What Causes High-Resistance Faults in Cables?
The hardest cable faults to find aren't the dramatic short circuits — they're the quiet high-resistance faults that hide from test equipment and trip networks at the worst moments.
Understanding what CAUSES these faults is the first step to locating them. Here are the five most common causes, how they develop, and how to find them.
Cause 1: Water Treeing (XLPE Cables)
What it is: microscopic tree-shaped water channels that grow inside XLPE insulation under moisture + electrical stress.
How it develops: over 5-15 years, water and ions penetrate tiny insulation defects. Electric field stress grows tree-like channels. The insulation weakens gradually — the fault resistance starts high and only breaks down under operating voltage.
Why it's hard to find: the fault only conducts at high voltage, so a low-voltage TDR sees nothing.
How to locate it: HV conditioning to force breakdown, then TDR + pinpointing. Water-tree faults are typically intermittent until they fail completely.
Cause 2: Thermal Aging
What it is: long-term heat exposure degrades insulation chemistry.
How it develops: cables run above rated temperature for years (overload, poor ventilation, high ambient). The insulation becomes brittle, loses dielectric strength, and develops high-resistance weak points.
Why it's hard to find: degradation is gradual and distributed — no single dramatic fault point until final failure.
How to locate it: thermal imaging during load can reveal hot spots. When a fault develops, HV conditioning + TDR works. Focus on known hot areas first.
Cause 3: Moisture Ingress at Joints and Terminations
What it is: water entering failed joints, terminations, or damaged sheaths.
How it develops: a tiny sheath breach lets moisture wick along the cable. At joints, the water creates tracking paths and corrosion — forming high-resistance faults that grow over time.
Why it's hard to find: the fault is often at a joint you can't see, and the resistance is voltage-dependent.
How to locate it: joint locations are usually documented. Test near known joints first, use HV conditioning, then pinpoint acoustically.
Cause 4: Loose or Corroded Connections
What it is: not an insulation fault but a connection fault — loose lugs, corroded terminals, poor crimps.
How it develops: thermal cycling loosens connections; moisture corrodes them. The connection resistance rises, creating a high-resistance point that behaves like a fault.
Why it's hard to find: it's often inside a box or termination you can't inspect without opening it.
How to locate it: thermal imaging under load is the best tool — hot connections show clearly. TDR can sometimes see the impedance change on shorter cables.
Cause 5: Partial Discharge Damage
What it is: internal micro-discharges that erode insulation over time.
How it develops: voids or defects in insulation initiate partial discharges. Each discharge erodes the insulation slightly — over years, the erosion forms a tracking path that eventually becomes a high-resistance fault.
Why it's hard to find: partial discharge is intermittent and may not appear under a simple TDR test.
How to locate it: partial discharge testing detects the activity early. Once a fault forms, HV conditioning + multi-pulse + pinpointing is the field approach.
The Common Thread: High Resistance = Invisible to Low-Voltage Testing
Every cause above shares one problem: the fault's high resistance makes it invisible to a standard TDR test.
The physics: a TDR's low-voltage pulse passes through a high-resistance fault as if it weren't there. The fault only becomes locatable after HV conditioning breaks it down.
This is why professional fault location kits always include an HV supply alongside the TDR:
The G35 HV supply and a complete fault location system handle all five causes above.
Prevention Checklist
- Track cable load and operating temperature
- Inspect and test joints during scheduled maintenance
- Monitor for partial discharge on critical circuits
- Keep sheaths sealed — moisture is the #1 accelerator
- Document cable routes and joint locations (saves hours later)
- Baseline TDR traces on critical cables for comparison
FAQ
What causes high-resistance faults in cables? Water treeing, thermal aging, moisture ingress at joints, loose/corroded connections, and partial discharge damage — all create faults that only conduct at elevated voltage.
Why can't my TDR see a high-resistance fault? The TDR's low-voltage pulse passes through high resistance without reflecting. The fault needs HV conditioning to break down into a locatable low-resistance arc.
Are high-resistance faults dangerous? Yes — they're intermittent and unpredictable, and can escalate to complete failure or flashover. They're the hardest faults to locate precisely because they hide from standard testing.
How do I locate a high-resistance fault? Condition it with an HV supply, re-test with the TDR, then pinpoint acoustically. A complete fault location system bundles all three tools.