Introduction to BS 6708 Type FS4 Trailing Cable

The BS 6708 Type FS4 trailing cable represents a specialized category of non-armoured, individually metallic screened flexible cables engineered specifically for demanding mining and quarrying applications. This cable type is distinguished by its unique flat configuration design, where three phase cores and one pilot core—each equipped with composite individual screens—are laid up around an elastomeric cradle in contact with each other. This construction methodology provides optimal electrical performance while maintaining the flexibility essential for dynamic mining operations.

According to BSI Knowledge documentation, BS 6708 establishes comprehensive requirements for flexible cables used at mines and quarries, ensuring that these critical power transmission components meet stringent safety and performance criteria. The Type FS4 designation specifically identifies non-armoured cables with individually metallic screened cores in flat configuration, engineered for overhead catenary applications where reliable power delivery is paramount.

The development of Type FS4 cables reflects decades of mining industry experience and addresses the unique electrical and mechanical challenges present in potentially explosive underground environments. As detailed by AmplexMining.com.au, the British Standard BS 6708 establishes comprehensive specifications for flexible cables intended for mining applications, with each type number designating specific construction and performance characteristics that have been refined through extensive field testing and operational feedback.

Purpose and Primary Applications

Overhead Catenary Systems

The primary application for BS 6708 Type FS4 cables is as supplying cables in overhead catenary systems within mining operations. These systems provide continuous power to mobile mining equipment through suspended cable arrangements that allow equipment movement while maintaining electrical connectivity. The flat configuration of Type FS4 cables specifically addresses the mechanical requirements of these systems, where cables must maintain consistent spacing and controlled bending characteristics.

Power Supply for Mining Machinery

Type FS4 cables serve as critical power supply connections for various mining equipment including:

• Coalcutters and Continuous Miners: Providing the substantial power requirements (up to 37A for 4mm² conductors) needed for cutting mechanisms
• Mobile Drilling Equipment: Supporting drilling operations with reliable power transmission
• Conveyor Systems: Powering material transport equipment in underground galleries
• Ventilation Equipment: Ensuring continuous operation of critical safety systems

Operating Environments

The cables perform reliably across diverse mining environments including underground coal mines, metalliferous mines, quarries, and open-pit operations. According to technical documentation from ReelingCable.com, mining cables must maintain their electrical and mechanical properties throughout temperature ranges from -40°C to +80°C in fixed installations, with flexible installations typically operating between -20°C and +60°C ambient temperatures.

Cable Construction and Material Structure

The Type FS4 cable construction represents sophisticated materials engineering where each layer serves specific functions contributing to overall performance characteristics essential for mining applications:

Technical Specifications and Electrical Parameters

General Cable Properties

Detailed Dimensional and Electrical Data

Electrical Performance Analysis

The conductor resistance values comply with IEC 60228 Class 5 requirements, with the 4×2.5mm² configuration exhibiting 7.82 Ω/km maximum DC resistance at 20°C. This resistance value accounts for the fine-stranded construction necessary for flexibility while maintaining acceptable power transmission efficiency. For extended cable runs, voltage drop calculations become critical:

The screen resistance values (7.40 Ω/km for 2.5mm² and 8.48 Ω/km for 4mm²) are specified to ensure adequate earth fault current capacity as detailed in BS 6708 Annex C regarding short-circuit current carrying capability of flexible cable screens.

Standards Compliance and Certifications

Primary Standard: BS 6708

The BS 6708 standard, officially titled “Flexible cables for use at mines and quarries,” provides comprehensive specifications for mining cables. According to GlobalSpec Standards Database, this standard specifies flexible cable types, their electrical characteristics, construction, identification, and testing requirements. Key referenced standards include:

• IEC 60228: Conductors of insulated cables (Class 5 flexible conductors)
• IEC 60724: Short-circuit temperature limits of electric cables
• BS 7655-1.2: Cross-linked elastomeric insulating compounds
• BS 7655-2.3: Cross-linked elastomeric sheathing compounds
• EN/IEC 60332-1-2: Flame retardance testing requirements

Type FS4 Specific Requirements

Within the BS 6708 framework, Type FS4 cables are classified as non-armoured cables having individually metallic screened cores with rated voltage of 640/1100V, specifically designated for overhead catenary systems and similar applications. This distinguishes them from other BS 6708 types such as:

Testing and Certification Requirements

BS 6708 mandates comprehensive testing protocols including:

• Voltage Test: 3kV applied voltage for Type FS4
• Insulation Resistance Test: Minimum values specified per conductor size
• Wrapping Test: Mechanical integrity verification under bending stress
• Conductor Resistance: Compliance with IEC 60228 Class 5 limits
• Tensile Strength: Outer sheath and insulation material verification
• Ozone Resistance: Critical for chloroprene sheath longevity

Comparison with VDE 0250-812 German Standard

Understanding the relationship between BS 6708 Type FS4 and the German standard DIN VDE 0250 Part 812 provides valuable insights into international mining cable specifications. According to DIN Media documentation, VDE 0250-812 (published May 1985) specifies requirements for rubber-insulated flexible cables NSSHÖU series used in power installations.

Key Technical Differences

Voltage Rating and Application Scope

The primary distinction lies in voltage rating: BS 6708 Type FS4 operates at 640/1100V, providing higher power transmission capability compared to VDE 0250-812’s 450/750V rating. This higher voltage capacity makes Type FS4 more suitable for powering larger mining equipment directly, while VDE-compliant cables often require additional transformation equipment for high-power applications.

Configuration and Mechanical Properties

Type FS4’s flat configuration represents a specific design optimization for overhead catenary applications, where consistent cable orientation and controlled bending characteristics are essential. In contrast, VDE 0250-812 cables typically employ circular construction with specific lay-up ratios (approximately 6×D) optimized for trailing operations behind moving equipment. According to Tratos Group technical specifications, VDE-compliant trailing cables feature steel/copper reinforced braiding between inner and outer sheaths specifically designed for free trailing operation over considerable distances.

Screening and Electromagnetic Compatibility

While both standards require metallic screening, the implementation differs. Type FS4 employs individual tinned copper braided screens over each phase and pilot core, optimizing electromagnetic symmetry in flat configuration. VDE 0250-812 cables may utilize concentric monitoring conductors with spinning of steel/Cu-tinned wires, providing continuous integrity monitoring capabilities. The NSSHCGEÖU designation under VDE standards specifically indicates the presence of concentric earth conductors with enhanced monitoring features.

Material Specifications

Both standards specify EPR insulation and chloroprene sheathing, though compound formulations may differ. VDE 0250-812 references specific compound types (e.g., 3GI3 for insulation, 5GM5 for sheath) according to DIN VDE 0207 Part 20, while BS 6708 references BS 7655 compound specifications. These compounds provide equivalent performance characteristics with minor variations in processing and material sourcing.

Tensile Strength and Mechanical Performance

VDE 0250-812 cables designed for trailing applications typically specify permanent tensile strength of 15 N/mm² with maximum tensile loads ranging from 1,125N to 10,800N depending on conductor size. Type FS4 cables, optimized for overhead catenary use, experience different mechanical stress profiles emphasizing vertical loading and controlled bending rather than longitudinal tension. The minimum bending radius specifications (100mm for 4×2.5mm² Type FS4) reflect these differing operational requirements.

Safety Thresholds and Critical Parameters

Electrical Safety Parameters

Voltage Withstand Capability

The 3kV test voltage specification provides a safety factor of approximately 2.7× above the maximum operating voltage (1100V). This margin ensures cable integrity during:

• Equipment starting transients (motor inrush currents)
• Switching operations and load transfers
• Atmospheric overvoltage events
• Minor insulation degradation over service life

Current Carrying Capacity Derating

The specified current ratings (28A for 4×2.5mm², 37A for 4×4mm²) assume 25°C ambient temperature. Derating factors must be applied for elevated temperatures:

Short-Circuit Thermal Limits

The 250°C maximum short-circuit conductor temperature establishes the thermal limit before insulation degradation occurs. The permissible short-circuit current (I²t) can be calculated using the adiabatic equation referenced in IEC 60724:

Mechanical Safety Thresholds

Minimum Bending Radius

Exceeding the minimum bending radius (100mm for 4×2.5mm², 120mm for 4×4mm²) causes:

• Conductor strand breakage and increased resistance
• Insulation cracking and reduced dielectric strength
• Screen damage affecting earth continuity
• Outer sheath deformation compromising environmental protection

Installation Temperature Limits

Installation should not be attempted below 5°C without cable pre-conditioning. At low temperatures, the chloroprene sheath becomes less flexible, increasing risk of cracking during installation. Similarly, installation at temperatures exceeding 60°C may cause permanent sheath deformation under tension.

Environmental Safety Considerations

Explosive Atmosphere Compliance

Type FS4 cables are suitable for zones where explosive gases (methane, coal dust) may accumulate. Safety features include:

• Spark Prevention: Individual screening minimizes electromagnetic interference and controls fault currents
• Fire Resistance: Chloroprene’s self-extinguishing properties (EN/IEC 60332-1-2 compliant) prevent fire propagation
• Earth Continuity: Braided copper screens provide low-impedance earth path for fault clearance
• Static Dissipation: Properly earthed screens dissipate static charge accumulation

Chemical Exposure Limits

While chloroprene sheathing provides excellent resistance to oils, greases, and many chemicals, exposure limits exist:

• Continuous oil immersion: Not recommended (surface exposure acceptable)
• Strong acids/alkalis: Limited exposure only
• Aromatic hydrocarbons: Avoid prolonged contact
• UV radiation: Extended exposure causes surface degradation

Installation Guidelines and Best Practices

Pre-Installation Verification

Before installation, verify the following parameters:

1. Cable Inspection: Check for shipping damage, sheath integrity, and proper identification marking
2. Insulation Resistance: Measure between conductors and between conductors and screen (minimum 100MΩ·km typical)
3. Screen Continuity: Verify continuous screen connection throughout cable length
4. Temperature Verification: Ensure ambient temperature is within 5°C to 60°C installation range
5. Route Planning: Confirm all bending radii exceed minimum specifications (100-120mm)

Overhead Catenary Installation

For Type FS4 cables in overhead catenary systems:

• Use flat cable clamps designed for rectangular cross-sections
• Install strain relief at termination points
• Maintain consistent cable orientation throughout route
• Provide adequate clearance from moving equipment (minimum 150mm recommended)
• Include expansion loops to accommodate thermal cycling

Termination Procedures

Proper termination is critical for reliable performance:

1. Strip outer sheath: Remove 150-200mm while avoiding damage to screen braiding
2. Separate screens: Carefully unbraid individual core screens
3. Ground screen connections: Connect all screens to common earth point with minimum 10mm² bonding conductor
4. Strip insulation: Remove EPR insulation using appropriate stripping tools (avoid knife cuts that may nick conductors)
5. Install connectors: Use compression or mechanical connectors rated for tinned copper and appropriate current capacity
6. Seal termination: Apply heat-shrink or compound-filled termination boots to prevent moisture ingress

Testing After Installation

Post-installation testing requirements:

• Continuity Test: Verify all conductor and screen connections
• Insulation Resistance: Minimum 1MΩ at 1000VDC for 640/1100V rated cables
• Earth Loop Impedance: Verify compliance with protective device coordination
• High-Voltage Test: If required by local regulations (typically 80% of factory test voltage = 2.4kV for 1 minute)
• Phase Rotation: Confirm correct phase sequence

Common Problems and Frequently Asked Questions

Q1: What causes premature cable failure in Type FS4 installations?

Answer: The most common causes include:

• Exceeding bending radius: Installation with bends tighter than 100-120mm causes conductor and insulation damage
• Overcurrent conditions: Operating above rated current (28A/37A) causes thermal degradation of EPR insulation
• Mechanical abrasion: Contact with moving equipment or sharp edges damages outer sheath
• Chemical exposure: Prolonged contact with incompatible substances degrades chloroprene
• Improper termination: Poor screen bonding leads to localized heating and insulation breakdown

Q2: How do I verify screen integrity for earth fault protection?

Answer: Screen resistance testing should verify:

• Continuity throughout cable length (no open circuits)
• Resistance values within specification (7.40-8.48 Ω/km)
• Bonding connections at both cable ends
• Proper connection to protective earth system

For a 100m cable run, expected screen resistance would be approximately 0.74-0.85Ω. Values significantly higher indicate damaged braiding or poor termination.

Q3: Can Type FS4 cables be used in continuously moving applications?

Answer: Type FS4 cables are specifically designed for overhead catenary systems with relatively static installations. For continuously moving applications (cable reels, trailing behind mobile equipment), consider circular mining cables with higher flex ratings such as BS 6708 Type 7 or VDE 0250-812 compliant NSSHCGEÖU cables that feature reinforced construction specifically designed for repeated flexing cycles (typically rated for thousands of bending cycles).

Q4: What maintenance schedule is recommended for Type FS4 installations?

Answer: Recommended maintenance intervals:

• Weekly: Visual inspection for physical damage, support integrity
• Monthly: Termination temperature checks using infrared thermography
• Quarterly: Insulation resistance testing
• Annually: Complete electrical testing including earth loop impedance verification
• As needed: Following any fault events, physical impacts, or operational anomalies

Q5: How does Type FS4 compare to Type 7 for coalcutter applications?

Answer: While both cable types operate at 640/1100V with individual screening, key differences exist:

• Configuration: Type FS4 is flat form; Type 7 is circular
• Primary Use: Type FS4 for overhead catenary; Type 7 for direct trailing behind coalcutters
• Flexibility: Type 7 offers superior bending performance for trailing applications
• Earth Core: Type 7 includes separate earth core; Type FS4 relies on screens

For coalcutter trailing applications, Type 7 or Type 7M cables are generally preferred. Type FS4 excels when feeding power from overhead distribution to fixed connection points.

Q6: What are the storage requirements for Type FS4 cables?

Answer: Proper storage extends cable service life:

• Store on cable drums or in coils respecting minimum bending radius
• Maintain dry environment (relative humidity <85%)
• Avoid direct sunlight exposure (UV degradation)
• Temperature range: -20°C to +50°C
• Protect cable ends from moisture ingress using end caps
• Maximum recommended storage: 5 years before re-testing

Q7: Can Type FS4 cables be spliced or repaired?

Answer: While technically possible, field splicing of mining cables should be minimized due to:

• Difficulty maintaining original screening effectiveness
• Risk of moisture ingress at splice location
• Potential weak point for mechanical and electrical failure
• Regulatory restrictions in some jurisdictions

If splicing is unavoidable, use only approved mining cable splice kits designed for screened cables, and ensure compliance with local mining electrical regulations. Many operations mandate cable replacement rather than repair.