Underground Cable Systems and Installation

Underground Cable Systems and Installation

Chapter Introduction

Underground cable systems are an essential component of the electrical distribution network. As a powerline technician, you will be called upon to install, splice, terminate, and maintain these systems. This chapter covers all the knowledge required for the Red Seal exam, including cable types, installation methods, voltage calculations, Canadian Electrical Code (CE Code) rules, and safe work procedures.

Types of Underground Cables

Extruded Dielectric Cables

Extruded dielectric cables are the most common in modern underground distribution networks. They consist of an aluminum or copper conductor, a conductor semi-conductive shield, cross-linked polyethylene (XLPE) or ethylene-propylene rubber (EPR) insulation, an insulation semi-conductive shield, and a metallic or non-metallic sheath.

Key characteristics:

Paper-Insulated Lead-Covered Cables (PILC)

Although less common in new installations, PILC cables still exist in many existing networks. They use oil-impregnated paper as the dielectric and require specialized splicing techniques.

Advantages: excellent thermal stability, long service life

Disadvantages: require complex joints, sensitive to moisture, require oil drainage

Gas-Insulated Cables

Used for very high voltages (225 kV and above), these cables use SF₆ (sulfur hexafluoride) as the insulator. Their installation is highly specialized and rarely encountered in standard distribution work.

Cable Identification

Marking and Labeling

Each underground cable must be identified according to the CE Code, Chapter V, Rule 5-102. Identification must include:

Colour Codes

Underground cable outer jackets use a standardized colour code:

Voltage Calculations and Voltage Drop

Voltage Drop Formula

Voltage drop in an underground cable is calculated using the formula:

ΔV = √3 × I × L × (R cos φ + X sin φ) for three-phase systems

ΔV = 2 × I × L × (R cos φ + X sin φ) for single-phase systems

Where:

Calculation Example

Problem: A three-phase 500 kcmil aluminum cable with XLPE insulation is installed over a length of 800 m. The load current is 350 A with a power factor of 0.85. The conductor resistance is 0.120 Ω/km and the reactance is 0.150 Ω/km. Calculate the voltage drop.

Solution:

ΔV = √3 × 350 × 0.8 × (0.120 × 0.85 + 0.150 × sin(cos⁻¹ 0.85))

sin φ = √(1 - 0.85²) = √(1 - 0.7225) = √0.2775 = 0.527

ΔV = √3 × 350 × 0.8 × (0.102 + 0.079)

ΔV = √3 × 350 × 0.8 × 0.181

ΔV = 1.732 × 350 × 0.8 × 0.181

ΔV = 87.8 V

Percentage: (87.8 / 25,000) × 100 = 0.35% (acceptable, as it is less than 3%)

Table of Typical Resistances

Installation Methods

Direct Burial

Direct burial is the most economical method for distribution cables. It involves burying the cable directly in a trench, without conduit.

Requirements according to the CE Code, Chapter V, Rule 8-200:

Conduit Installation

Conduit installation provides superior mechanical protection and facilitates future cable replacement.

Types of conduit:

Installation requirements:

Cable Pulling

Pulling cables through conduits requires special attention to avoid damaging the insulation.

Pulling tension calculation:

T = L × W × f

Where:

Pulling tension limits:

Pulling techniques:

Splices and Joints

Types of Splices

Underground splices fall into two main categories:

Straight splices (joints): connect two cable lengths end to end

Tee splices (branch): connect a branch cable to a main cable

Components of a Splice

A properly made splice includes:

Splice Procedure for XLPE Cable

Step 1: Preparation — cut the cable at a right angle, remove the outer jacket to the required length

Step 2: Remove the metallic shield and the insulation semi-conductive layer

Step 3: Clean the insulation with an appropriate solvent

Step 4: Install the connector by compression (use the correct die)

Step 5: Reconstruct the semi-conductive layer with semi-conductive tape

Step 6: Apply the splice insulation (tape or heat-shrink tube)

Step 7: Reconstruct the metallic shield

Step 8: Apply the outer protection

Important: The insulation semi-conductive layer must be removed without damaging the insulation. Use a special knife with an adjustable cutting depth.

Post-Splice Testing

After each splice, the following tests are required:

Cable Terminations

Indoor Terminations

Indoor terminations are used in substations, transformer chambers, and enclosures. They can be dry-type or with shield termination.

Components:

Outdoor Terminations

Outdoor terminations must withstand environmental conditions (rain, pollution, UV).

Types:

Termination Installation Procedure

Canadian Electrical Code Rules

Chapter V — Applicable Rules

The CE Code, Chapter V, contains the specific rules for underground installations. The main rules are:

Rule 8-200: Cable burial depth

Rule 8-202: Mechanical protection of cables

Rule 8-204: Spacing between cables

Rule 8-206: Underground conduits

Rule 8-208: Manholes

Table of Burial Depths

Thermal Considerations

Cable Ampacity

The ampacity of an underground cable depends on several factors:

Correction Factors

Correction factor for soil temperature:

Correction factor for grouping:

Soil Thermal Resistivity

Fault Location

Location Methods

Time Domain Reflectometer (TDR) method: sends a pulse and measures the reflection time to determine the distance to the fault. Accuracy of ±1% of total length.

Wheatstone bridge method: compares the resistances of two conductors to locate a solid fault. Accuracy of ±2%.

Step voltage method: applies a pulsed voltage and measures the potential gradient at the ground surface. Accuracy of ±0.5 m.

Location Procedure

Safety During Underground Work

Specific Hazards

Electrical hazard: underground cables can remain energized even after disconnection (residual capacitance). Always verify the absence of voltage with an appropriate detector.

Fire hazard: PILC cables contain oil that can ignite. Have an appropriate fire extinguisher nearby.

Gas hazard: underground chambers can contain methane, hydrogen sulfide, or oxygen deficiency. Test the atmosphere before entering.

Collapse hazard: trenches must be shored or have safe slopes according to occupational health and safety regulations.

Mandatory Safety Procedures

Common Pitfalls to Avoid

Pitfall 1: Confusing the burial depth for direct burial (600 mm) with that for conduits under driveways (900 mm). The CE Code, Chapter V, Rule 8-200, specifies different depths depending on the type of installation.

Pitfall 2: Forgetting the correction factor for soil temperature when calculating ampacity. A cable installed in soil at 30 °C must be derated by 11% from its rated value.

Pitfall 3: Using the single-phase voltage drop formula for a three-phase system. The three-phase formula uses √3, not 2.

Pitfall 4: Neglecting the insulation semi-conductive layer during splicing. An improperly removed or reconstructed semi-conductive layer causes partial discharges and premature failure.

Pitfall 5: Confusing copper and aluminum resistance. Aluminum has a resistance approximately 1.6 times higher than copper for the same size.

Pitfall 6: Forgetting the warning tape at 300 mm above the cable. This requirement of Rule 8-202 is often tested on the exam.

Pitfall 7: Using a lubricant that is not compatible with the cable jacket. Some lubricants can attack PVC or polyethylene.

Pitfall 8: Not accounting for the minimum bending radius during pulling. A radius that is too tight damages the insulation invisibly.

Pitfall 9: Confusing the tests required after splicing. The voltage test must be performed at 3 × the rated voltage, not at the rated voltage.

Pitfall 10: Entering an underground chamber without testing the atmosphere. Gas accumulation is a deadly hazard that is often underestimated.

Summary

Underground cable systems are a critical component of electrical distribution networks. To succeed on the Red Seal exam, you must master:

Key points to remember:

Exam tips:

This chapter prepares you for both the theoretical and practical questions on the exam. Mastering these concepts, combined with your practical experience, will allow you to approach the exam with confidence.