The measurement of cable insulation resistance is a crucial test for evaluating the insulation performance of cables and determining whether they can operate safely. The core of this test is to measure the insulation resistance values between the cable conductors and the insulation layer, between the conductors and the sheath, and between different conductors using an ohmmeter (multimeter). This helps determine if there are issues such as moisture, aging, or damage to the insulation.
1. Preparations before measurement
Equipment selection
Select the corresponding specification of an ohmmeter based on the cable voltage level:
Low-voltage cables (≤1kV): Use a 500V ohmmeter with a measurement range of 0~500MΩ
Medium and high-voltage cables (6kV~35kV): Use a 2500V ohmmeter
High-voltage cables (≥110kV): Use a 5000V or 10000V ohmmeter
Ohmmeter condition check process: Short-circuit “L” (line end) and “E” (ground end), rotate the handle at a constant speed, the pointer should point to 0; in an open circuit state, rotate the handle, the pointer should point to ∞.
Cable pre-treatment
Power off, test for electricity, and ground the cable: Completely disconnect the cable ends from the power source and load, use an electric tester to confirm there is no electricity, then ground the cable conductors for discharge for 5~10 minutes (for high-voltage cables, extend the discharge time), release residual charges
Clean the surface: Remove oil, moisture, and dust from the cable ends and the surface of the insulation layer to avoid interference with the measurement results
Remove the original grounding: Disconnect the grounding connections of the cable sheath and shielding layer to ensure an independent measurement circuit
2. Wiring method for measurement
Cable insulation resistance measurement mainly includes two types of measurements: conductor – insulation – sheath (or ground) and conductor-to-conductor insulation. The following explains the wiring method using a common three-core cable as an example:
Conductor – sheath (ground) measurement
Short-circuit the three conductors at one end of the cable and connect them to the L terminal of the ohmmeter
Connect the metal sheath, shielding layer, or armor layer of the cable to the E terminal of the ohmmeter
Wrap the G terminal (shielding terminal) of the ohmmeter around the surface of the cable insulation layer (near the conductor end) to reduce surface leakage current interference
Conductor-to-conductor insulation measurement
Select any two conductors, connect one to the L terminal, another to the E terminal, and leave the third conductor suspended or grounded
Measure the insulation resistance between each pair of conductors (such as A-B, A-C, B-C)
3. Measurement operation steps
After wiring, rotate the ohmmeter handle at a constant speed of 120r/min
Read the insulation resistance value after continuous rotation for 60 seconds (for low-voltage cables, you can also read the value for 15 seconds to calculate the absorption ratio)
After the measurement, disconnect the L terminal wiring first, then stop rotating the handle to prevent the residual charges in the cable from reversing and damaging the ohmmeter
Ground the cable conductors again to ensure safety of the operation
Record the complete measurement data, including environmental temperature, humidity, cable model and specification, measurement voltage, and insulation resistance value
4. Result judgment criteria
Insulation resistance value requirements
Low-voltage cables: At room temperature (20℃), the insulation resistance should be ≥1MΩ, and the difference in resistance between each phase should not exceed 2 times
6kV cables: Using a 2500V ohmmeter to measure, the insulation resistance should be ≥10MΩ
High-voltage cables: There is no fixed lower limit for insulation resistance values. It is necessary to compare with historical data or the measurement data of cables of the same batch to ensure no significant decrease
Absorption ratio judgment
Absorption ratio K = R60s/R15s. For oil-paper insulated cables, K ≥ 1.3 is qualified; for rubber and plastic insulated cables, absorption ratio is generally not evaluated, and the focus is on the insulation resistance value.
Other judgments
The insulation resistance values between conductors should be basically consistent, with no significant differences
If the insulation resistance value is extremely low (close to 0), it indicates that there are severe problems such as short circuits, moisture, or damage to the insulation layer
5. Safety and precautions During the measurement process, no one is allowed to work at both ends of the cable, and a “Do Not Close Switch, Work In Progress” warning sign should be hung.
In thunderstorm weather, outdoor cable measurements are strictly prohibited to prevent the risk of lightning strike.
The output voltage of the megohmmeter is relatively high. When operating, insulated gloves should be worn to avoid touching the connection terminals.
After the measurement, the cable must be fully discharged, especially for high-voltage cables, as the residual charge may cause an electric shock accident.
The insulation resistance is greatly affected by temperature. For every 10℃ increase in temperature, the insulation resistance approximately decreases by half. When recording data, the environmental temperature should be noted. If necessary, the measurement values should be converted to the standard value at 20℃.
Post time: Jan-09-2026