Fire-resistant cables are lifelines for ensuring power connectivity in buildings and industrial facilities under extreme conditions. While their exceptional fire performance is critical, moisture ingress poses a hidden yet frequent risk that can severely compromise electrical performance, long-term durability, and even lead to the failure of their fire-protection function. As experts deeply rooted in the field of cable materials, ONE WORLD understands that cable moisture prevention is a systemic issue spanning the entire chain from the selection of core materials like insulation compounds and sheathing compounds, to installation, construction, and ongoing maintenance. This article will conduct an in-depth analysis of moisture ingress factors, starting from the characteristics of core materials such as LSZH, XLPE, and Magnesium Oxide.
1. Cable Ontology: Core Materials and Structure as the Foundation of Moisture Prevention
The moisture resistance of a fire-resistant cable is fundamentally determined by the properties and synergistic design of its core cable materials.
Conductor: High-purity Copper or Aluminum conductors are chemically stable themselves. However, if moisture penetrates, it can initiate persistent electrochemical corrosion, leading to a reduced conductor cross-section, increased resistance, and consequently becoming a potential point for local overheating.
Insulation Layer: The Core Barrier Against Moisture
Inorganic Mineral Insulation Compounds (e.g., Magnesium Oxide, Mica): Materials like Magnesium Oxide and Mica are inherently non-combustible and resistant to high temperatures. However, the microscopic structure of their powder or mica tape laminations contains inherent gaps that can easily become pathways for water vapor diffusion. Therefore, cables using such insulation compounds (e.g., Mineral Insulated Cables) must rely on a continuous metal sheath (e.g., copper tube) to achieve hermetic sealing. If this metal sheath is damaged during production or installation, moisture ingress into the insulating medium like Magnesium Oxide will cause a sharp decrease in its insulation resistivity.
Polymer Insulation Compounds (e.g., XLPE): The moisture resistance of Cross-Linked Polyethylene (XLPE) stems from the three-dimensional network structure formed during the cross-linking process. This structure significantly enhances the density of the polymer, effectively blocking water molecule penetration. High-quality XLPE insulation compounds exhibit very low water absorption (typically <0.1%). In contrast, inferior or aged XLPE with defects can form moisture-absorption channels due to molecular chain breakage, leading to permanent degradation of insulation performance.
Sheath: The First Line of Defense Against the Environment
Low Smoke Zero Halogen (LSZH) Sheathing Compound: The moisture resistance and hydrolysis resistance of LSZH materials directly depend on the formulation design and compatibility between its polymer matrix (e.g., polyolefin) and inorganic hydroxide fillers (e.g., Aluminum Hydroxide, Magnesium Hydroxide). A high-quality LSZH sheathing compound must, while providing flame retardancy, achieve low water absorption and excellent long-term hydrolysis resistance through meticulous formulation processes to ensure stable protective performance in damp or water-accumulating environments.
Metal Sheath (e.g., Aluminum-Plastic Composite Tape): As a classic radial moisture barrier, the effectiveness of Aluminum-Plastic Composite Tape highly depends on the processing and sealing technology at its longitudinal overlap. If the seal using hot-melt adhesive at this junction is discontinuous or defective, the integrity of the entire barrier is significantly compromised.
2. Installation and Construction: The Field Test for the Material Protection System
Over 80% of cable moisture ingress cases occur during the installation and construction phase. The quality of construction directly determines whether the cable’s inherent moisture resistance can be fully utilized.
Inadequate Environmental Control: Performing cable laying, cutting, and jointing in environments with relative humidity exceeding 85% causes water vapor from the air to rapidly condense on the cable cuts and exposed surfaces of insulation compounds and filling materials. For Magnesium Oxide mineral insulated cables, the exposure time must be strictly limited; otherwise, the Magnesium Oxide powder will rapidly absorb moisture from the air.
Defects in Sealing Technology and Auxiliary Materials:
Joints and Terminations: The heat-shrink tubes, cold-shrink terminations, or poured sealants used here are the most critical links in the moisture protection system. If these sealing materials have insufficient shrinkage force, inadequate adhesion strength to the cable sheathing compound (e.g., LSZH), or poor inherent aging resistance, they instantly become shortcuts for water vapor ingress.
Conduits and Cable Trays: After cable installation, if the ends of conduits are not tightly sealed with professional fire-resistant putty or sealant, the conduit becomes a “culvert” accumulating moisture or even stagnant water, chronically eroding the cable’s outer sheath.
Mechanical Damage: Bending beyond the minimum bending radius during installation, pulling with sharp tools, or sharp edges along the laying route can cause invisible scratches, indentations, or micro-cracks on the LSZH sheath or Aluminum-Plastic Composite Tape, permanently compromising their sealing integrity.
3. Operation, Maintenance, and Environment: Material Durability Under Long-Term Service
After a cable is commissioned, its moisture resistance relies on the durability of the cable materials under long-term environmental stress.
Maintenance Oversights:
Improper sealing or damage to cable trench/well covers allows rainwater and condensation water direct entry. Long-term immersion severely tests the hydrolysis resistance limits of the LSZH sheathing compound.
Failure to establish a periodic inspection regime prevents the timely detection and replacement of aged, cracked sealants, heat-shrink tubes, and other sealing materials.
Aging Effects of Environmental Stress on Materials:
Temperature Cycling: Diurnal and seasonal temperature differences cause a “breathing effect” within the cable. This cyclic stress, acting long-term on polymer materials like XLPE and LSZH, can induce micro-fatigue defects, creating conditions for moisture permeation.
Chemical Corrosion: In acidic/alkaline soil or industrial environments containing corrosive media, both the polymer chains of the LSZH sheath and metal sheaths can suffer chemical attack, leading to material powdering, perforation, and loss of protective function.
Conclusion and Recommendations
Moisture prevention in fire-resistant cables is a systematic project requiring multi-dimensional coordination from the inside out. It starts with the core cable materials – such as XLPE insulation compounds with a dense cross-linked structure, scientifically formulated hydrolysis-resistant LSZH sheathing compounds, and Magnesium Oxide insulation systems relying on metal sheaths for absolute sealing. It is realized through standardized construction and the rigorous application of auxiliary materials like sealants and heat-shrink tubes. And it ultimately depends on predictive maintenance management.
Therefore, sourcing products manufactured with high-performance cable materials (e.g., premium LSZH, XLPE, Magnesium Oxide) and featuring robust structural design is the fundamental cornerstone for building moisture resistance throughout a cable’s entire life cycle. Deeply understanding and respecting the physical and chemical properties of each cable material is the starting point for effectively identifying, assessing, and preventing moisture ingress risks.
Post time: Nov-27-2025