The core goal of cable temperature monitoring is to catch anomalies before overheating evolves into fire. Distributed Temperature Sensing (DTS) measures temperature every meter along the full cable length with 0.8 m spatial resolution, ±0.5°C accuracy, 0.1°C temperature resolution and 50°C/min response sensitivity — the most mature technical route for cable galleries, tunnels and mezzanine scenarios today. This article puts point probes, thermal imaging and fiber temperature sensing in the same table, and explains how to choose, how to deploy, and how to integrate with fire protection.
Why Cable Fires Are Frightening: Overload, Aging and "Only Known Afterward"
Power cables are densely laid in tunnels, utility galleries and cable mezzanines. Once a fire starts, it spreads fast, is hard to suppress and causes wide outages. What makes it worse is that cable fires often give no warning — a cable that looks intact on the outside may have been overheating internally for years.
There are three root causes of cable heating. Overload is the most common. When load exceeds the ampacity design value, conductor temperature keeps rising and the insulation accelerates its aging under heat, forming a vicious cycle of "hotter → more aging → more heating." Joint degradation is another frequent trigger. The mid-span cable joint is a weak point in workmanship; poor crimping raises contact resistance, and the local temperature rise can be tens of degrees above the surrounding — a high-probability starting point for fire. External heat sources also matter: high-temperature media leaking from adjacent pipes and heat dissipated by electrical equipment can push the cable's ambient temperature past its design ceiling.
The trouble is this: the precursor to a cable fire is temperature, but every point along the cable can be a fault site, and hot spots appear randomly over tens of kilometers of route. Trying to watch an entire line with a few probes fails at the level of logic.
Why the Monitoring Requirement Is Special: A Linear Asset Meets Random Hot Spots
The monitoring profile of cable temperature is completely different from point-type equipment like transformers and switchgear.
Length. A feeder cable runs from a few hundred meters to several kilometers; the monitored object is the whole line, not a single point. Random position. Joint positions are relatively fixed, but overload heating can occur anywhere along the conductor, and external heat sources are impossible to anticipate. Time sensitivity. From overheating to fire may take only minutes to tens of minutes; temperature data must update at the second level — after-the-fact logging is meaningless.
These three characteristics together dictate that cable temperature monitoring must cover the full route, at meter-level resolution, in real time. Distributed Temperature Sensing naturally matches all three — which is why it has become the mainstream approach for cable scenarios.
Technical Route Comparison: Point Probes, Thermal Imaging and Fiber Temperature Sensing
Four routes in one table make the trade-offs obvious.
| Dimension | Point probes | Thermal imaging | Distributed Temperature Sensing (DTS) |
|---|---|---|---|
| Coverage | Single-point measurement | Imaging within field of view | Continuous meter-by-meter along the line |
| Spatial resolution | Depends on probe density | Imaging resolution | 0.8 m |
| Temperature accuracy | Higher | Affected by environment | ±0.5°C |
| Fire-warning capability | Misses gaps between probes | Requires someone watching the screen | Full-route coverage, automatic alarm |
| Installation cost | More probes, more cost | Limited coverage per unit | Cable deployment, one-time |
| Maintenance | Probes fail year by year | Periodic calibration needed | Maintenance-free fiber, break localization |
| Suitable scenarios | Key-joint spot measurement | Open-space patrol | Linear scenarios: tunnels, galleries, mezzanines |
Thermal imaging is not without value — its strength is open spaces and mobile patrol. But underground cable tunnels are perpetually dark and dusty, limiting imaging range and accuracy. Point-probe cost rises linearly with density; watching a few hundred joints means a few hundred probes, and the probes' own aging failures are hard to detect in time. DTS covers the whole line with one fiber, making every meter a measurement point — a generational gap at the route level.
How DTS Deploys: Laid Along the Cable, Every Meter Measuring
The engineering implementation of Distributed Temperature Sensing is simpler than expected.
The temperature-sensing fiber is laid in the same trench or on the same rack as the cable, typically close to the cable surface and lashed along the tray; one sensing cable covers the temperature field of every cable on that layer. The interrogator connects at one end of the fiber; laser pulses travel along it, the Raman component of the backscattered light varies in intensity with temperature, and the system reconstructs the temperature of every segment from that. A 0.8 m spatial resolution means one measurement point every 0.8 m — a 1 km cable gallery is equivalent to more than 5,000 measurement points, with no blind zone in between.
Response speed is a hard metric for fire warning. DTS's 50°C/min response sensitivity, combined with second-level data refresh, triggers warnings at the onset of temperature rise, rather than waiting for a smoke detector to react. The system supports two-tier alarming over the whole line — temperature threshold and rate-of-rise threshold. Slow heating triggers the temperature threshold; rapid anomalies trigger the rate threshold — both typical fault modes are covered.
The differences at the laying stage deserve more attention. For new cable tunnels, the sensing cable can be laid together with the power cable in a single construction pass. For existing, in-service galleries, the fiber must be added without de-energizing the cable, using a secondary-laying process with tighter requirements on the construction window and fiber protection. Where an existing gallery is space-constrained or has many tray layers, the sensing cable's routing must be planned in advance — close to the heat source yet clear of crossing work zones. Field experience here affects the final measurement quality more than equipment specs do.
Decoding the Key Specs: 0.8 m, ±0.5°C and 50°C/min
These numbers are often recited at selection time, but many people don't fully grasp what each one constrains.
0.8 m spatial resolution is the minimum spacing at which the system can resolve a temperature step along the fiber. It directly determines "whether you can find that overheating joint." The industry norm is around 1 m; 0.8 m means stronger resolution of short-distance joint overheating, and hot spots are less likely to be smoothed away. ±0.5°C accuracy is the absolute error of the temperature reading; it determines whether the system can resolve a 0.5°C-level rising trend — insufficient accuracy buries slow-developing faults in noise. 0.1°C temperature resolution is the smallest temperature change the system can distinguish between two adjacent readings, and is a different metric from accuracy — the former is how precisely it measures, the latter is how finely it can see a change; LandSub Global DTS holds both at ±0.5°C and 0.1°C respectively. 50°C/min response sensitivity is the system's sensitivity to the rate of temperature change, mapping to rapid temperature rises caused by sudden short circuits or poor contact — the most critical time window for fire warning.
One more easily overlooked point at selection time: sensing range. Multimode systems typically cover up to 20 km, single-mode up to 30 km. For longer routes or higher-loss scenarios, calculate the signal budget against the actual route to avoid end-of-line sensitivity degradation.
Integrating with Cable-Gallery Fire Protection: Warning Is Only the First Step
The value of temperature monitoring is fully released inside a closed response loop.
After a DTS warning triggers, the response can be tiered: at the onset of a temperature rise, push an O&M work order and dispatch staff for on-site inspection; when the temperature crosses the second threshold, trigger ventilation and audible-visual alarms; in an emergency, trigger zoned fire suppression. The data feeds into the cable gallery's integrated monitoring platform, working alongside video, smoke detection and gas detection subsystems, so fire-duty staff see the full picture on one screen.
A word of caution on interface standardization. When purchasing, confirm whether the interrogator exposes data to the upper-level platform over standard protocols (such as Modbus or IEC 61850); a closed protocol will leave later integration in a passive position. This should be pinned down at the tender stage, not negotiated after the equipment arrives on site.
Selection Advice and Common Misconceptions
A few judgments for colleagues running cable-temperature projects.
Misconception one: looking only at accuracy, not spatial resolution. Accuracy decides how precisely you measure; resolution decides whether you can find it. For cable scenarios, resolution is often the more fatal shortcoming. Misconception two: laying the sensing cable carelessly. The contact between the sensing cable and the power cable, and the lashing spacing, directly affect heat-transfer efficiency — a suspended fiber and a well-attached one can read several degrees apart. Misconception three: ignoring the system's self-diagnosis. Fiber-break localization and interrogator self-checks determine long-term availability; there are no shortage of cases where a broken fiber went unnoticed and the system was only discovered "long dead" halfway through an incident.