When operating electrical networks, the main criterion for ensuring their reliability and electrical safety is the state of insulation of conductive elements. To measure the insulation resistance, a megohmmeter device is used, working with which requires knowledge of safety precautions. Voltage from 200 to 2500 V, which occurs during the operation of the device, is life-threatening.
Content
- Reasons for a decrease in insulation resistance
- The device and how it works
- Connecting probes
- Measurement order
- Security
Reasons for a decrease in insulation resistance
External factors affecting the elements of electrical circuits can lead to defects and disruptions in the operation of electrical installations. The main reasons for the decrease in insulation resistance:
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heating of elements, which reduces the dielectric properties of materials; - improper use of devices;
- high humidity;
- dust and dirt covering instrument cases;
- mechanical damage to the wiring.
For the timely detection of violations in the operation of electrical networks, it is required to use an insulation resistance meter.
The device and how it works
The principle of operation of the megohmmeter is based on Ohm's law. This explains the name, which has the prefix "mega", referring to the unit of resistance "Ohm" (ten to the sixth power of Ohm), and "meter" in Greek means measurement. Therefore, the correct spelling is a megohmmeter, not a megohmmeter. The device is used to measure large values of resistance.
Ohm's law for a section of a circuit expresses the relationship between the strength of the current flowing in a conductor, the voltage and resistance of the conductor. I = U / R, where I is the current, U is the voltage, and R is the resistance.
The megohmmeter generates a calibrated high voltage that is applied to the tested section of the circuit. With the help of an ammeter, the current strength is measured by these values and the resistance of the circuit is determined. The device includes:
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DC generator; - measuring probes, which are wires with lugs;
- a set of current limiting resistors;
- a switch that allows you to switch different resistors;
- digital screen or scale of measurement.
The devices are manufactured in various designs. The oldest analog devices, also called dials, are equipped with hand-operated generators, driven by the rotation of the handle. Modern digital devices are equipped with rechargeable batteries or built-in power supplies. The measurement results are displayed on the screen of a digital device or on an analog scale. Many modern products have additional functions: storing measurement results in the device memory, communication with a computer, screen backlight.
Connecting probes
The device has three connectors for measuring probes. Connectors signed:
- line - "L";
- screen - "E";
- land - "Z".
The device includes three probes. One of them has two tips on one side. This probe is connected to the shielded sheath of the cable when the leakage current needs to be measured. It is inserted into the connector. "NS". Single probes are inserted into the correspondingly labeled sockets.
If the line resistance is measured without taking into account the shielding, then only two single probes are used. One is installed in the "L" connector, and the other in the "Z" connector. The other ends of the wires are connected by means of "crocodiles" to the current-carrying conductors in the case of a breakdown test between them. If a "ground fault" is being tested, then the "crocodiles" are connected to the ground and the live wire.
Measurement order
Before testing, the mains must be de-energized, all connected devices must be turned off and all plugs must be removed from the sockets. When measuring in the lighting network, unscrew all the bulbs so that they do not burn out from the applied high voltage. The circuits under test must be grounded. To start using a megohmmeter, you need:
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Set the required voltage value. It depends on the type of test object and is determined from the tables. - Connect the probes.
- Remove the ground from the item under test.
- Twist the handle of a dynamo machine for an analog device or press the "test" button for a digital one. The knob must be rotated until the light signal appears. And when working with a digital device, you should wait until the numbers on the screen stabilize.
After completing the measurements, you need to stop rotating the knob of the analog device or press the button to complete the measurements on the digital device.
Security
In production conditions, only specialists of the third and higher admission groups for electrical safety are allowed to work with a megohmmeter. When taking measurements on your own, the following rules must be observed:
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When working, you must use dielectric gloves. - Only hold the probes by the insulated handles bounded by the stops. Fingers must be all the way to avoid contact with live parts.
- Before starting measurements, make sure that there are no people near the line, if necessary, you can hang out warning posters.
- Remove residual voltage using a portable ground. Do this after each measurement.
- Grounding can only be disconnected after installing the probes.
- After each measurement, remove the residual voltage by touching the probes to each other.
The life and health of people depends on the observance of these rules. Insulation resistance testing is the most important condition for maintaining electrical equipment and cables in good working order. These works help prevent accidents on power grids and repair them in time.
