Medium-voltage cables fail, and when they do, operators reach for two families of instruments: cable fault locators and partial discharge (PD) testers. They are sometimes confused with each other, but they do different jobs at different times. Understanding the difference saves money, downtime and wasted effort.
The two jobs
Cable fault location: finding the failure
When a cable has already failed – an insulation breakdown, a joint failure, or a short – the network is down and the fault must be found fast. Cable fault location uses techniques such as:
- Time domain reflectometry (TDR) – sends a pulse down the cable and measures reflections to locate impedance changes and faults
- Surge/arc reflection – applies a high-voltage surge to break down the fault and locates it by the resulting reflection
- Sheath fault location – locates faults in the cable sheath (armour-to-earth faults) on unjacketed cable
Fault location answers one question: where is the failure, right now?
PD testing: finding the risk before failure
Partial discharge testing detects insulation defects while the cable is still energised or energisable – before they become a fault. Oscillating wave testing (OWTS) applies a damped AC voltage and measures PD activity, localising it to a section or joint position. PD testing answers a different question: where is the insulation degrading, before it fails?
When to use each
| Scenario | Tool |
|---|---|
| Cable has failed; restore supply as fast as possible | Fault locator (TDR, arc reflection) |
| New cable; prove installation quality before energisation | PD testing (OWTS acceptance) |
| Operating cable; assess insulation health and plan maintenance | PD testing (OWTS diagnosis) |
| After repair or re-jointing; confirm the repair | PD testing, then fault locator if a new fault appears |
How they complement each other
A mature cable maintenance programme uses both:
- Prevent with PD testing – find and repair defects before failure, avoiding outages entirely.
- React with fault location – when a failure still occurs, restore supply quickly by locating and repairing the fault precisely.
- Learn from both – repeated PD activity at the same joint family, or repeat faults on a route, feed the replacement programme.
Many utilities own both instrument families: the fault locator for the day of failure, the PD test set for the years before it.
About HUWOR
Zhuhai Huawang Technology Co., Ltd. (HUWOR) manufactures both cable fault locators (TDR and arc reflection systems) and oscillating wave PD test sets for MV cables, supporting utilities, contractors and testing companies worldwide with OEM/ODM supply and application guidance. Contact the HUWOR sales team for a cable maintenance configuration.