV Type TC cables must be separated from low-voltage conductors by a sheet metal barrier.

  • Conductors in concrete/masonry require permanent markers every $3\,m$.

  • Current ratings for shielded cables ($5\,kV$ to $46\,kV$) are found in Tables D17A to D17N.

• Radii of Bends (Rule 36-102): Sharp bends cause cable failure. Use Table 15 for minimum radii.

  • Example: For a $31.75\,mm$ diameter tape-shielded cable, the multiplier is 12.

  • $Calculation: 12 \times 31.75\,mm = 381\,mm$ (measured at the innermost surface).

• Shielding Requirements (Rule 36-104): Required for thermoset insulation operating above $2\,kV$ phase-to-phase, with exceptions for underground cables up to $3\,kV$ (if ozone-resistant) and specific isolated indoor installations up to $5\,kV$.

• Spacing and Clearances (Rules 36-108, 36-110):

  • Exposed conductors (busbars) must have minimum clearances found in Tables 30 and 31.

  • Example (Table 31): On a $13800\,V$ system, the max system voltage is $15\,kV$, requiring $400\,mm$ minimum separation outdoors.

  • Vertical clearances for unguarded parts (Table 32) depend on traffic. For a $19920\,V$ phase-to-ground system ($25\,kV$ class) over vehicular traffic, minimum clearance is $5.2\,m$.

  • Joints in Sheathed Cables (Rule 36-114): Must be mechanically and electrically equivalent to the original cable and maintain bonding path continuity.

• Control and Protection:

  • Overcurrent Protection (Rule 36-204): Required on every ungrounded conductor. Fuses must have group-operated, visible break, load-interrupting devices ahead of them.

  • Interlocking (Rule 36-208): Fuse compartments must be interlocked with the supply-side disconnect so the door only opens when de-energized.

  • Disconnecting Means (Rule 36-214): Must allow visible verification of contact position (Open). In feedback scenarios, disconnects must be on both sides of the switch/breaker.

• Grounding and Bonding (Rules 36-300 to 36-312):

  • Station Ground Electrode: Minimum four ground rods, spaced at least the rod length apart, interconnected by $No.\,2/0\,AWG$ bare copper grid.

  • Grid Installation: Loop around equipment, buried max $600\,mm$ below rough grade and min $150\,mm$ below finished grade.

  • Ground Resistance (Rule 36-304): Asphalt or crushed gravel must cover the station area and extend at least $1\,m$ past the station ground electrode.

  • Bond Sizes for Stations (Rule 36-308):

    • Metal structures: $No.\,2/0\,AWG$ Cu.

    • Apparatus on structures: $No.\,2/0\,AWG$ Cu.

    • Water main nearby: $No.\,2/0\,AWG$ Cu connecting every $12\,m$.

    • Equipment (raceways/switchboards): $No.\,4\,AWG$ Cu.

  • Fence Enclosures (Rule 36-312): Metallic fences must be bonded and located at least $1\,m$ inside the station ground grid perimeter.

General Isolation Procedures for HV Equipment

1. Designate a supervisor for the crew.

2. Supervisor operates or directs the operation of isolating switches.

3. Open, lock, and tag all necessary switches (Tag detail: isolation time, date, names of operator and requester).

4. Operator informs supervisor once switches are tagged.

5. Supervisor records information in the logbook.

6. Supervisor informs the requester that isolation is complete.

7. Potential Testing: Test for energy. Use a device to ensure it is safe to ground (check for induction/static).

8. Protective Grounds: Install grounding jumpers or chains as per supply authority rules.

9. Work proceeds only after protective grounds are in place.

10. After work, supervisor ensures all employees are clear.

11. Supervisor directs removal of all protective grounding.

12. Re-confirm employees are clear after grounding removal.

13. Remove isolation tags and log the time, date, and name.

14. Close isolation switches to return to normal operation.

Glossary of Terms

• Belted Type Cable: Multiple conductor cable with an insulation layer over assembled insulated conductors.

• Corona: Result of air molecule ionization into ozone caused by a strong electrical field; appears as a glow and results in energy loss.

• Cross-linking: Process converting thermoplastic material to thermoset material.

• Dielectric Strength: Highest voltage a dielectric can withstand before breakdown; expressed in volts/mil.

• Dielectric Loss: Rate at which electric energy is lost to heat in a dielectric subjected to a changing field.

• Elasticity: Ability of a substance to return to its original shape after deformation.

• Elastomer: Natural or synthetic compound exhibiting high elasticity (stretches to $2\times$ length and returns quickly).

• Flashover: Arcing caused by high-voltage difference between the insulation shield and the exposed conductor.

• Ionization: High voltages creating ozone from atmospheric molecules or air gaps, leading to insulation failure.

• Shielding: Reduces voltage stress to levels below corona-threshold, protects from induced potentials, and reduces shock hazard.

• Thermoplastic: Plastic that softens repeatedly under heat.

• Thermoset: Plastic that does not soften under heat; may discolour or crack instead.