Distributed fiber-optic temperature sensing (DTS) cables can be installed underground to provide continuous, real-time monitoring of cable temperature, hotspots, and thermal performance.

Cable Selection

For underground temperature monitoring, single-mode optical fibers are typically preferred due to their low attenuation and long-distance performance, compliant with ITU-T G.652/654/655 standards, with bend-insensitive fibers (G.657) recommended for tight routing ( ). Loose-tube constructions are common for DTS applications, allowing fibers to float freely within the tube, minimizing strain and compression, and can be gel-filled or dry-blocked for water protection. For harsh environments, Fiber-In-Metal-Tube (FIMT) cables provide high tensile strength and crush resistance ( ).

Installation Methods

1. Direct Burial: Cables can be buried directly in trenches with protective conduits or sand bedding to prevent mechanical damage. Depth and protective layers should comply with local regulations and environmental conditions ( ).
2. Conduit Installation: Using PVC or HDPE conduits provides additional protection against moisture, soil movement, and mechanical stress.
3. Embedded in Cable Systems: DTS fibers can be integrated into existing power cable sheaths or alongside the cable in a dedicated monitoring cable, ensuring accurate thermal readings ( ).

Monitoring Considerations

• Temperature Measurement: DTS systems measure temperature along the fiber using Raman or Brillouin scattering, providing near-continuous monitoring of the cable length ( ).
• Hotspot Detection: Proper placement ensures detection of potential hotspots, cold spots, and thermal bottlenecks, which are critical for ampacity management and cable lifetime prediction ( ).
• Integration with Control Systems: DTS and other fiber-optic sensing systems (DAS, DTSS) can be integrated into centralized monitoring platforms for real-time alerts and historical data analysis ( ).

Environmental and Mechanical Protection

• Water Blocking: Gel-filled or super-absorbent polymer powders prevent water ingress without affecting optical performance ( ).
• Mechanical Protection: Additional layers such as aramid fibers, stainless steel wires, or plastic sheaths protect against strain, crushing, and chemical exposure ( ).
• Thermal and Strain Considerations: Loose-tube fibers minimize strain effects, but for strain or deformation monitoring, tight-buffered or bonded fibers may be required ( ).

Best Practices

• Consult with fiber sensing suppliers to select the appropriate cable type and installation method for the specific underground environment ( ).
• Ensure proper trenching, bedding, and backfilling to avoid mechanical stress and maintain fiber integrity.
• Plan for access points and splicing locations to facilitate maintenance and calibration.
• Use DTS in combination with thermal models to estimate conductor temperature accurately, as direct measurement of the conductor is not feasible ( ). By following these guidelines, underground optical cables can provide reliable, continuous temperature monitoring, enabling early detection of anomalies, optimized load management, and extended cable lifespan ( ).