How Water Ingress Causes Cable Failure

Water is essential for life, but for power cables, it is a silent, relentless destroyer. A single drop of moisture entering a cable can initiate a cascade of degradation mechanisms that, over months or years, lead to insulation breakdown, short circuits, and catastrophic failure. Water ingress is one of the most common—and most preventable—causes of cable failure worldwide. This article explains how water enters cables, what damage it causes, and how engineers fight back.

1. The Entry Points: How Water Gets In

Water does not need a large opening to enter a cable. Even microscopic pathways are enough.

A. Damaged Outer Jacket
The outer sheath is the cable's first line of defence. Cracks, abrasion, cuts from tools, or damage from rodents create direct entry points. Once water passes the jacket, it reaches the metallic shield, armour, and eventually the insulation.

B. Poorly Sealed Cable Ends
Cable ends that are not properly sealed during storage, transport, or installation allow water to enter the conductor strands. Water can wick along the stranded conductor for hundreds of metres.

C. Defective Joints and Terminations
Joints and terminations are the most vulnerable points. If the sealing mastic is inadequate, if the cold-shrink tubing does not fully contract, or if the heat-shrink adhesive does not flow properly, water finds its way in.

D. Porosity of Materials
Some sheath materials (notably certain grades of PVC) are not completely impermeable to water vapour. Over decades, moisture can diffuse through the sheath, especially in hot, humid environments.

E. Condensation
Temperature cycling causes moisture-laden air inside a cable or accessory to condense. This is a particular problem in cables with poor end seals or in accessory housings that are not hermetically sealed.

2. The Path of Destruction: Longitudinal Water Migration

Once water enters a cable, it does not stay in one place. It migrates along the cable through two main pathways:

A. Along the Conductor Strands
In a stranded conductor, the gaps between strands form continuous channels. Water can travel along these channels for kilometres, especially under the influence of capillary action and thermal pumping (expansion and contraction of air).

B. Between the Insulation and Shield
The interface between the insulation and the semi-conductive screen or metallic shield is another pathway for water migration. This is particularly damaging because it directly attacks the insulation.

The consequence: A single pinhole in a jacket can allow water to travel hundreds of metres, causing widespread damage far from the original breach.

3. Water Treeing: The Slow Killer

In XLPE (cross-linked polyethylene) insulated cables, water ingress leads to one of the most insidious degradation mechanisms: water treeing.

What are water trees?
Water trees are microscopic, tree-like structures that grow from defects (contaminants, voids, or shield protrusions) into the insulation under the combined influence of moisture and electrical stress. They are not electrically conductive themselves, but they are paths of degraded, weakened polymer.

How do they form?

• Moisture enters the insulation through a defect or permeable area.

• The electric field concentrates at the defect.

• Water molecules are forced into microscopic voids, creating local pressure and stress.

• The polymer chains break down, forming a tree-like structure that grows over time.

The consequence:
Water trees grow slowly—over years or decades. When they reach a critical size, they can initiate electrical breakdown, especially during voltage surges. Once a water tree is established, it cannot be reversed. The cable is permanently degraded.

4. Corrosion: Eating Away the Metal

Water promotes electrochemical corrosion of metallic components inside a cable.

A. Conductor Corrosion
Copper and aluminium corrode when exposed to water and oxygen. The corrosion products have higher resistance, creating hot spots. In severe cases, the conductor can be completely eaten away.

B. Shield and Armour Corrosion
Steel armour wires rust rapidly in water. Rust expands in volume, cracking the outer jacket and creating even more entry points for water. Copper shields can corrode to the point where they no longer provide electromagnetic protection or a fault current path.

C. Galvanic Corrosion
If dissimilar metals are in contact (e.g., copper conductor and aluminium connector), water acts as an electrolyte, accelerating galvanic corrosion. The less noble metal (aluminium) corrodes preferentially.

5. Partial Discharge: The Final Nail

Water in voids or at interfaces lowers the dielectric strength of the material and can initiate partial discharge (PD) .

How water promotes PD:

• Water in a void creates a conductive path, concentrating the electric field.

• The PD inception voltage drops significantly when moisture is present.

• Each PD event releases energy, eroding the insulation and creating carbonized tracks.

The vicious cycle:
PD creates more damage, which allows more water to enter, which intensifies PD. The process accelerates until complete breakdown occurs.

6. Freeze-Thaw Damage

In cold climates, water that has entered a cable can freeze. Water expands by about 9% when it freezes, and this expansion can:

• Crack insulation – The expanding ice forces open cracks in the insulation or jacket.

• Displace stress control elements – A stress cone or Hi-K layer may be pushed out of position.

• Loosen connectors – The mechanical force can deform the connector or lug.

• Rupture joint casings – The pressure can split the outer casing of a joint.

When the ice melts, the water refills the new, larger voids. The next freeze does even more damage. After several cycles, the cable or accessory can be severely damaged even without any electrical load.

7. The Electrical Consequences: From Leakage to Breakdown

Water ingress leads to a progression of electrical problems:

The progression can take years or occur in a matter of days, depending on the severity of the ingress and the operating conditions.

8. How Engineers Prevent Water Ingress

Prevention is the key to reliable cable systems. Engineers use a range of technologies to keep water out:

A. Water-Blocking Layers

• Water-swellable tapes – Swell on contact with water, blocking further migration.

• Water-swellable powders – Applied to conductors or fillers.

• Filled cores – The interstices between conductors are filled with a water-blocking compound (gel or petroleum jelly).

B. Robust Outer Sheaths

• PE (Polyethylene) – Excellent moisture resistance, used for direct burial and submarine cables.

• Lead sheath – Completely impermeable, used for critical high-voltage applications.

• Multi-layer sheaths – Combining materials for optimal protection.

C. Sealed Accessories

• Cold-shrink terminations and joints – Radial pressure creates a void-free, watertight seal.

• Adhesive-lined heat-shrink – Melts and flows to seal gaps.

• Resin-filled joints – Encapsulated in waterproof resin.

• Mastic tape – Applied at cable jacket entries.

D. Proper Installation

• Seal cable ends immediately – Use end caps during storage and installation.

• Avoid installation in wet conditions – Work under shelter if possible.

• Test after installation – Perform insulation resistance and sheath integrity tests.

9. Detection and Monitoring

Early detection of water ingress can prevent catastrophic failure.

For critical installations, permanent monitoring systems (e.g., fibre optic moisture sensors) can provide real-time alerts.

Water ingress is one of the most destructive forces in cable systems. It attacks insulation, corrodes metals, creates water trees, and initiates partial discharge. The damage is often progressive and invisible until failure occurs—sometimes years after the initial breach.

The good news is that water ingress is preventable. Through robust sheath materials, water-blocking technologies, sealed accessories, and careful installation, engineers can keep water out and ensure that cables operate reliably for decades. The next time you see a cable—underground, underwater, or on a pole—remember: inside that jacket, an invisible battle is being fought against water. The best cables are the ones that win that battle.