When testing insulation resistance with manual insulation resistance tester, when is the most accurate reading?

Manual insulation resistance tester is the most accurate when shaking the meter at a constant speed (1200 rpm).

The shaking table, also called megohmmeter, is mainly used to measure the insulation resistance of electrical equipment. It consists of alternator voltage doubling rectifier circuit, meter head and other components. When the vibration table vibrates, DC voltage will be generated. When a certain voltage is applied to the insulating material, an extremely weak current will flow in the insulating material, which consists of three parts, namely, capacitance current, absorption current and leakage current. The ratio of DC voltage generated by shaking table to leakage current is insulation resistance. The test to check whether the insulation material is qualified by shaking table is called insulation resistance test, which can find out whether the insulation material is damp, damaged and aged, and thus find out the equipment defects.

Commonly used vibration tables are ZC-7, ZG- 1 1, ZC-25, etc. The rated voltages of megohmmeters are 250, 500, 1000, 2500 volts, etc. , the measuring range is 500, 1000 and 2000 mω.

1 shaking test of insulation resistance of distribution transformer

Before the distribution transformer is put into operation after installation or maintenance, and after long-term shutdown, the winding insulation resistance should be measured with a shaking table with a voltage of1000 ~ 2500 V. The measured value and measured oil temperature should be recorded in the transformer file. When in use, the measured value is compared with the insulation resistance value of the distribution transformer to judge the insulation state of the transformer. If the insulation resistance of the distribution transformer suddenly drops to 50% or less of the initial value, it should be replaced and overhauled.

The accuracy of measuring insulation resistance is closely related to the measurement method and weather conditions during measurement. Attention should be paid to the following items when measuring:

1. 1 measuring conditions. Select the weather with temperature above 5℃ and humidity below 70% for measurement;

1.2 Measure the voltage. Measuring the insulation resistance of distribution transformer should use 1000V or 2500V shaking table. When comparing the insulation resistance of transformer, the shaking table used should be the interphase voltage level;

1.3 measurement project. Insulation resistance of primary winding to secondary winding, primary winding to ground (iron core or shell of distribution transformer) and secondary winding to ground shall be measured;

1.4 measurement safety. Before and after measurement, the outlet terminals of the primary side and the secondary side of the distribution transformer should be grounded and discharged respectively, and the zero line should be disconnected to release the residual static charge on the winding to ensure safety.

Shaking test of insulation resistance of high and low voltage arrester

Shake and measure the insulation resistance of high-voltage valve arrester, using 1000V or 2500V shaking table; Use a 500V or 1000V shaking table to shake and measure the low-voltage 380V arrester, and connect the two test lines of the shaking table to the upper and lower terminals of the arrester respectively. See the following table for the insulation resistance standard of high-voltage valve arrester:

The insulation resistance of low-voltage 380V arrester should not be less than 1 mω.

3 shaking test of insulation resistance of insulator

Measure the insulation resistance of low-voltage insulators with a 500~ 1000V shaking table, and its value should not be lower than 1mω. The vibration measurement method is as follows:

3. 1 pin insulator. Connect the test wire (-) of the vibration table to the iron foot, and connect the connecting wire (+) to the metal rod of the screwdriver. Testers carry insulating gloves and hold the wooden handle of the screwdriver so that the metal tip (knife) of the screwdriver slides around the top and neck of the pin insulator. When sliding, shake the table to measure insulation.

3.2 bobbin insulator and butterfly insulator. When measuring, a circle of copper wire can be tied on the circumference of the middle neck, one wire of the shaking table is connected to the copper wire, and the other wire of the shaking table is connected to the iron frame or iron shoulder of the insulator.

3.3 It takes time to shake the new bobbin insulator alone. When measuring, one wire of the shaking table is contacted with the outer surface of the insulator, and the other test wire is tied to the long metal rod of the big screwdriver, and the end of the metal rod is inserted into the hole of the inner neck of the insulator to avoid repeated sliding up and down, left and right, so as to measure its insulation resistance. The method of measuring butterfly insulator is the same as this.

4 Power cable shaking test

4. 1 Remove the power supply and all external connections of the tested cable, and at the same time, ground all core wires of the tested cable, and discharge 1 min.

4.2 Wipe off the dirt on the surface of cable terminal head.

4.3 Determine the specifications of shaking table, and use 1kV shaking table for cables with working voltage below 1kV, and use 2500V shaking table for cables with working voltage above 1kV.

4.4 When testing a three-core cable, one core should be connected with (+), and the other two cores should be grounded together with the lead sheath. If the measured cable may cause measurement error due to surface leakage, a protective ring should also be connected.

4.5 Shake the shaking table to the rated speed (1 200rpm), and the insulation resistance shall be the reading when the resistance value is stable after shaking1min. When measuring, the rotating speed of the shaking table should not be lower than 80% of the rated rotating speed.

4.6 After testing one core of the cable, fully discharge it, and then test other cores in the same way.

Measurement of phase-shifting capacitor

During the operation of the phase-shifting capacitor, the insulation resistance should be measured regularly. Insulation resistance can be divided into insulation resistance between two poles and insulation resistance between two poles and shell. Because there is capacitance between the two poles of the capacitor and the shell, special attention should be paid to the shaking method, otherwise it is easy to damage the shaking table.

500V shaking table can be used to shake the low-voltage capacitor, and its insulation resistance should be not less than 20MΩ. Before shaking test, the capacitor should be discharged first. When shaking, first shake the shaking table to the specified speed, connect the shaking table wires to the two poles of the capacitor after the pointer is balanced, and continue to rotate the shaking table. At first, due to the charging of the capacitor, the pointer will drop, and then slowly rise to a stable value. At this time, the reading is the inter-electrode insulation resistance of the capacitor. After reading, remove the shaking wire first, and then stop shaking, otherwise the instrument will be burnt out due to capacitor discharge. After shaking, the capacitor should be discharged immediately.

Because the capacitor consists of series and parallel capacitive elements, the insulation deterioration of a single element will not significantly reduce the insulation resistance of the whole capacitor, so it is difficult to find the defects of the inter-electrode insulation resistance. Therefore, this test is generally not done, only the insulation measurement of the shell of a single capacitor is done.

6 Measurement of insulation resistance of motor

The three-phase winding of the motor has six terminals, which are respectively connected to the six columns of the junction box. When shaking test, first open the motor junction box, remove the connector, shake test the insulation resistance between phases (windings), then connect the motor winding to the line terminal L and the shell to the terminal E, and measure the resistance of each phase winding to the ground (or to the shell).

7 matters needing attention in using shaking table

7. 1 When selecting the voltage level of shaking table, it should be noted that1000 ~ 2500V shaking table should be selected for equipment, motor windings and power transformer windings with rated voltage above 500 V; To measure the insulation resistance of low-voltage electrical equipment below 500 volts, a shaking table with a rated voltage of 500 volts should be used. ..

7.2 In order to prevent personal and equipment accidents and obtain accurate measurement results, the power supply must be cut off before measurement, and the tested equipment should be fully discharged.

7.3 The wires connected between the instrument terminal and the equipment under test cannot use twisted-pair insulated wires and stranded wires, but single stranded wires should be used for separate connection, so as to avoid errors caused by poor insulation of stranded wires.

7.4 The surface of the measured object should be clean and dry to reduce errors.

7.5 Before measurement, conduct open circuit and short circuit tests on the shaking table to check whether the shaking table is in good condition. When testing, first open two connecting wires, shake the handle, and the pointer should be in the ∞ position, then short the two connecting wires, and gently shake the handle, otherwise the shaking table is faulty and needs to be overhauled.

7.6 When measuring, place the shaking table smoothly. L terminal is connected to the measured object, and E terminal is grounded. The speed of shaking the handle should be gradually increased from slow to slow, and the speed should be kept at about 120r/min. If the device under test is short-circuited, when the pointer is set to "0", stop shaking the handle immediately to avoid burning the instrument.

7.7 When measuring the insulation resistance of electrical equipment with large capacitance, after measuring the insulation resistance, disconnect the L terminal first, and then slow down and loosen the handle of the shaking table, so as to avoid damaging the instrument by charging the tested equipment backwards to the shaking table.

7.8 It is forbidden to use shaking table when there is lightning or near high-voltage equipment to avoid danger.

7.9 After the measurement, fully discharge the tested equipment.