Introduction to BS 6708 Type 20 Mining Cable

The BS 6708 Type 20 mining cable represents a specialized category of galvanized steel pliable wire armoured flexible cables engineered for the most demanding mining and quarrying applications. This cable type is characterized by its robust construction featuring three unscreened cores laid up around an elastomeric cradle without contacting each other, protected by galvanized steel pliable armour that provides exceptional mechanical protection while maintaining the flexibility essential for mobile mining equipment applications.

According to BS 6708:1998 standard documentation, Type 20 and Type 21 cables are specifically categorized as cables with galvanized steel pliable wire armouring having unscreened cores and a rated voltage of 640/1100V. This classification distinguishes them from screened cable types (such as Types 7, 201, 211) that incorporate individual metallic screens around each core, and positions them as the preferred choice for applications requiring maximum mechanical protection without the electromagnetic interference shielding requirements.

The development of Type 20 cables reflects the mining industry’s need for cables that can withstand severe mechanical abuse while maintaining reliable electrical performance in potentially explosive atmospheres. As detailed by AmplexMining.com.au’s comprehensive mining cable guide, mining cables must possess exceptional ruggedness to survive crushing forces from heavy equipment, abrasion from constant contact with rough surfaces, and the mechanical stress of being dragged, twisted, and flexed thousands of times during their service life. The galvanized steel pliable armour in Type 20 cables specifically addresses these requirements while maintaining sufficient flexibility for trailing cable applications.

The unscreened core design of Type 20 cables offers distinct advantages in applications where electromagnetic interference shielding is not critical. This construction results in reduced cable weight and diameter compared to equivalent screened cables, important considerations for mobile equipment where every kilogram of cable weight affects operational efficiency. According to Cleveland Cable Company specifications, Type 20 cables are manufactured with conductor sizes ranging from 2.5mm² to 150mm², enabling power delivery for applications ranging from auxiliary equipment to major excavating machinery.

Purpose and Primary Applications

Core Application Areas

BS 6708 Type 20 mining cables serve as critical power transmission lifelines for heavy-duty mining equipment operating in the most challenging environments. Their primary applications include:

Excavating Machinery

• Electric Shovels: Requiring power delivery up to 320A (150mm² configuration) for digging operations
• Draglines: Large-scale earthmoving equipment with substantial power requirements
• Bucket-wheel Excavators: Continuous mining operations demanding reliable power transmission
• Face Shovels and Hydraulic Excavators: Mobile earth-moving equipment in surface mining

Crushing and Processing Equipment

• Primary Jaw Crushers: High-power motors requiring cables with excellent mechanical protection
• Secondary Cone Crushers: Continuous operation machinery in aggressive environments
• Mobile Crushing Plants: Relocatable equipment requiring trailing cable configurations
• Screening and Sizing Equipment: Processing machinery with variable power demands

Ancillary Mining Equipment

• Conveyor Drive Systems: Material transport infrastructure requiring robust power supply
• Pumping Stations: Dewatering equipment in underground and surface operations
• Ventilation Systems: Critical safety equipment in underground mining
• Loading Equipment: Material handling machinery at mining faces

Operating Environments

The operating environments for Type 20 cables span both underground and surface mining operations:

Underground Mining Applications

In underground coal mines, Type 20 cables power equipment in development headings, longwall panels, and roadway installations. The cables must withstand:

• Methane concentrations up to 5% by volume (Lower Explosive Limit)
• Coal dust accumulation creating explosive atmospheres
• High humidity levels (often exceeding 95% RH)
• Roof convergence and ground movement stresses
• Temperature ranges from 15°C to 45°C in deep mines

Surface Mining Operations

Surface applications present different but equally challenging conditions:

• Extreme temperature variations (-20°C to +85°C ambient)
• UV radiation exposure from solar radiation
• Mechanical damage from mobile equipment operations
• Chemical exposure from mineral processing operations
• Abrasion from rocky terrain and material handling

According to ReelingCable.com technical specifications, professional mining cables for heavy-duty mobile equipment must provide fire retardancy, oil resistance, and temperature performance ranging from -50°C to +80°C. Type 20 cables achieve comparable performance characteristics through their EPR insulation and chloroprene sheath combination, making them suitable for both underground mining, surface mining, and quarry operations.

Quarrying Applications

Quarrying operations utilize Type 20 cables for:

• Dimension Stone Extraction: Granite, marble, and limestone quarries
• Aggregate Production: Sand, gravel, and crushed stone facilities
• Mineral Extraction: Industrial minerals and chemical feedstock production

Cable Construction and Material Structure

The Type 20 cable construction represents sophisticated engineering where each layer serves specific protective and functional purposes. The multi-layer design provides the mechanical robustness essential for mining applications while maintaining electrical performance under adverse conditions.

Dimensional Specifications and Physical Parameters

The dimensional specifications of BS 6708 Type 20 cables reflect careful optimization for mining applications, balancing mechanical protection with practical installation requirements. The following comprehensive table presents the complete dimensional data across the full range of available conductor sizes:

Dimensional Analysis and Engineering Implications

Minimum Bending Radius

The minimum bending radius specifications range from 243mm (2.5mm²) to 786mm (150mm²), representing approximately 9× the overall cable diameter. This ratio is critical for:

• Preventing conductor strand breakage during installation
• Avoiding armour wire damage and distortion
• Maintaining bedding compound integrity
• Ensuring outer sheath does not crack or deform permanently

Cable Weight Considerations

Cable weights ranging from 990 kg/km (2.5mm²) to 12,500 kg/km (150mm²) have significant implications for installation and operation:

Strand Configuration Optimization

The conductor stranding progression demonstrates careful engineering optimization:

• Small conductors (2.5-4mm²): 0.25-0.30mm diameter wires for maximum flexibility
• Medium conductors (6-35mm²): 0.30-0.40mm diameter wires balancing flexibility and manufacturing
• Large conductors (50-150mm²): 0.40-0.50mm diameter wires maintaining practical strand counts

The transition to 0.50mm wires at 70mm² and above keeps strand counts manageable (360-740 strands) while maintaining Class 5 flexibility requirements.

Electrical Characteristics and Performance Parameters

The electrical performance of BS 6708 Type 20 cables is meticulously specified to ensure reliable power transmission in demanding mining applications. The following comprehensive table presents the critical electrical parameters:

Voltage Drop Analysis and System Design

The three-phase voltage drop values (mV/A/m) provide critical information for electrical system design in mining applications. For extended cable runs, voltage drop calculations determine equipment performance:

Conductor Resistance Compliance

The conductor resistance values comply with IEC 60228 Class 5 requirements for tinned copper conductors. The resistance values account for:

• Reference temperature of 20°C
• Tinning process resistance increase (typically 2-3%)
• Stranding factor for fine-wire construction

Temperature correction for conductor resistance at operating temperatures:

Where α₂₀ = 0.00393 /°C for copper

Armour Earth Fault Current Capability

The armour resistance values are critical for earth fault protection coordination. The armour must carry fault current long enough for protective devices to operate. For a phase-to-armour fault:

Where Z_loop includes source impedance, phase conductor resistance, and armour resistance.

Current Carrying Capacity Derating

The continuous current ratings assume specific installation conditions. Derating factors must be applied for:

Standards Compliance and Certification Requirements

Primary Standard: BS 6708

BS 6708:1998 “Flexible cables for use at mines and quarries” provides the comprehensive framework for Type 20 cables. According to Standards.ie BS 6708:1998 documentation, this standard specifies requirements for cables with galvanized steel pliable wire armouring rated at various voltages up to 3800/6600V, with Types 20, 21, 62, 63, 70, 71, 201, 211, 321 and 331 falling under Section 7 of the standard.

Referenced International Standards

Testing Requirements

Comparison with VDE 0250-812 German Standard Mining Cables

Understanding the relationship between BS 6708 Type 20 and German VDE 0250 Part 812 mining cables provides valuable insight into international mining cable specifications and enables proper cable selection for global mining operations.

Key Technical Differences

Voltage Rating and Power Capacity

BS 6708 Type 20 cables operate at 640/1100V, providing higher voltage capability compared to standard VDE 0250-812 cables rated at 450/750V. This higher voltage rating enables:

• Reduced current for equivalent power transfer (P = √3 × V × I × pf)
• Smaller conductor sizes for same power capacity
• Extended cable run lengths with acceptable voltage drop
• Direct connection to 1000V mining distribution systems

Mechanical Protection Philosophy

Type 20 cables feature galvanized steel pliable wire armour as mandatory construction, providing:

• Superior impact and crush resistance
• Protection against cable cuts and abrasion
• Rodent and vermin protection
• Integrated earth fault current path

VDE 0250-812 cables may or may not include armour, with unarmoured variants (NSSHÖU) optimized for maximum flexibility in trailing applications. The DIN VDE 0250 Part 812 standard, according to DIN Media, specifies requirements for rubber insulated flexible cables NSSHÖU, focusing on elastomeric flexibility rather than armoured protection.

Earth Fault Protection Strategy

Temperature Performance

Both cable types utilize EPR insulation with similar thermal characteristics. However, notable differences exist:

Application Suitability

Choose BS 6708 Type 20 when:

• Maximum mechanical protection is required
• Cable will be subject to impact, crushing, or severe abrasion
• Installation in rocky terrain or areas with falling debris
• Higher voltage distribution (1000V systems) is utilized
• Armoured cable is mandated by local mining regulations

Choose VDE 0250-812 when:

• Maximum flexibility for continuous reeling/trailing is needed
• Dedicated earth continuity monitoring is required
• Extreme cold temperature operation (-40°C) is anticipated
• Weight minimization is critical
• European regulatory compliance is mandatory

Safety Thresholds and Critical Precautions

Electrical Safety Parameters

Voltage Withstand and Testing

The 3kV test voltage provides safety margins for the 640/1100V operating voltage:

Current Overload Protection

The maximum conductor temperature of 90°C continuous must not be exceeded. Overload conditions cause:

• Accelerated insulation aging (Arrhenius equation: every 10°C increase halves insulation life)
• Thermal expansion stresses on terminations
• Risk of fire initiation in cable trays
• Softening of bedding compound

Short-Circuit Thermal Limits

The 200°C (maximum 5 seconds) short-circuit temperature limit defines the permissible let-through energy (I²t):

Mechanical Safety Thresholds

Minimum Bending Radius Compliance

The minimum bending radii (243mm to 786mm depending on size) must be respected during:

• Drum winding and storage
• Installation pulling operations
• Route planning around obstructions
• Termination box entry points
• Operational flexing during equipment movement

Armour Integrity Requirements

The galvanized steel pliable armour must maintain:

• Continuous electrical bonding for earth fault protection
• Mechanical integrity for crush and impact protection
• Corrosion resistance through zinc coating preservation
• Proper termination bonding at both cable ends

Environmental Safety Precautions

Explosive Atmosphere Compliance

Type 20 cables for use in potentially explosive atmospheres must comply with:

• Zone classification requirements (Zone 1, Zone 2 for gas; Zone 21, Zone 22 for dust)
• Temperature class limitations (T1-T6 classifications)
• Equipment protection concepts (Ex d, Ex e, Ex ia/ib)
• Cable gland and termination requirements for hazardous areas

Installation Temperature Restrictions

Installation Best Practices and Guidelines

Pre-Installation Verification

Cable Handling and Pulling

Drum Handling

• Cable drums must be stored and transported with axis horizontal
• Never roll drum in direction that would loosen outer wraps
• Use appropriate lifting equipment for drum weights (750 kg for small drums to 5,000+ kg for large cables)
• Provide adequate braking during payout to maintain controlled tension

Pulling Force Limitations

Maximum pulling force calculations must consider conductor strength and pulling method:

Where:

• k = safety factor (typically 0.5 for pulling)
• n = number of conductors (3 for Type 20)
• S = conductor cross-section (mm²)
• σ = conductor tensile strength (approximately 200 N/mm² for copper)

Termination and Bonding Procedures

Armour Bonding Requirements

The galvanized steel armour must be properly terminated to provide earth fault current path:

1. Strip outer chloroprene sheath (150-250mm depending on gland size)
2. Clean armour wires of bedding compound residue
3. Install appropriate cable gland with armour retention cone
4. Ensure metallic contact between armour and gland body
5. Bond gland to earth bar with minimum 10mm² copper conductor
6. Verify armour resistance is within specified limits

Conductor Termination

• Use compression or mechanical lugs rated for tinned copper
• Strip EPR insulation without damaging conductor strands
• Apply anti-oxidant compound to conductor surface (prevents surface oxidation)
• Torque connections to manufacturer specifications
• Apply heat-shrink boots for moisture sealing
• Maintain phase identification throughout termination

Post-Installation Testing

Common Problems and Frequently Asked Questions

Q1: What is the difference between Type 20 and Type 21 BS 6708 cables?

Answer: Both Type 20 and Type 21 are galvanized steel pliable wire armoured cables with unscreened cores rated at 640/1100V. The primary difference lies in the number of cores:

• Type 20: 3 power cores (three-phase supply without separate earth core)
• Type 21: 4 cores (three-phase supply plus dedicated fourth core, typically earth)

According to Demka Electrical Suppliers, both types feature maximum operating temperature of 90°C and maximum short-circuit temperature of 200°C (max. 5 sec), with three or four unscreened cores laid up around an elastomeric cradle without contacting each other. Type 21 is preferred when a dedicated earth conductor is required by regulations or design specifications.

Q2: Why does Type 20 use armour as the earth conductor instead of a dedicated core?

Answer: The armour-as-earth approach offers several advantages:

• Large cross-sectional area: Steel armour provides substantial fault current capacity
• 360° protection: Armour surrounds all cores, providing comprehensive mechanical shield
• Weight optimization: No additional copper for earth conductor reduces cable mass
• Cost efficiency: Steel armour serves dual purpose (mechanical and electrical)

However, armour resistance (0.41-4.2 Ω/km) is higher than equivalent copper conductors. For long runs or high fault current requirements, verify that armour resistance allows adequate fault current for protective device operation. Some installations may require supplementary earth conductors.

Q3: Can Type 20 cables be used for continuous trailing (reeling) applications?

Answer: Type 20 cables are not optimized for continuous reeling applications. The galvanized steel pliable armour, while providing excellent mechanical protection, has limitations for repeated flexing:

• Armour wires may fatigue after extensive flexing cycles
• Weight (990-12,500 kg/km) increases reeling equipment stress
• Armour adds stiffness compared to unarmoured cables
• Bending radius requirements (243-786mm) limit drum sizing

For continuous reeling applications, consider BS 6708 Type 7 (screened, non-armoured) or VDE 0250-812 NSSHCGEÖU cables specifically designed for repeated flexing. Type 20 is suitable for semi-permanent installations where cables are relocated infrequently.

Q4: What maintenance schedule is recommended for Type 20 installations?

Answer: Recommended maintenance intervals based on operating environment severity:

Q5: How do I calculate voltage drop for extended cable runs?

Answer: Use the three-phase voltage drop values from the electrical specifications table:

Q6: What causes premature armour corrosion and how can it be prevented?

Answer: Armour corrosion causes include:

• Moisture ingress: Water entering through damaged sheath or poor terminations
• Chemical attack: Acidic mine water or processing chemicals
• Galvanic corrosion: Contact with dissimilar metals (copper terminations)
• Zinc coating damage: Mechanical abrasion removing protective layer

Prevention measures:

• Use IP68-rated cable glands for water ingress protection
• Apply corrosion-inhibiting compounds at terminations
• Install cables above expected water levels where possible
• Perform regular visual inspections for sheath damage
• Replace cables showing signs of armour corrosion before failure

Q7: Can Type 20 cables be spliced or repaired in the field?

Answer: Field splicing of Type 20 cables is strongly discouraged due to:

• Difficulty achieving equivalent armour protection at splice
• Risk of moisture ingress at joint location
• Reduced mechanical strength at splice point
• Regulatory restrictions in hazardous areas
• Warranty and liability implications

If splicing is absolutely necessary (due to cable damage), use only approved armoured cable joint kits specifically designed for mining applications, and ensure compliance with local mining electrical regulations. Many mining operations mandate complete cable replacement rather than field repair.