The procedure for making insulation test measurements with a megohmmeter
Insulation resistance is an important characteristic of the insulation condition of electrical equipment. Therefore, resistance measurement is performed during all insulation condition checks. Insulation resistance is measured with a megohmmeter.
Electronic megohmmeters of type F4101, F4102 for voltages of 100, 500 and 1000 V. have found wide application. Megoometers of types M4100 / 1 — M4100 / 5 and MS -05 are still used for voltages of 100, 250, 500, 1000 in commissioning and operational practice. and 2500 V. The error of the F4101 device does not exceed ± 2.5%, and of the M4100 type devices - up to 1% of the length of the working part of the scale. The F4101 device is powered by 127-220 V AC or 12 V DC source. The M4100 type devices are powered by built-in generators.
The choice of the type of megohmmeter is made depending on the nominal resistance of the object (power cables 1 — 1000, switching equipment 1000 — 5000, power transformers 10 — 20,000, electric cars 0.1 — 1000, porcelain insulators 100 — 10,000 MΩ), its parameters and nominal voltage.
As a rule, to measure the insulation resistance of equipment with a nominal voltage of up to 1000 V (secondary switching circuits, motors, etc.), megohmmeters are used Rated voltage 100, 250, 500 and 1000 V, and in electrical installations with a nominal voltage of more than 1000 V, a megohmmeter is used for 1000 and 2500 V.
When making measurements with megohmmeters, the following order of operations is recommended:
1. Measure the insulation resistance of the connecting wires, the value of which should not be less than the upper measurement limit of the megohmmeter.
2. Set the measurement limit; if the value of the insulation resistance is unknown, then in order to avoid «off-scale» of the pointer of the meter, it is necessary to start with the largest measurement limit; when choosing a measurement limit, one should be guided by the fact that the accuracy will be greatest when reading readings in the working part of the scale.
3. Make sure there is no voltage on the test object.
4. Disconnect or short circuit all low insulation or low test voltage parts, capacitors and semiconductors.
5. Ground the circuit under test while connecting the device.
6.Pressing the «high voltage» button in devices powered by the network or turning the handle of the inductor megohmmeter generator at a speed of about 120 rpm, 60 seconds after the start of the measurement, fix the resistance value on the scale of the device.
7. When measuring the insulation resistance of high-capacity objects, take the reading after the needle has completely come to rest.
8. After the end of the measurement, especially for equipment with a large capacity (for example, long cables), before disconnecting the ends of the device, it is necessary to remove the accumulated charge by applying a mass.
When the result of the insulation resistance measurement can be distorted by surface leakage currents, for example, due to the wetting of the surface of the insulating parts of the installation, the conductive electrode is applied to the insulation of the object connected to the terminal of the megohmmeter E.
The connection of the conducting electrode E is determined by the condition of creating the largest potential difference between the mass and the place of connection of the screen.
In the case of measuring the insulation of a cable insulated from the ground, the clamp E is connected to the cable shield; when measuring the insulation resistance between the windings of electrical machines, the clamp E is connected to the body; when measuring the resistance of the transformer windings, the clamp E is connected under the skirt of the output insulator.
Measurement of insulation resistance of power supply and lighting is carried out with switches on, fuses removed, electrical receivers, devices, apparatus and lamps turned off.
It is strictly forbidden to measure the insulation of a line if it passes at least for a small section near another powered line and during a lightning storm on overhead power lines.