Can Power Cables Be Monitored 24/7? The Rise of Online Cable Condition Monitoring

From Periodic Testing to Continuous Insight: How Sensors Are Transforming Cable Management

Introduction

Until recently, the only way to know the condition of an underground power cable was to shut it down, drive to the site, connect test equipment, and run a few hours of tests. That approach—periodic, offline, and disruptive—works, but it leaves a dangerous gap: between tests, a cable can degrade, develop a partial discharge, or overheat without anyone knowing. The question is no longer whether we can do better, but how.

Online (or continuous) cable condition monitoring uses permanently installed sensors to measure key cable parameters 24 hours a day, 7 days a week—while the cable is energized and in service. This article explains what online monitoring is, why it is gaining traction, which technologies are available, what they can and cannot detect, and how to decide whether continuous monitoring makes sense for your cable system.

1. What Is Online Cable Condition Monitoring?

Online condition monitoring is the continuous, automated measurement of cable health parameters without taking the cable out of service. Sensors are permanently installed at cable joints, terminations, or along the cable route. They feed data—via wired, wireless, or fiber optic links—to a central monitoring unit or cloud platform, where trends are analyzed and alarms are raised when thresholds are exceeded.

Unlike offline testing, which provides a snapshot at one moment in time, online monitoring provides a streaming picture. It can detect slow degradation over months and sudden changes in seconds—something a periodic test can never do. The cable never leaves service, and maintenance teams are alerted the moment something changes, not weeks or months later when the next scheduled test comes around.

2. Why Move from Periodic Testing to Continuous Monitoring?

2.1 Cables Are Aging

A large portion of the underground cable fleet in service today is 25–40 years old—the age at which insulation degradation accelerates. Periodic tests every 1–3 years may miss a fast-growing defect that fails between tests. Continuous monitoring closes that gap.

2.2 The Cost of Unplanned Outages

An unplanned cable fault in a critical feeder can cost tens of thousands of dollars per hour in lost production, customer penalties, and emergency repairs. Online monitoring catches developing faults before they become failures, converting unplanned outages into scheduled maintenance windows.

2.3 Data-Driven Maintenance

Operators are increasingly moving from time-based maintenance (fix on a schedule) to condition-based maintenance (fix when the data says it is needed). Continuous data is the fuel that makes this possible. Trend curves show not just whether a cable is healthy today, but whether it is getting worse—and how fast.

2.4 New Cables Are Being Laid in Ducts

As urban underground networks expand, more cables are being installed in densely packed ducts under city streets. Access for offline testing is difficult, time-consuming, and disruptive. Online monitoring eliminates the need to dispatch crews to every cable every year.

3. Key Online Monitoring Technologies

3.1 Partial Discharge (PD) Monitoring

Partial discharge is the most sensitive early indicator of insulation defects in medium and high voltage cables. Online PD sensors are typically high-frequency current transformers (HFCTs) clamped around the earth strap at cable joints and terminations. They continuously listen for the high-frequency pulse signatures of internal discharges. When PD activity exceeds a baseline, the system raises an alarm—often years before the defect would cause a breakdown.

3.2 Distributed Temperature Sensing (DTS)

DTS uses a fiber optic cable installed alongside or within the power cable to measure temperature along the entire length, at every meter. A laser pulse sent down the fiber scatters light; the frequency shift indicates local temperature. DTS detects hotspots caused by overloading, poor joints, or blocked ducts—the cable equivalent of a fever thermometer reading every centimeter.

3.3 Online Tan Delta / Dielectric Loss Monitoring

Tan Delta (dissipation factor) monitoring continuously measures the dielectric loss of the cable insulation while in service. A rising Tan Delta trend indicates moisture ingress, thermal aging, or water treeing. Online systems use coupling capacitors and precision sensors to measure the small loss factor without interrupting service.

3.4 Sheath and Circulating Current Monitoring

Cable sheath (metallic shield) circulating currents reveal sheath faults, grounding problems, and phase unbalance. Current transformers at grounding points continuously measure these currents. A sudden change indicates sheath damage—a precursor to main insulation failure.

3.5 Fiber Optic Current and Strain Sensing

Specialized fiber optic sensors can also measure load current (via Faraday rotation) and mechanical strain (via Brillouin scattering). This is particularly useful for monitoring cables that are subject to ground movement, thermal expansion, or physical stress.

4. What Can Online Monitoring Detect?

What it cannot do: online monitoring does not replace offline electrical tests. It complements them. An online PD system detects that something is wrong; an offline VLF or Tan Delta test then quantifies how bad it is and whether immediate action is needed.

5. Benefits and Limitations

Benefits

Limitations

6. Is Online Monitoring Right for Your Cable?

Online monitoring is not necessary for every cable. It is most cost-effective when:

For low-voltage distribution cables, routine offline testing remains the more economical approach. The decision should be based on risk: where the cost of failure justifies the investment, online monitoring pays for itself in avoided outages.

Conclusion

Yes—power cables can now be monitored 24/7. Online condition monitoring has moved from a niche luxury to a practical, cost-effective tool for critical cable circuits. PD sensors, distributed temperature sensing, and continuous Tan Delta monitoring provide the visibility that periodic offline tests simply cannot: a real-time, trend-based picture of cable health that catches developing faults before they become failures.

Online monitoring does not replace offline testing—it completes it. The most reliable cable programs use both: continuous sensors on the most critical cables, periodic offline tests on the rest, and a central data platform that brings it all together. The era of testing cables only when something goes wrong is ending; the era of watching them continuously has begun.

About XZH TEST

XZH TEST specializes in electrical testing and cable diagnostic equipment, including VLF test systems, partial discharge detectors, TDR fault locators, and online monitoring solutions. Products are designed for field durability, measurement accuracy, and seamless integration with utility and industrial asset management systems—helping operators move from reactive maintenance to predictive, data-driven cable management.

Website: XZH TEST