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.
The following is the detailed operation procedure and precautions for measuring cable insulation resistance:
1. Preparations before measurement
Equipment selection
Select the appropriate specification of an ohmmeter based on the voltage level of the cable: for low-voltage cables (≤1kV), use a 500V ohmmeter with a measurement range of 0~500MΩ; for medium and high-voltage cables (6kV~35kV), use a 2500V ohmmeter; for high-voltage cables (≥110kV), use a 5000V or 10000V ohmmeter.
Check the state of the ohmmeter as follows: short-circuit “L” (line end) and “E” (ground end), rotate the handle at a constant speed, and the pointer should point to 0; in the open circuit state, rotate the handle, and the pointer should point to ∞.
Preparation of the cable
Disconnect the cable ends from the power source and load completely, use an electric tester to confirm there is no electricity, then ground and discharge the cable conductors for 5~10 minutes. The discharge time for high-voltage cables should be appropriately extended to release residual charges.
Clean the oil stains, moisture, and dust on the cable end connections and the surface of the insulation layer to avoid interference from surface leakage on the measurement results.
Disconnect the grounding connections of the cable sheath and shielding layer to ensure the measurement circuit remains independent.
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. Taking a common three-core cable as an example, the specific wiring method is as follows:
Conductor – sheath (ground) measurement
Short-circuit the three conductors at one end of the cable, 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) around the surface of the cable insulation layer (near the conductor end) to reduce the interference of surface leakage current.
Conductor-to-conductor insulation measurement
Select any two conductors, connect one to the L terminal, another to the E terminal, and keep the third conductor suspended or grounded. Measure the insulation resistance between each pair of conductors in this way, covering all conductor combinations.
3. Measurement operation steps
After wiring, rotate the handle of the ohmmeter 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 at 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.
After the measurement, ground and discharge the cable conductors again to ensure safety during the operation.
Record the measurement data completely, including the ambient temperature, humidity, cable model and specification, measurement voltage, and insulation resistance value.
4. Result judgment criteria
Insulation resistance value requirements
For low-voltage cables at normal temperature (20℃), the insulation resistance should be ≥1MΩ, and the difference in resistance between each phase should not exceed 2 times.
For 6kV cables, using a 2500V ohmmeter for measurement, the insulation resistance should be ≥10MΩ.
There is no fixed lower limit requirement for the insulation resistance value of high-voltage cables, but it should be compared with historical data or the measurement data of the same batch of cables to ensure no obvious downward trend.
Absorption ratio judgment
The absorption ratio calculation formula is K = R60s/R15s. For oil-paper insulated cables, the absorption ratio K ≥ 1.3 is considered qualified; for rubber and plastic insulated cables, the absorption ratio is generally not evaluated, and the focus is on the value of insulation resistance.
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 problems such as short circuits, moisture, or damage in the cable insulation layer.
V. Safety and Precautions
During the measurement process, no one is allowed to work at both ends of the cable. A warning sign reading “Do not close the switch, work in progress” should be hung.
During thunderstorm weather, it is strictly prohibited to measure outdoor cables to prevent the risk of lightning strikes.
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 the measurement data, the environmental temperature should be noted. If necessary, the measurement values should be converted to the standard value at 20℃.
Post time: Jan-12-2026