Cables serve as the physical carriers for power transmission and signal transmission. Their insulation performance is a key factor determining the safety, stability, and long-term operation of the entire system. Insulation resistance testing, as a non-destructive electrical test, quantitatively assesses the ability of insulating materials to prevent current from passing through by measuring the leakage current generated after applying a direct current voltage. This test can effectively detect defects such as moisture absorption, aging, mechanical damage, and surface contamination of the insulation, and is one of the most fundamental and commonly used items in cable handover tests, preventive tests, and fault diagnosis. Correctly conducting the test and scientifically analyzing the results have significant engineering significance for ensuring equipment lifespan and system reliability.
1. Test Principle and Method
(1) Test Principle
Insulation resistance testing is based on Ohm’s Law. When a direct current voltage is applied to the insulating medium, the current flowing through consists of three parts: capacitive current, absorption current, and leakage current. As the application time of the voltage increases, the capacitive current and absorption current rapidly decay, and the leakage current stabilizes. At this point, the ratio of the applied direct current voltage to the stable leakage current is the insulation resistance value.
(2) Main Test Methods
Voltage – Current Method (High Resistance Meter Method) is suitable for precise laboratory measurements. A high-impedance DC voltage source is used to apply voltage to the sample, and a highly sensitive current meter (such as an electrometer or piciohm meter) is employed to measure the tiny current in the insulating medium, thereby calculating the resistance value. The measurement range is wide, typically ranging from 10⁴Ω to 10⁶Ω.
DC comparison method is mainly used for factory quality control and on-site rapid detection. The core principle is to connect the insulation resistance of the tested cable with a high-precision standard resistor in the same circuit. By comparing the voltage drop or current of the two, the resistance value of the tested cable can be indirectly determined. The operation is simple, and the measurement range covers from 10⁵Ω to 2×10¹⁵Ω.
(III) Testing Instruments
In engineering applications, megohmmeters are commonly used. They are classified by power supply type as hand-cranked, battery-powered, and external power supply. The rated output voltage is selected based on the cable voltage level, typically 500V, 1000V, 2500V, and 5000V.
II. Key Test Conditions and Operating Specifications
The insulation resistance value is significantly affected by environmental temperature, humidity, sample length, and charging time. It is necessary to strictly follow standardized operating requirements to ensure the results are repeatable and comparable.
(1) Environmental Control
Type tests and arbitration tests should be conducted in a standard environment: temperature 20℃ (allowable deviation ±5℃), relative humidity ≤80%. For every 10℃ increase in temperature, the polymer insulation resistance may decrease exponentially. When comparing historical data, the measured values at different temperatures should be converted to the same reference temperature.
(2) Sample Preparation
The effective length of the test sample for finished cables should be ≥10 meters (except as otherwise specified in the product standards). When removing the outer sheath, metal armor, or shielding layer, use specialized tools to carefully peel off without scratching the internal insulation layer. Avoid changing the surface leakage path to ensure the measured value is not significantly偏低.
(3) Wiring Method
Correct wiring is the key to obtaining accurate data. Different cable wiring logic is as follows：
Cables with metal sheaths or total shielding layers： Connect the conductor to the metal sheath / shielding layer.
Single-core cables without sheaths： Use the immersion electrode method. Seal both ends of the sample and immerse it in water to measure the resistance between the conductor and the electrode.
Multi-core cables without sheaths： Measure the insulation resistance of each wire core to the other wire cores and the ground wire (if any).
(4) Reading Time
According to the standard， the insulation resistance value should be read exactly one minute after applying the rated test voltage. This time point can avoid transient interference from capacitive current and absorption current， ensuring that the value mainly reflects the stable leakage current component and guaranteeing data consistency.
III. Result Judgment and Analysis
The insulation resistance value is approximately inversely proportional to the cable length and decreases with temperature increase. The qualification determination should be comprehensively evaluated by combining standardized conversion， longitudinal comparison， and lateral balance， rather than relying solely on a single value.
(1) Reference Criteria
Specific limits follow the product standards， design drawings， or test procedures for corresponding voltage levels. Typical engineering references are as follows：
10kV cross-linked polyethylene power cables： 20℃， length ≤500 meters， insulation resistance ≥400MΩ.
Polyethylene insulated communication cables： 20℃， insulation resistance constant ≥6000MΩ・km.
Polyvinyl chloride insulated communication cables： 20℃， insulation resistance constant ≥120MΩ・km.
(2) Core Judgment Principles
The value compliance principle: The test value must be higher than the lower allowable value specified in the standard or design document. This is the basic threshold for commissioning.
Absorption ratio and polarization index. For large-capacity and high-voltage-grade cables, additional measurements are required:
Absorption ratio: The ratio of insulation resistance at 60 seconds to that at 15 seconds.
Polarization index: The ratio of insulation resistance at 10 minutes to that at 1 minute.
The absorption ratio and polarization index of damp or severely aged insulation will be significantly lower.
Vertical comparison principle (trend analysis) – Compare the current measurement values after converting them to the reference temperature of the historical tests. If the insulation resistance has decreased by more than 30% compared to the previous measurement, even if the current value is acceptable, it indicates that the insulation may be deteriorating at an accelerated rate. Therefore, the monitoring period should be shortened or further diagnosis should be conducted.
Horizontal comparison principle (balance analysis) For three-phase cables, the insulation resistance values of each phase need to be compared. Normally, the values of the three phases are basically the same; if one phase is significantly lower and the imbalance coefficient (larger value / smallest value) is greater than 2.5, it is highly suspected that there is a local defect in that phase, and it needs to be thoroughly investigated.
Post time: Jul-22-2026