Unlike mechanical equipment, cables do not announce their degradation with noise or vibration. They quietly deteriorate over years until a sudden insulation failure causes a trip, an earth fault, or in worst cases a fire. A structured cable maintenance programme catches degradation before it becomes failure.
Why Cable Insulation Degrades
PVC insulation has a design life of 20, 25 years under normal conditions. Several factors accelerate degradation:
- Heat: Every 10°C above rated operating temperature halves insulation life (Arrhenius rule). A cable running at 80°C continuous instead of 70°C loses half its life expectancy.
- UV exposure: Standard PVC degrades in direct sunlight. Outdoor cables must use UV-resistant sheathing.
- Oil and chemical contact: Certain PVC grades are not resistant to oils, solvents or acids. Chemical attack makes the jacket brittle and porous.
- Mechanical stress: Repeated vibration, bending or rodent damage creates micro-cracks that allow moisture ingress.
- Moisture: Water at terminations and in damaged jackets reduces insulation resistance and accelerates oxidation of copper conductors.
Maintenance Schedule
| Frequency | Activity | Tool / Method |
|---|---|---|
| Monthly | Visual inspection of cable trays, entries, glands | Visual |
| Monthly | Check for oil / moisture at termination points | Visual |
| Annually | Thermographic scan of panel terminations | Thermal camera |
| Annually | Insulation resistance (IR) test on critical feeders | 500V or 1kV Megger |
| 3-yearly | Full IR test on all cables, compare to baseline | Megger with trend data |
| On any fault | Fault location test + IR test after repair | TDR, Megger |
Insulation Resistance (IR) Testing, The Core Tool
The Megger test applies a DC voltage to the cable insulation and measures the resulting leakage current. The result (in MΩ or GΩ) indicates the condition of the insulation. The key rule: trend matters more than absolute value.
LV cables (1.1kV): >1 MΩ (minimum), >100 MΩ (good)
MV cables (3.3, 11kV): >1,000 MΩ (minimum)
A cable that measured 500 MΩ three years ago and now reads 5 MΩ is a cable to watch, even though 5 MΩ is technically "above minimum." Trending downward rapidly is the warning sign.
Polarisation Index (PI), For MV Cables
For medium voltage cables, measure IR at 1 minute and at 10 minutes after applying the test voltage. The ratio (PI = IR10/IR1) indicates insulation quality:
- PI < 1.0: Dangerous, replace the cable
- PI 1.0, 2.0: Poor, investigate
- PI 2.0, 4.0: Good
- PI > 4.0: Excellent
Thermographic Inspection, Your Annual Priority
An infrared thermal camera scan of all switchboard and distribution board terminations while the system is live and under load identifies hot joints before they fail. A termination running 20, 30°C above its neighbour has elevated resistance, usually from a loose connection, an undersized lug, or corrosion. Fix it at the next planned shutdown; it will not fix itself.
Thermographic inspection is the most cost-effective maintenance activity in an electrical system, it requires no downtime and catches problems months before failure.
Visual Inspection Checklist
- Cable tray fill, is it over 40% full? Overcrowded trays trap heat and raise cable temperature
- Missing or damaged gland seals at panel entries
- Cable resting on sharp edges, the metalwork cuts through the outer sheath over time
- Cables touching hot pipe surfaces, identify and reroute or shield
- Oil or coolant dripping onto cables, the chemicals attack the PVC jacket
- Cables with outer sheath cracked or split, mark for replacement at next shutdown
- Rodent damage, particularly in food processing, textile and warehouse environments
When to Replace Rather Than Repair
Joints and repairs on LV power cables are acceptable for isolated physical damage. Do not attempt to repair degraded insulation with insulation tape, it is a temporary measure, not a repair. Joints on MV cables should be done by a specialist jointer with correct accessories.