1、 Characteristics of insulation materials themselves

1. Material Types
   The resistivity of different insulation materials varies greatly:

   • Cross linked polyethylene (XLPE): high insulation resistance (usually ≥ 10 ¹⁴Ω· cm), strong aging resistance, suitable for high temperature and high voltage scenarios;

   • Polyvinyl chloride (PVC): moderate insulation resistance (about 10 ¹² -10 ¹³ Ω· cm), but susceptible to temperature and humidity;

   • Rubber (such as nitrile rubber): has low insulation resistance (about 10 ¹¹¹ -10 ¹Ω· cm), but is flexible and suitable for mobile applications.

2. Material purity and process

   • If impurities (such as metal particles, moisture, bubbles) are mixed into the material, it will reduce the insulation resistance (impurities may become conductive channels);

   • Production process defects, such as uneven insulation layer thickness and insufficient cross-linking, can lead to weak local insulation and decreased resistance.

3. Aging degree
   After long-term use, insulation materials will age due to oxidation, thermal decomposition, chemical corrosion, etc., causing molecular structure damage. The insulation resistance will significantly decrease with increasing aging degree

   • For example, PVC will decompose into HCl at high temperatures, causing the insulation layer to become brittle and the resistance to plummet; XLPE will degrade and its insulation performance will decrease when exposed to ultraviolet radiation for a long time.

2、 The impact of environmental conditions

1. ambient humidity

   • Humidity is one of the most significant factors affecting insulation resistance. In humid environments, the surface of the insulation layer will adsorb moisture, forming a water film (water itself has weak conductivity, but its conductivity increases after dissolving impurities in the air), resulting in a significant decrease in surface insulation resistance;

   • If there are micropores or cracks in the insulation layer, moisture will seep into the interior, reducing the volume insulation resistance (for example, when the humidity increases from 30% to 90%, the insulation resistance of PVC cables may decrease by 1-2 orders of magnitude).

2. ambient temperature

   • The resistivity of insulating materials decreases with increasing temperature (most organic materials have a negative temperature coefficient). For example, when the temperature rises from 20 ℃ to 60 ℃, the insulation resistance of XLPE may decrease by more than 50%;

   • High temperature can also accelerate material aging (such as an exponential increase in oxidation reaction rate with increasing temperature), and long-term high temperature environments can lead to an irreversible decrease in insulation resistance.

3. Pollution and corrosive media

   • Pollutants such as dust, oil, salt spray, etc. in the environment will adhere to the surface of the cable, absorb moisture and form a conductive layer, reducing the surface insulation resistance;

   • Corrosive gases (such as sulfur dioxide, ammonia) or liquids (such as acid-base solutions) can corrode the insulation layer, damage the material structure, and cause a decrease in insulation resistance (for example, the insulation resistance of cables in chemical workshops is usually lower than that in normal environments).

3、 The impact of cable operation status

1. Working voltage and overvoltage

   • When operating at rated voltage for a long time, the insulation layer will undergo "electrochemical aging" (under the action of an electric field, trace ions in the material migrate, forming local conductive channels), resulting in a slow decrease in insulation resistance;

   • Short term overvoltage (such as operating overvoltage, lightning induced voltage) may cause local breakdown or damage to the insulation layer (forming small conductive channels without full breakdown), resulting in a sudden drop in insulation resistance.

2. Load current and temperature rise

   • When the load current of the cable is too high, the heating of the conductor will cause the temperature of the insulation layer to rise (such as the temperature may exceed the allowable upper limit of the material during overload), resulting in a temporary decrease in insulation resistance;

   • Frequent overloading can accelerate insulation aging and continuously reduce resistance (for example, if a motor control cable is overloaded for a long time, the insulation resistance may decrease from the initial 1000M Ω to below 100M Ω).

3. laying method

   • When buried underground, if the soil is moist or there are corrosive substances present, the insulation layer is susceptible to erosion;

   • When laying through pipes, if there is water accumulation or poor ventilation inside the pipes, it can cause the cables to become damp, the temperature to rise, and the insulation resistance to decrease;

   • When laying overhead, the insulation layer is prone to aging and cracking due to the influence of ultraviolet radiation, rain and snow, and the resistance decreases.

4、 External damage and maintenance

1. mechanical damage
   If the cable is subjected to compression, puncture, or excessive bending during construction or operation, it may cause damage to the insulation layer (even if the appearance is not noticed, cracks may have appeared inside), resulting in a decrease in insulation resistance (the damaged area may form a local conductive path).
2. Joint processing quality
   Improper insulation treatment of cable joints (such as terminal heads and intermediate joints) (such as poor sealing or insufficient filling of insulation glue) can lead to the intrusion of moisture or impurities, resulting in insulation resistance at the joint being much lower than that of the cable body (for example, the normal insulation resistance of a cable is 500M Ω, but it may drop to 10M Ω when the joint is poorly treated).
3. Improper maintenance

   • Using conductive liquid (such as wiping with water) when cleaning cables can temporarily reduce surface insulation resistance;

   • Long term lack of testing and maintenance makes it difficult to detect insulation aging or damage in a timely manner, resulting in a continuous decrease in resistance that goes unnoticed.

This article is generated by AI