How to measure cable insulation resistance with a megohmmeter? Take a look at the detailed testing process
1. Preparation work
(1) To measure the insulation resistance of 10kV power cables, a 2500V megohmmeter (with a test line) needs to be selected. Before measurement, check the megger to see whether the pointer is normal, whether the clamp lead is well connected with the clamp terminal, and the measuring lead should be insulated with a shielding layer.
(2) Prepare the grounding rod and be ready to discharge at any time. After the power cable is cut off, discharge it first and then start measuring.
(3) Check the markings on the power cable, such as whether the rated insulation voltage in the model matches the voltage level of the megger. When measuring it, it is necessary to understand the insulation value range of the equipment.
(4) Use a dry and clean soft cloth to wipe away any dirt near the cable core.
(5) Before starting the measurement, use a rope to lift the measured cable head by about 1.5m, and the other end of the cable should also be facing upwards. It is best to stand it up to prevent the cable head from touching the wall and grounding, and a dedicated person is required to take care of it.
2. Testing process
The main test items are the insulation resistance values between phases and to ground, namely U-V, W, and ground; V-U, W, ground; W-U, V, ground, three times in total.
(1) Perform wiring as required to ensure accuracy. If the insulation resistance of the phase to ground is measured by shaking, add a shield to the measured phase and connect it to the G terminal of the megohmmeter; Connect the two cores of the non measured phase to the metal sheath of the cable and then ground them together. At the same time, connect the wires that are grounded together to the E terminal of the megohmmeter; Connect a test wire to the L terminal of the megohmmeter, and the other end of the test wire (L wire) will not be connected to the core at this time.
(2) One person should wear insulated gloves and hold the insulated part of the L test line with their hand. The other person should rotate the handle of the megohmmeter to make the generator speed reach 120r/min, and contact the L line with the wire core. After 1 minute (after the pointer stabilizes), record the insulation resistance value.
(3) Remove the L-wire from the core, stop rotating the handle, and then discharge it. After the discharge is completed, the measurement of one phase cable core has been completed.
Repeat the above steps twice and measure the insulation resistance of the other two-phase cable cores separately. If the measured value is not within the qualified range, it indicates that the insulation resistance of the measured power cable is not qualified; If the three measurement results are within the standard range, it indicates that the insulation resistance of the power cable is qualified.
The standard provisions for determining whether the insulation resistance of power cables is qualified are as follows:
1. 10kV power cables with a length of 500m or less should be measured using a 2500V megohmmeter. When the cable temperature is 20 ℃, the insulation resistance value should generally not be less than 400M Ω.
2. The insulation resistance value should be almost consistent between three phases; If inconsistent, the imbalance coefficient shall not exceed 2.5.
3. The value measured this time should not decrease by more than 30% when converted to the same temperature as the previous measurement.
