Ask a cable engineer where the network fails and the answer is consistent: not in the cable itself, but at the joints and terminations. The factory cable is made under controlled conditions, but every joint is assembled in the field, and every termination sits at the interface between the cable and the equipment – two places where the electric field is highest and the workmanship is least controlled. This article explains why joints and terminations fail, and how partial discharge measurement finds the defect in time.
Why joints and terminations fail
The failure drivers are the same everywhere:
- Field workmanship – a joint depends on surface preparation, cleanliness and exact dimensions; a single contamination or an off-dimension cut creates a weak point
- Electric field concentration – the termination ends the cable screen, and the field must be graded; a grading defect produces corona and PD
- Moisture ingress – a failed seal lets water into the interface, which tracks along the insulation over months
- Thermal and mechanical stress – load cycling moves the joint and the termination, opening micro-gaps
Every one of these mechanisms produces partial discharge before it produces failure. The discharge starts small and grows – which is exactly what makes it detectable and why early detection pays.
Detecting PD at the termination
Terminations sit at the end of the cable where it connects to switchgear, transformers or overhead lines. The practical detection methods:
- HFCT sensors around the cable screen or earth strap – the standard for continuous monitoring, capturing the discharge current pulse (for example the HUWOR HMJ1010-HCT, 0.5-30 MHz)
- TEV sensors on the termination compartment of the switchgear, where the discharge also couples to the enclosure
- Ultrasonic sensors for the airborne component at the termination, which is often accessible
For a transformer or switchgear termination, an HFCT around the screen connection gives a continuous, broadband view of the discharge activity flowing out of the cable.
Detecting PD at the joint
Joints are buried or enclosed, which makes them harder to reach:
- Oscillating wave testing (OWTS) during an outage locates the discharge along the whole cable and shows which joint is faulty – the standard diagnosis for a suspected joint
- HFCT sensors at both ends of the cable, and at accessible intermediate joint bays, provide continuous coverage where joints are accessible
- Acoustic detection at joint bays, where the operator can reach the surface and localise the source
The combination is the same as for the whole cable: continuous monitoring at the accessible points, and OWTS diagnosis to locate the defect when the trend rises.
Building joint and termination monitoring into the programme
A practical programme for a cable-heavy network:
- Install HFCT sensors at the critical terminations (transformer and switchgear connections) and monitor continuously
- Include the joint bays that are accessible in the patrol route with a handheld detector
- When a termination or a joint flags, confirm with an OWTS test at the next outage to locate and quantify
- Log the results and compare the trend at each joint over the years
The cost is concentrated on the terminations that matter most, while the patrol and OWTS layers cover the rest of the route.
Summary
Joints and terminations are where cable networks fail, and PD is the common early signal. HFCT monitoring at terminations, OWTS location at suspected joints and acoustic confirmation on accessible sections cover the full picture. HUWOR supplies the HFCT sensors, the HMJ9031 OWTS tester and handheld detectors as one family. Contact the HUWOR sales team for a joint and termination monitoring plan.