Four-Core Galvanized Steel Pliable Wire Armoured Cable with Dedicated Earth Conductor for Mining Excavating and Crushing Equipment in Explosive Atmosphere Applications — Comprehensive Technical Analysis
What is BS 6708 Type 21 Trailing Cable?
Four-Core Galvanized Steel Pliable Wire Armoured Cable with Dedicated Earth Conductor for Mining Excavating and Crushing Equipment in Explosive Atmosphere Applications — Comprehensive Technical Analysis
Introduction to BS 6708 Type 21 Mining Cable
The BS 6708 Type 21 mining cable represents an advanced variant of galvanized steel pliable wire armoured flexible cables specifically engineered for demanding mining and quarrying applications requiring a dedicated fourth earth conductor. This four-core configuration distinguishes Type 21 from the three-core Type 20, providing enhanced safety features and compliance with electrical regulations that mandate separate protective earth conductors in mining installations.
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. The critical distinction is the core count: Type 20 provides three cores (relying on armour for earth return), while Type 21 provides four cores with a dedicated earth conductor providing enhanced fault protection and regulatory compliance.
The development of Type 21 cables addresses the mining industry’s evolving safety requirements where separate earth conductors ensure more reliable protective earth systems. As documented 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. Type 21 cables achieve this while providing the additional safety assurance of a dedicated earth path.
With copper weights ranging from 96 kg/km (4×2.5mm²) to 5,760 kg/km (4×150mm²), Type 21 cables demonstrate significant investment in conductor material, reflecting their robust construction for heavy-duty mining applications. The additional fourth core increases cable diameter and weight compared to Type 20 equivalents, but provides critical safety advantages that justify these increases in demanding applications.
Type 21 vs Type 20: Comparative Analysis
Understanding the differences between BS 6708 Type 21 and Type 20 cables is essential for proper cable selection. Both cable types share the same fundamental construction principles but differ in core configuration and earthing philosophy:
| Parameter | Type 20 (3-Core) | Type 21 (4-Core) | Difference |
|---|---|---|---|
| Core Configuration | 3×Phase cores | 3×Phase + 1×Earth core | +1 dedicated earth |
| Earth Return Path | Via steel armour only | Dedicated copper conductor + armour | Dual earth path |
| 25mm² Max. Diameter | 47.0 mm | 49.8 mm | +2.8 mm (+6.0%) |
| 25mm² Cable Weight | 3,350 kg/km | 4,030 kg/km | +680 kg/km (+20.3%) |
| 25mm² Copper Weight | 720 kg/km | 960 kg/km | +240 kg/km (+33.3%) |
| 25mm² Min. Bend Radius | 423 mm | 449 mm | +26 mm (+6.1%) |
| Earth Fault Current Path | Limited by armour resistance | Full copper conductor capacity | Superior fault clearing |
| Earth Continuity Monitoring | Armour resistance measurement | Direct conductor continuity test | Simpler verification |
Quantitative Comparison: 120mm² Configuration
Physical Dimensions:
• Type 20: Ø77.0mm, 10,650 kg/km, 3,456 kg Cu/km
• Type 21: Ø89.0mm, 13,450 kg/km, 4,608 kg Cu/km
• Diameter Increase: +12.0mm (+15.6%)
• Weight Increase: +2,800 kg/km (+26.3%)
• Copper Increase: +1,152 kg/km (+33.3%)
Electrical Parameters:
• Current Rating: 295A (identical for both types)
• Conductor Resistance: 0.164 Ω/km (identical)
• Type 20 Armour Resistance: 0.74 Ω/km
• Type 21 Armour Resistance: 0.47 Ω/km
• Type 21 Earth Conductor Resistance: 0.164 Ω/km
Earth Fault Current Advantage:
Type 21 provides parallel earth paths: copper conductor (0.164 Ω/km) in parallel with armour (0.47 Ω/km), resulting in effective earth resistance of approximately 0.121 Ω/km—significantly lower than Type 20’s armour-only resistance of 0.74 Ω/km.
Selection Criteria: When to Choose Type 21
Choose Type 21 when:
- Local mining regulations mandate separate earth conductors
- Maximum earth fault current capacity is required
- Earth continuity monitoring systems require dedicated conductor
- Critical equipment demands highest safety standards
- Insurance or certification requirements specify four-core cables
- Longer cable runs where armour resistance becomes limiting factor
Type 20 may be acceptable when:
- Regulations permit armour-only earth systems
- Weight minimization is critical
- Space constraints limit cable diameter
- Short cable runs with adequate armour conductivity
- Cost optimization is primary consideration
Purpose and Primary Applications
Core Application Areas
BS 6708 Type 21 mining cables serve as critical power transmission infrastructure for heavy-duty mining equipment, with the dedicated earth conductor providing enhanced safety assurance in the following applications:
Primary Power Distribution
- Excavating Machinery: Electric shovels, draglines, and bucket-wheel excavators requiring up to 320A continuous current (150mm² configuration)
- Crushing Equipment: Primary jaw crushers, secondary cone crushers, and mobile crushing plants with high starting currents
- Material Handling: Conveyors, stackers, and reclaimers with variable load demands
- Processing Equipment: Screening systems, feeders, and sizing equipment
Underground Mining Applications
Type 21 cables excel in underground environments where enhanced earth protection is critical:
- Development Headings: Continuous miners and roadheaders
- Longwall Operations: Armoured face conveyors and shearer loaders
- Auxiliary Systems: Ventilation fans and dewatering pumps
- Safety-Critical Equipment: Emergency systems requiring reliable earth protection
Type 21 cables are specifically designed for use in:
• Deep mines where explosive gases (methane CH₄ concentrations up to 5% LEL) may accumulate
• Areas with combustible dust (coal dust with particle sizes <75μm)
• Surface mining operations with extreme environmental exposure
• Temperature range: -20°C to +85°C (fixed installation)
• Installation temperature: +5°C to +60°C
• Maximum conductor temperature: 90°C continuous operation
Quarrying and Surface Mining
Surface applications benefit from Type 21’s robust construction:
- Dimension Stone Quarries: Granite, marble, and limestone extraction with mobile equipment
- Aggregate Production: Sand, gravel, and crushed stone processing facilities
- Open-Pit Mining: Large-scale earth-moving operations with electric shovels
- Mineral Processing: Concentration plants and material handling systems
According to ReelingCable.com technical specifications, professional mining cables for heavy-duty mobile equipment must provide fire retardancy, oil resistance, and robust mechanical protection. Type 21 cables achieve these requirements through their EPR insulation, chloroprene sheath, and galvanized steel pliable armour combination, making them suitable for underground mining, surface mining, and quarry operations requiring the highest safety standards.
Cable Construction and Material Structure
The Type 21 cable construction represents sophisticated multi-layer engineering where each component serves specific protective and functional purposes. The addition of the fourth earth core requires optimized layup geometry while maintaining the mechanical robustness essential for mining applications.
All four conductors (three phase + one earth) utilize electrolytic, stranded, tinned copper wire conforming to IEC 60228 Class 5 specifications. This classification provides maximum flexibility through fine-wire construction:
• 4×2.5mm²: 50 strands × 0.25mm diameter = 3.13mm² actual
• 4×25mm²: 196 strands × 0.40mm diameter = 24.6mm² actual
• 4×70mm²: 360 strands × 0.50mm diameter = 70.7mm² actual
• 4×150mm²: 740 strands × 0.50mm diameter = 145.5mm² actual
Material Properties:
• Electrical Conductivity: ≥58 MS/m (100% IACS)
• Tinning: Hot-dip or electroplating (2-3% resistance increase)
• Tensile Strength: ~200 N/mm² (annealed copper)
• Elongation: ≥30% (ensures flexibility without breakage)
EPR insulation provides exceptional electrical isolation with superior mechanical and thermal properties. According to Eland Cables technical reference, EPR offers high dielectric strength, excellent ozone and weathering resistance, and a wide thermal range from -55°C to 150°C.
• Dielectric Strength: >20 kV/mm
• Volume Resistivity: >10¹⁴ Ω·cm
• Dielectric Constant (εr): 2.5-3.5 @ 1 kHz
• Dissipation Factor (tan δ): <0.01 @ 1 kHz
• Temperature Rating: 90°C continuous, 200°C short-circuit
Insulation Thickness (per core):
• 4×2.5mm² to 4×4mm²: 1.4 mm
• 4×6mm² to 4×16mm²: 1.5 mm
• 4×25mm² to 4×50mm²: 1.6-1.7 mm
• 4×70mm² to 4×95mm²: 1.8-2.0 mm
• 4×120mm² to 4×150mm²: 2.2-2.3 mm
Four unscreened cores (three phase + one earth) are laid up around an elastomeric cradle without contacting each other. This configuration differs from Type 20’s three-core arrangement:
• Geometric Symmetry: Square/rhomboid configuration provides balanced mechanical stress distribution
• Thermal Balance: Earth core provides heat sink effect, improving thermal performance
• Electrical Symmetry: Four-core arrangement reduces electromagnetic unbalance
• Fault Isolation: Earth core provides physical separation between phases
• Identification: Distinct earth core color coding (green/yellow) for installation verification
The elastomeric cradle (EPR-based compound) maintains consistent core spacing throughout cable length, ensuring uniform electrical characteristics and accommodating differential thermal expansion during operation.
Rubber-based bedding compound provides critical interface protection between cable cores and metallic armour:
• 4×2.5mm² to 4×4mm²: 2.0 mm
• 4×6mm² to 4×50mm²: 2.5 mm
• 4×70mm² to 4×95mm²: 2.5-3.8 mm
• 4×120mm² to 4×150mm²: 3.8-5.0 mm
Functions:
• Mechanical cushioning preventing armour damage to insulation
• Fire barrier retarding flame propagation along cores
• Moisture barrier preventing water migration
• Accommodation of differential thermal expansion
The galvanized steel pliable armour comprises seven-wire strands wound helically to provide mechanical protection while maintaining flexibility. According to Tratos Group armoured cable specifications, pliable wire armour (PWA) cables are specifically designed for mining applications requiring armoured protection with occasional movement capability.
• Configuration: 7-wire strands (concentric helical winding)
• Wire Diameters:
– 4×2.5mm² to 4×4mm²: 7/0.45 mm
– 4×6mm² to 4×16mm²: 7/0.71 mm
– 4×25mm² to 4×120mm²: 7/0.90 mm
– 4×150mm²: 7/1.25 mm
• Galvanization: Hot-dip zinc ≥40 g/m²
• Material: Low-carbon steel (350-550 N/mm² tensile)
• Coverage: ≥85% mechanical protection
The armour provides dual functionality in Type 21 cables: primary mechanical protection and secondary earth fault current path in parallel with the dedicated copper earth conductor.
The heavy-duty chloroprene outer sheath provides comprehensive environmental protection. According to Tratos Cable Academy, chloroprene offers self-extinction properties, excellent resistance to solvents and oils, and superior weathering characteristics—qualities specifically mandated for colliery trailing cables.
• 4×2.5mm² to 4×4mm²: 2.5 mm
• 4×6mm² to 4×16mm²: 3.8-3.9 mm
• 4×25mm² to 4×50mm²: 4.3-5.1 mm
• 4×70mm² to 4×120mm²: 5.7-6.8 mm
• 4×150mm²: 7.3 mm
Performance Characteristics:
• Tensile Strength: 10-15 MPa (minimum)
• Elongation at Break: 300-400%
• Abrasion Resistance: Superior to natural rubber
• Temperature Range: -40°C to +120°C (intermittent)
• Oil Resistance: Good (mineral oils and greases)
• Flame Retardance: Self-extinguishing (EN 60332-1-2)
Dimensional Specifications and Physical Parameters
The dimensional specifications of BS 6708 Type 21 cables reflect the enhanced construction required for four-core armoured mining cables. The following comprehensive table presents complete dimensional data:
| Cross Section (mm²) | Cores | Stranding (mm) | Conductor Dia. (mm) | Insulation Thick. (mm) | Inner Sheath (mm) | Armour Size | Outer Sheath (mm) | Max. OD (mm) | Min. Bend Radius (mm) |
|---|---|---|---|---|---|---|---|---|---|
| 4×2.5 | 4×2.5 | 50/0.25 | 2.10 | 1.4 | 2.0 | 7/0.45 | 2.5 | 28.6 | 258 |
| 4×4 | 4×4 | 56/0.30 | 2.70 | 1.4 | 2.0 | 7/0.45 | 2.5 | 30.1 | 271 |
| 4×6 | 4×6 | 84/0.30 | 3.30 | 1.5 | 2.5 | 7/0.71 | 3.8 | 38.8 | 350 |
| 4×10 | 4×10 | 80/0.40 | 4.40 | 1.5 | 2.5 | 7/0.71 | 3.8 | 41.0 | 369 |
| 4×16 | 4×16 | 126/0.40 | 5.50 | 1.5 | 2.5 | 7/0.71 | 3.9 | 44.0 | 396 |
| 4×25 | 4×25 | 196/0.40 | 7.10 | 1.6 | 2.5 | 7/0.90 | 4.3 | 49.8 | 449 |
| 4×35 | 4×35 | 276/0.40 | 8.40 | 1.6 | 2.5 | 7/0.90 | 4.7 | 56.1 | 505 |
| 4×50 | 4×50 | 396/0.40 | 10.10 | 1.7 | 2.5 | 7/0.90 | 5.1 | 61.8 | 557 |
| 4×70 | 4×70 | 360/0.50 | 11.90 | 1.8 | 2.5 | 7/0.90 | 5.7 | 71.4 | 643 |
| 4×95 | 4×95 | 475/0.50 | 13.90 | 2.0 | 3.8 | 7/0.90 | 6.3 | 78.6 | 708 |
| 4×120 | 4×120 | 629/0.50 | 15.80 | 2.2 | 3.8 | 7/0.90 | 6.8 | 89.0 | 801 |
| 4×150 | 4×150 | 740/0.50 | 17.00 | 2.3 | 5.0 | 7/1.25 | 7.3 | 95.3 | 858 |
Weight and Material Analysis
| Cross Section (mm²) | Cable Weight (kg/km) | Copper Weight (kg/km) | Copper % of Total | Weight per 100m (kg) | Cost Implication (Relative) |
|---|---|---|---|---|---|
| 4×2.5 | 1,150 | 96 | 8.3% | 115 | Base |
| 4×4 | 1,320 | 154 | 11.7% | 132 | 1.15× |
| 4×6 | 2,350 | 230 | 9.8% | 235 | 2.04× |
| 4×10 | 2,740 | 384 | 14.0% | 274 | 2.38× |
| 4×16 | 3,170 | 614 | 19.4% | 317 | 2.76× |
| 4×25 | 4,030 | 960 | 23.8% | 403 | 3.50× |
| 4×35 | 5,220 | 1,344 | 25.7% | 522 | 4.54× |
| 4×50 | 6,400 | 1,920* | 30.0% | 640 | 5.57× |
| 4×70 | 8,060 | 2,688 | 33.4% | 806 | 7.01× |
| 4×95 | 10,650 | 3,648 | 34.3% | 1,065 | 9.26× |
| 4×120 | 13,450 | 4,608 | 34.3% | 1,345 | 11.70× |
| 4×150 | 15,300 | 5,760 | 37.6% | 1,530 | 13.30× |
*Note: 4×50mm² copper weight appears to have a typo in source data (192 kg/km); estimated value of 1,920 kg/km used based on scaling.
Dimensional Analysis
Examples:
• 4×25mm²: 449mm ÷ 49.8mm = 9.0×
• 4×95mm²: 708mm ÷ 78.6mm = 9.0×
• 4×150mm²: 858mm ÷ 95.3mm = 9.0×
The consistent 9× ratio ensures uniform mechanical protection across all cable sizes, preventing conductor strand breakage, armour distortion, and insulation damage.
Electrical Characteristics and Performance Parameters
The electrical performance specifications of BS 6708 Type 21 cables are meticulously engineered to ensure reliable power transmission in demanding mining applications while providing enhanced earth fault protection through the dedicated fourth conductor:
| Cross Section (mm²) | Continuous Current Rating @ 25°C (A) | Conductor Resistance (Ω/km) | Armour Resistance (Ω/km) | Effective Earth Resistance (Ω/km)* | Earth Fault Advantage |
|---|---|---|---|---|---|
| 4×2.5 | 28 | 8.21 | 4.2 | 2.78 | 33.8% lower |
| 4×4 | 37 | 5.09 | 3.9 | 2.21 | 43.3% lower |
| 4×6 | 46 | 3.39 | 2.0 | 1.26 | 37.0% lower |
| 4×10 | 63 | 1.95 | 1.9 | 0.96 | 49.5% lower |
| 4×16 | 85 | 1.24 | 1.7 | 0.72 | 57.6% lower |
| 4×25 | 110 | 0.795 | 1.5 | 0.52 | 65.3% lower |
| 4×35 | 135 | 0.565 | 1.1 | 0.37 | 66.4% lower |
| 4×50 | 170 | 0.393 | 0.96 | 0.28 | 70.8% lower |
| 4×70 | 205 | 0.277 | 0.80 | 0.21 | 73.8% lower |
| 4×95 | 250 | 0.210 | 0.73 | 0.16 | 78.1% lower |
| 4×120 | 295 | 0.164 | 0.47 | 0.12 | 74.5% lower |
| 4×150 | 320 | 0.132 | 0.41 | 0.10 | 75.6% lower |
*Effective Earth Resistance calculated as parallel combination of conductor and armour: Reff = (Rcond × Rarmour) ÷ (Rcond + Rarmour)
Earth Fault Current Analysis
Type 20 (3-core, armour only):
Armour resistance: 0.80 Ω/km × 0.2km = 0.16 Ω
Phase resistance: 0.210 Ω/km × 0.2km = 0.042 Ω
Loop resistance: 0.16 + 0.042 = 0.202 Ω
Fault current (640V phase): 640 ÷ 0.202 = 3,168A
Type 21 (4-core, parallel paths):
Effective earth: 0.16 Ω/km × 0.2km = 0.032 Ω
Phase resistance: 0.210 Ω/km × 0.2km = 0.042 Ω
Loop resistance: 0.032 + 0.042 = 0.074 Ω
Fault current (640V phase): 640 ÷ 0.074 = 8,649A
Improvement: 173% higher fault current for faster protective device operation
Voltage Drop Calculations
For Type 21 cables, voltage drop can be calculated using standard three-phase formula. Since all four conductors have identical construction, the fourth earth conductor has the same resistance as phase conductors:
Where X (reactance) ≈ 0.08 Ω/km for mining cables. For typical power factor of 0.85:
Cable: 4×70mm², Length: 150m, Load: 180A, pf = 0.85
R = 0.277 Ω/km × 0.15 km = 0.0416 Ω
X = 0.08 Ω/km × 0.15 km = 0.012 Ω
cos φ = 0.85, sin φ = 0.527
ΔV = 1.732 × 180 × (0.0416 × 0.85 + 0.012 × 0.527)
ΔV = 311.8 × (0.0354 + 0.0063)
ΔV = 311.8 × 0.0417 = 13.0V
Percentage drop = (13.0 ÷ 1100) × 100 = 1.18%
Well within 5% limit for mining motor circuits.
Temperature Derating Factors
Current ratings assume 25°C ambient temperature. Apply derating factors for elevated temperatures:
| Ambient Temp. (°C) | Derating Factor | 4×95mm² Rated (A) | 4×150mm² Rated (A) |
|---|---|---|---|
| 25 (reference) | 1.00 | 250 | 320 |
| 35 | 0.94 | 235 | 301 |
| 45 | 0.87 | 218 | 278 |
| 55 | 0.79 | 198 | 253 |
| 65 | 0.71 | 178 | 227 |
| 75 | 0.61 | 153 | 195 |
Short-Circuit Withstand Capability
Maximum Short-Circuit Currents (1-second duration):
• 4×25mm²: √(143² × 25²) = 3,575A
• 4×70mm²: √(143² × 70²) = 10,010A
• 4×120mm²: √(143² × 120²) = 17,160A
• 4×150mm²: √(143² × 150²) = 21,450A
Standards Compliance and Certification
Primary Standard: BS 6708:1998
According to Standards.ie BS 6708:1998 documentation, Type 21 cables fall under Section 7: “Cables with galvanized steel pliable wire armouring, rated voltages up to 1900/3300V (types 20, 21, 62, 63, 70, 71, 201, 211, 321 and 331).”
• Section 7: Steel pliable wire armoured cables
• Voltage Rating: 640/1100V (U₀/U)
• Test Voltage: 3kV AC
• Core Configuration: Type 21 = 4 unscreened cores
• Application: Explosive atmospheres (IEC 29.260.20)
Supporting Annexes:
• Annex A: Approved codes of practice
• Annex B: Recommended current ratings
• Annex C: Short-circuit current capability
Referenced International Standards
| Standard | Title | Application |
|---|---|---|
| IEC 60228 | Conductors of insulated cables | Class 5 flexible conductor requirements |
| IEC 60724 | Short-circuit temperature limits | 200°C maximum for EPR (5 sec) |
| BS 7655-1.2 | Elastomeric insulating compounds | EPR compound specifications |
| BS 7655-2.3 | Elastomeric sheathing compounds | Chloroprene sheath requirements |
| EN 60332-1-2 | Flame propagation test | Single cable vertical flame test |
| IEC 60502-1 | Power cables 1kV to 3kV | General construction requirements |
Testing and Certification Requirements
☐ Conductor resistance verification (IEC 60228 Class 5)
☐ Voltage withstand: 3kV AC for 5 minutes
☐ Insulation resistance: ≥100 MΩ·km @ 20°C
☐ Partial discharge: <10 pC @ 1.5 × U₀
☐ Bending test: Minimum radius compliance
☐ Hot set test: EPR cross-linking verification
☐ Tensile/elongation: EPR and chloroprene
☐ Armour galvanization: ≥40 μm zinc coating
☐ Flame retardance: EN 60332-1-2 pass
☐ Core identification: Color coding verification
☐ Conductor resistance per unit length
☐ High-voltage test @ 3kV AC
☐ Insulation resistance (each core to armour)
☐ Armour continuity verification
☐ Earth conductor identification (green/yellow)
☐ Dimensional compliance check
☐ Visual inspection for defects
Comparison with VDE 0250-812 German Standard
Understanding the relationship between BS 6708 Type 21 and German DIN VDE 0250 Part 812 mining cables enables proper cable selection for international mining operations:
BS 6708 Type 21
- ✓ Rated Voltage: 640/1100V
- ✓ Cores: 4 (3P + 1E)
- ✓ Insulation: EPR
- ✓ Protection: Galvanized steel pliable armour
- ✓ Sheath: Heavy-duty chloroprene
- ✓ Temperature: -20°C to +85°C
- ✓ Short-Circuit: 200°C (5 sec)
- ✓ Test Voltage: 3kV
- ✓ Earth System: Dedicated Cu + armour parallel
- ✓ Armour Configuration: 7-wire strands
VDE 0250-812 NSSHÖU
- ✓ Rated Voltage: Up to 450/750V
- ✓ Cores: Various configurations
- ✓ Insulation: EPR (compound 3GI3)
- ✓ Protection: Optional armour
- ✓ Sheath: Chloroprene (compound 5GM5)
- ✓ Temperature: -40°C to +80°C
- ✓ Short-Circuit: 250°C
- ✓ Test Voltage: 2-3kV
- ✓ Earth System: Concentric monitoring conductor
- ✓ Armour Configuration: Steel/Cu braid (if equipped)
Key Technical Differences
Voltage Rating Comparison
BS 6708 Type 21’s 640/1100V rating versus VDE 0250-812’s 450/750V provides:
For 150kW motor load (pf = 0.85):
At 750V (VDE):
I = 150,000 ÷ (1.732 × 750 × 0.85) = 135.8A
Requires 70mm² cable (rated 205A)
At 1100V (BS 6708):
I = 150,000 ÷ (1.732 × 1100 × 0.85) = 92.6A
Adequate with 50mm² cable (rated 170A)
Result: Potential one size reduction in conductor, saving ~25% in cable cost
Mechanical Protection Philosophy
Type 21 cables feature mandatory galvanized steel pliable wire armour providing:
- Impact resistance from falling rocks and equipment
- Crush protection in high-traffic mining areas
- Rodent and vermin protection
- Dual earth path (copper conductor + armour parallel)
VDE 0250-812 cables may be unarmoured (NSSHÖU) for maximum flexibility, or include optional armour configurations. According to DIN Media, DIN VDE 0250-812 specifies rubber insulated flexible cables NSSHÖU optimized for trailing applications requiring maximum flexibility rather than armoured protection.
Earth Conductor Configuration
| Feature | BS 6708 Type 21 | VDE 0250-812 NSSHCGEÖU |
|---|---|---|
| Earth Conductor | Full-size copper (same as phase) | Concentric monitoring conductor |
| Earth Path | Parallel: Cu core + steel armour | Concentric PE + steel/Cu braid |
| Monitoring | Resistance measurement | Dedicated pilot cores available |
| Fault Current | High (full conductor capacity) | Variable (depends on configuration) |
Application Suitability
Choose BS 6708 Type 21 when:
- Maximum mechanical protection required
- Regulations mandate separate earth conductor
- Highest earth fault current capacity needed
- 1000V distribution systems utilized
- British/Commonwealth mining regulations apply
Choose VDE 0250-812 when:
- Maximum flexibility for continuous reeling required
- Continuous earth monitoring via pilot cores needed
- Extreme cold operation (-40°C) anticipated
- Weight minimization critical
- European regulatory compliance mandatory
Safety Thresholds and Critical Precautions
BS 6708 Type 21 cables operate in potentially explosive atmospheres. Improper installation, operation, or maintenance can result in ignition of flammable gases, fires, electrical shock fatalities, or catastrophic equipment damage. All work must be performed by competent personnel in accordance with applicable mining safety legislation, manufacturer specifications, and local electrical codes.
Electrical Safety Parameters
Voltage Withstand and Safety Margins
• Rated Voltage: 640/1100V (U₀/U)
• Test Voltage: 3,000V AC
• Safety Factor: 3,000 ÷ 1,100 = 2.73×
This margin accommodates:
• Switching transients (up to 2× nominal)
• Lightning-induced surges
• Motor starting effects
• Insulation degradation over service life
• Manufacturing tolerances
Overcurrent Protection Requirements
Exceeding this temperature causes:
• Accelerated insulation aging (halved life per 10°C increase)
• Thermal expansion stress on terminations
• Fire risk in cable assemblies
• Bedding compound softening
Protection Device Coordination:
• 120% rated current: Operate <2 hours
• 150% rated current: Operate <30 minutes
• 200% rated current: Operate <2 minutes
• 500% rated current: Operate <10 seconds
Earth Continuity Verification
The dedicated fourth earth conductor provides enhanced safety verification:
Type 21 Advantage: Two independent earth paths can be verified:
1. Copper earth conductor: Direct continuity test
2. Steel armour: Resistance measurement
3. Combined effectiveness: Parallel path calculation
Example (4×95mm², 300m run):
• Earth conductor: 0.210 Ω/km × 0.3 = 0.063 Ω
• Armour: 0.73 Ω/km × 0.3 = 0.219 Ω
• Parallel: (0.063 × 0.219) ÷ (0.063 + 0.219) = 0.049 Ω
Total earth loop impedance (including source): ~0.15 Ω
Earth fault current: 640 ÷ 0.15 = 4,267A
Protective device operating time: <0.1 second
Mechanical Safety Thresholds
Bending Radius Compliance
Exceeding minimum bending radius (258mm to 858mm) causes:
• Immediate: Conductor strand breakage, armour wire distortion
• Progressive: Water ingress, insulation damage, increased resistance
• Catastrophic: Phase-to-earth faults, cable fires, explosions
Critical Points:
• Cable drum winding
• Route changes around obstructions
• Entry to termination boxes
• Support structure transitions
Installation Temperature Restrictions
• Below 5°C: EPR and chloroprene become brittle, risking cracking
• Above 60°C: Materials become too soft, causing permanent deformation
Pre-condition cables stored in cold environments before installation. Allow cables stored in hot conditions to cool before handling.
Explosive Atmosphere Compliance
Type 21 cables for explosive atmospheres must comply with:
• Zone classification (Zone 1, 2 for gas; Zone 21, 22 for dust)
• Temperature class limitations (T1-T6)
• Equipment protection concepts (Ex d, Ex e, Ex ia/ib)
• Certified cable glands and terminations
Maximum Surface Temperature:
With 90°C conductor rating and appropriate derating:
• T4 classification (135°C): Compliant
• T5 classification (100°C): Marginal—verify sheath temperature
• T6 classification (85°C): May require additional derating
Installation Guidelines and Best Practices
Pre-Installation Verification
☐ Verify cable drum condition (no damage, proper rotation direction)
☐ Inspect outer sheath for cuts, abrasions, deformation
☐ Check armour continuity with low-resistance ohmmeter
☐ Measure insulation resistance (each core to armour, core to core)
☐ Verify ambient temperature within 5°C-60°C range
☐ Confirm cable specification matches design requirements
☐ Review route for bending radius compliance
☐ Ensure proper equipment for cable weight handling
☐ Verify earth conductor identification (green/yellow marking)
Cable Handling and Pulling
Weight Considerations
Type 21 cables are significantly heavier than equivalent unarmoured cables:
• 4×25mm²: 100m = 403 kg—Manual handling possible with team
• 4×70mm²: 100m = 806 kg—Mechanical assistance required
• 4×120mm²: 100m = 1,345 kg—Heavy lifting equipment mandatory
• 4×150mm²: 100m = 1,530 kg—Crane or specialized drum trailer required
Cable drums may weigh 2-5 tonnes total with cable and drum weight.
Maximum Pulling Force
Where k = 0.5 (safety factor), n = 4 (all conductors), S = cross-section, σ ≈ 200 N/mm²
• 4×25mm²: F = 0.5 × 4 × 25 × 200 = 10,000 N
• 4×70mm²: F = 0.5 × 4 × 70 × 200 = 28,000 N
• 4×120mm²: F = 0.5 × 4 × 120 × 200 = 48,000 N
• 4×150mm²: F = 0.5 × 4 × 150 × 200 = 60,000 N
Termination and Bonding Procedures
Four-Core Termination
- Strip outer chloroprene sheath (150-250mm)
- Clean and prepare armour wires for gland retention
- Install appropriate armoured cable gland with compression cone
- Bond gland to earth bar (minimum 10mm² bonding conductor)
- Identify earth core (green/yellow) and verify continuity
- Connect phase conductors with proper phase rotation
- Connect earth conductor to equipment earth terminal
- Apply heat-shrink boots for moisture sealing
- Torque all connections to manufacturer specifications
Armour and Earth Bonding
Both earth paths must be properly bonded:
1. Copper earth conductor: Connected to equipment earth terminal
2. Steel armour: Bonded via cable gland to earth bar
Both paths should be tested independently before energization. Total earth loop impedance must be verified for protective device coordination.
Post-Installation Testing
1. Visual Inspection: Route compliance, gland tightness, identification
2. Continuity Test: All four conductors plus armour bond
3. Earth Conductor Verification: Dedicated earth path continuity
4. Insulation Resistance: Phase-to-phase, phase-to-earth, phase-to-armour
• Minimum acceptable: 1 MΩ
• Typical new cable: 100-1000 MΩ
5. Earth Loop Impedance: Both paths (conductor and armour)
6. Phase Rotation Test: Confirm correct sequence for motor loads
7. Documentation: Record all test results for future reference
Common Problems and Frequently Asked Questions
Q1: Why choose Type 21 over Type 20 if both meet BS 6708?
Answer: Type 21 provides several critical advantages:
- Enhanced earth fault protection: 65-78% lower effective earth resistance through parallel paths
- Regulatory compliance: Some jurisdictions mandate separate earth conductors
- Simplified testing: Dedicated earth conductor easier to verify than armour resistance
- Higher fault currents: Up to 173% higher fault current for faster protective device operation
- Insurance requirements: Some insurers require four-core cables for critical equipment
The additional cost (approximately 20-30% weight increase) is justified for safety-critical applications.
Q2: Can the fourth core be used for purposes other than earth?
Answer: No. The fourth core is specifically designated as earth conductor (green/yellow identification) and must not be used for:
- Neutral conductor (star-connected loads)
- Control signals
- Pilot wire monitoring
- Additional power circuit
Using the earth conductor for other purposes violates safety standards and creates serious electrical hazards. For applications requiring neutral, pilot cores, or additional conductors, select appropriate cable types (e.g., Type 506/512/518/524 for multi-core applications).
Q3: What causes armour corrosion in Type 21 cables?
Answer: Common causes include:
- Moisture ingress: Through damaged sheath or poor terminations
- Galvanic corrosion: Contact between steel armour and copper earth conductor at terminations
- Acidic mine water: Chemical attack on zinc galvanization
- Mechanical damage: Abrasion removing protective coating
Prevention measures:
- Use IP68-rated cable glands with proper sealing
- Apply anti-oxidant compound at terminations
- Install cables above water levels where possible
- Regular visual inspections for sheath damage
- Use bi-metallic separators at copper/steel interfaces
Q4: How do I size the cable for a specific motor load?
Answer: Follow this sizing procedure:
Motor: 110kW, 1000V, 3-phase, pf = 0.85, eff = 0.92
Cable length: 250m
Ambient temperature: 40°C
Step 1: Calculate full load current
IFL = 110,000 ÷ (1.732 × 1000 × 0.85 × 0.92) = 81.3A
Step 2: Apply derating factor
At 40°C: Factor = 0.91
Required rating: 81.3 ÷ 0.91 = 89.3A
Step 3: Select cable
4×25mm² rated 110A (inadequate margin)
4×35mm² rated 135A (adequate margin)
Select: 4×35mm²
Step 4: Verify voltage drop
ΔV = 1.732 × 81.3 × 0.25 × 0.565 = 19.9V
Percentage: (19.9 ÷ 1000) × 100 = 2.0% ✓
Q5: What maintenance schedule is recommended for Type 21 installations?
Answer: Recommended intervals based on environment severity:
| Inspection Type | Normal Environment | Severe Environment |
|---|---|---|
| Visual inspection | Monthly | Weekly |
| Termination temperature (IR) | Quarterly | Monthly |
| Earth continuity (Cu conductor) | Semi-annually | Quarterly |
| Insulation resistance test | Annually | Semi-annually |
| Complete electrical testing | Every 3 years | Annually |
| Armour condition assessment | Annually | Semi-annually |
Q6: Can Type 21 cables be spliced in the field?
Answer: Field splicing is strongly discouraged due to:
- Difficulty maintaining armour protection integrity at splice
- Risk of moisture ingress and insulation failure
- Earth conductor continuity may be compromised
- Regulatory restrictions in hazardous areas
- Warranty and liability implications
If absolutely necessary, use only approved armoured cable joint kits specifically designed for four-core mining cables, and ensure full compliance with local mining electrical regulations. Cable replacement is generally preferred over field repair.
Q7: What is the expected service life of Type 21 cables?
Answer: Under proper installation and maintenance conditions:
- Fixed installation: 20-30 years typical life expectancy
- Semi-mobile installation: 10-15 years with periodic movement
- Frequent relocation: 5-10 years due to mechanical fatigue
Factors affecting service life:
- Operating temperature (every 10°C above 90°C halves life)
- Mechanical stress (flexing, crushing, abrasion)
- Chemical exposure
- Moisture ingress
- Overcurrent events
- Quality of installation and terminations
Contact Anhui Feichun Special Cable Co., Ltd.
For technical inquiries, product specifications, quotations, and custom cable requirements for BS 6708 Type 21 four-core mining cables and other specialized mining cable solutions, please contact our expert engineering team:
[email protected]
[email protected]
+86 138 5512 3218
Anhui Feichun Special Cable Co., Ltd. (安徽飞纯特种电缆有限公司) specializes in manufacturing high-quality mining cables, armoured trailing cables, and specialty cables for demanding industrial applications. Our products comply with international standards including BS 6708, AS/NZS 2802, VDE 0250, and SANS 1520, serving mining operations worldwide with comprehensive technical support and custom cable solutions.
References and External Technical Resources
- BS 6708:1998 – Flexible cables for use at mines and quarries – British Standards Institution
- IEC 60228: Conductors of insulated cables – International Electrotechnical Commission
- DIN VDE 0250-812: Rubber insulated flexible cable NSSHÖU – DIN Media
- AmplexMining.com.au – Comprehensive Cable Guide for the Mining Industry
- ReelingCable.com – Professional Mine Reeling Cable Technical Specifications
- EN 60332-1-2: Tests on electric and optical fibre cables under fire conditions
- Eland Cables – EPR insulation technical benefits – Technical Documentation
- Tratos Group – Polychloroprene sheathing properties – Cable Academy
- Cleveland Cable Company – BS 6708 Mining Cable specifications
- Demka Electrical Suppliers – Type 20/21 technical data sheets
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