The conclusion is: yes, but under certain conditions.

Those working in electrical engineering have likely encountered this dilemma: if there's water in the cable trench, is it okay to directly submerge XLPE cables? Does underwater laying require additional waterproofing?

The answer is—XLPE cables themselves have a certain degree of water resistance, but whether they can be submerged for extended periods depends on their structural design and protection level.

Is XLPE insulation material itself susceptible to water damage?

XLPE (cross-linked polyethylene) can be used in humid environments because of its molecular structure. While the upper temperature limit for ordinary polyethylene is approximately 70°C, XLPE forms a three-dimensional network structure through chemical cross-linking, significantly increasing its crystallinity. This dense structure greatly reduces the rate of water molecule penetration. At 95% relative humidity, XLPE absorbs only a fraction of the water compared to ordinary PE.

In other words, the XLPE insulation layer itself has a very low water absorption rate, making it much more reliable than PVC in normal humid environments.

However, "low water absorption" does not mean "no water absorption." Water molecules slowly diffuse and penetrate into the insulation layer. Laboratory data shows that XLPE cables immersed in 25°C pure water for 30 days can experience a drop in insulation resistance to one-tenth or even less of their original value; if the water contains salt, the drop can exceed 90%. This demonstrates that the insulation material itself cannot withstand prolonged water immersion.

Cable waterproofing relies not on the insulation layer, but on a "three-layer structure."

Many people have a misconception: that the denser the insulation layer, the more waterproof it is. In fact, cable waterproofing is a systematic project, with roughly three lines of defense from the inside out:

The first line: conductor water barrier. Water seeping into the conductor gaps is the most troublesome issue. High-end waterproof cables fill the conductor gaps with water-blocking paste or yarn, which expands upon contact with water to seal the gaps.

The second line: radial water-blocking layer. Water-blocking tape is wrapped around the outside of the insulation layer, also based on the principle of water expansion. Some manufacturers extrude aluminum-plastic composite tape around the insulation layer, plus a polyethylene sheath, using high-temperature heat fusion to completely bond them together, forming a physical barrier.

The third and strongest line: the metal sheath. For high-voltage cables (e.g., 69kV and above), a metal sheath is directly applied—such as a hot-pressed aluminum sheath, lead sheath, or welded corrugated aluminum sheath. Metal is completely impermeable to water, so the radial waterproofing capability of this type of cable is "absolute."

Different voltage levels have vastly different waterproofing requirements. The waterproofing solutions for low-voltage cables (e.g., 600V or 1kV) are completely different from those for high-voltage cables.

Low-voltage and medium-voltage cables: Generally, an extruded HDPE outer sheath plus water-blocking tape is sufficient. HDPE sheaths have a density of over 0.94 g/cm³, with a tightly packed molecular structure and a very low water vapor permeability coefficient. Many submersible pump cables and deep well cables use this structure, specifically designed for long-term immersion in water.

High-voltage and ultra-high-voltage cables: A metal sheath is mandatory. Aluminum sheaths and lead sheaths provide complete sealing. Many submarine cables abroad use XLPE insulation with a metal sheath structure, commonly seen in projects like the North Sea wind farm.

Real-world test data tells you: How long can a well-waterproof XLPE cable last after immersion?

Theory is one thing, but actual data is more convincing.

An underwater engineering test: Cables with XLPE insulation operated continuously for 5 years in 3 meters of water without significant degradation in insulation performance or increased dielectric loss due to moisture.

A subway tunnel water seepage accident: A cable with a radial water-blocking structure maintained over 90% of its initial insulation resistance after being accidentally submerged for 24 hours.

Accelerated aging test: SINTEF research shows that XLPE cables essentially do not develop water trees when the relative humidity is below 70%; for systems with double sheaths, it may take over 50 years for the insulation layer relative humidity to reach 70%.

On the other hand: In a residential underground parking garage, after 36 hours of flooding, the insulation resistance of a three-phase cable dropped from 2000 MΩ to 2.3 MΩ. What's the difference? Most likely, it's due to insufficient waterproofing.

"Water trees" are the biggest enemy of XLPE cables when submerged. When discussing the issue of XLPE cables being submerged, "water tree aging" is unavoidable. This is a microscopic degradation phenomenon in XLPE insulation caused by the combined effects of moisture and an electric field.

The formation process of water trees is roughly as follows: Moisture seeps into the insulation layer through defects in the sheath. Under the influence of an electric field, water molecules combine with impurities in the insulation material to form an acidic microenvironment. Then, the electric field drives ion migration, causing molecular chains to break and forming tiny pores, which slowly expand into dendritic water tree channels.

Once a water tree forms, the insulation strength drops significantly—the dielectric strength of the water tree channel is only one-tenth to one-fifth of that of normal XLPE. If the water tree continues to develop, it may transform into an electrical tree under overvoltage, ultimately leading to insulation breakdown.

The impact of water trees on cable lifespan is very real: XLPE cables designed for a lifespan of 30 to 40 years when not submerged in water can have their lifespan shortened to 5 to 10 years if submerged for extended periods; if the water contains salt or acidic substances, the lifespan may even be less than 3 years.

Therefore, waterproofing and moisture resistance are not only about preventing short circuits, but also about ensuring cable lifespan.

In what scenarios can XLPE cables be safely submerged in water?

Submersible pump cables and deep well cables: These cables are designed for long-term water immersion. XLPE insulation combined with HDPE sheath or steel tape armor is perfectly adequate. There's even a dedicated category of "submersible XLPE cable" on the international market, used in drainage systems, sewage pumps, and deep well pumps.

Submarine cables: Submarine cables represent the pinnacle of XLPE waterproof applications. 500kV XLPE insulated submarine cables have been successfully used at depths of 200 meters abroad. However, submarine cables have specialized metal sheaths and corrosion-resistant designs, which cannot be directly replaced by ordinary XLPE cables.

Underground burial and water accumulation in cable trenches: This depends on the specific product. Ordinary XLPE power cables are generally fine for occasional short-term water immersion, but long-term water immersion requires waterproof cables. When selecting, look for IPX7 or higher waterproof ratings according to IEC 60529, or products labeled "water-blocking" or "submersible".

Practical Tips for Cable Laying

Even if you use waterproof cables, there are still a few details to pay attention to during on-site construction: Joints are the weakest point—no matter how well the cable itself is waterproofed, it's useless if the joints aren't sealed properly. A prefabricated waterproof outer shell filled with sealant is necessary to achieve IP68 protection.

Avoid prolonged immersion of cables—cable trenches should have drainage holes, and cable trays should have rain covers; these are basic requirements.

Buy cables according to international standards—IEC 60502-2 (extruded insulated cables with rated voltages from 1kV to 30kV), IEEE 404 (cable joint standards), and AEIC CS8 (medium and high voltage XLPE cables) all have specific requirements for moisture resistance. A qualified waterproof cable should pass immersion pressure tests and water permeability tests.

Summary
Can cross-linked polyethylene cables be submerged in water? Yes, but it depends on the type.

Ordinary XLPE power cables: Short-term, occasional immersion in water is acceptable, but long-term immersion is not recommended.

Waterproof XLPE cables: These cables feature a water-blocking structure, HDPE sheath, or metal sheath, specifically designed to withstand water immersion environments. Use them with confidence.

Two key points to consider: first, the extent of the cable's waterproof structure; and second, the quality of the joint sealing.

Finally, a word of caution: prevention is far more important than remediation when water gets into cables. Once water trees form in the insulation, the cable's performance is irreversible. Drying only removes free water, but the microscopic damage left by water trees cannot be repaired. Therefore, when choosing cables, don't just look at the price; consider the waterproof rating and structural configuration as well.