To assess the performance of the cable, wiring it is necessary to measure the insulation resistance. For this there is a special device - a megaommeter. It gives high voltage to the measured chain, measures the current flowing through it, and issues results on the screen or scale. How to use a megaometer and consider in this article.
Device and principle of operation
The megaommeter is a device for checking insulation resistance. There are two types of devices - electronic and shooter. Regardless of the type, any megaommeter consists of:
- Source of constant voltage.
- Current meter.
- Digital screen or measurement scale.
- Properts, by means of which the tension from the device is transmitted to the measured object.
This is how the shooter megaommeter (left) and electronic (right) looks like
In the shooting devices, the voltage is produced by the dynamo embedded in the case. It is driven by the meter - it twists the instrument handle with a certain frequency (2 turns per second). Electronic models take power from the network, but can work from batteries.
The operation of the megaommeter is based on the law of Ohm: i = u / r. The device measures the current that flows between two connected objects (two cable veins, livestock, etc.). Measurements are made of calibrated voltage, the value of which is known, knowing the current and voltage, can be found resistance: r = u / i, which makes the device.
Approximate Magommeter scheme
Before checking the probe is installed in the appropriate jacks on the device, after which they are connected to the measurement object. When testing, a high voltage is generated in the device, which is transmitted to the object being checked. The measurement results are displayed in the muma mega (IOM) on the scale or screen.
Work with megaommeter
When testing, the megaommeter produces a very high voltage - 500 V, 1000 V, 2500 V. In connection with this, measurements must be carried out very carefully. In the enterprises to work in the device, persons having an electrical safety group not lower than 3rd are allowed.
Before measuring the megaommeter, in the test chains are disconnected from the power supply. If you are going to check the condition of the wiring in the house or apartment, you need to turn off the switches on the shield or unscrew the plugs. After all semiconductor devices are turned off.
One of the options for modern megohmmeters
If you check the socket groups, take out the forks of all the devices that are included in them. If lighting chains are checked, light bulbs are turned off. They will not stand test voltage. When checking the insulation of the engines, they are also completely disconnected from the power supply. After that, grounding is connected to the tested circuits. To do this, a stranded wire in a cross section of at least 1.5 mm2 is attached to the "earth" tire. This is the so-called portable grounding. For a safer work, a free end with a bare conductor is attached to a dry wooden holder. But the bare end of the wire should be available - so that you can touch them to the wires and cables.
Requirements for secure working conditions
Even if you want at home to measure the resistance of the cable insulation, before using the megaommeter it is worth familiar with the safety requirements. The basic rules are several:
- Keep the probes only for isolated and restricted by the stops.
- Before connecting the device, turn off the voltage, make sure that there are no people nearby (throughout the measured route, if we are talking about cables).
How to use a megaommeter: Electrical Safety Rules
- Before connecting the probe, remove the residual voltage using the portable ground connection. And turn it off after the probe is installed.
- After each measurement, removing the residual voltage by connecting their bare parts together.
- After measuring to the measured duty, connect the portable ground, removing the residual charge.
- Work in gloves.
The rules are not very complex, but your safety depends on their implementation.
How to connect probe
The device usually has three slots for connecting probe. They are located at the top of the instruments and signed:
- E - screen;
- L-line;
- S - earth;
There are also three probe, one of which has two tips on one side. It is used when you need to exclude leakage currents and clings to the cable screen (if any). On the double divert of this probe there is a letter "E". That plug that comes from this removal and is installed in the corresponding nest. The second plug is installed in the slot "L" - line. In the Earth's nest always connects the same probe.
Properties for megaommeter
There are stops on our senses. When carrying out measurements with hands to take them for them so that your fingers are up to these stops. This is a prerequisite for safe work (remember the high voltage).
If you need to check only the insulation resistance without the screen, two single probe is put - one in the "Z" terminal, the other in the terminal "L". With the help of crocodiles clamps at the ends, we connect the probe:
- To test wires, if you need to check the breakdown between the cable veins.
- To the dwelling and "land", if we check the "breakdown to earth".
There is a letter "E" - this end is inserted into the nest with the same letter.
There are no other combinations. It is checked more often isolation and its breakdown, work with the screen is quite rare, as the shielded cables themselves in apartments and private houses are rarely used. Actually, it is not particularly difficult to use the megoometer. It is only important not to forget about the presence of high voltage and necessity Remove the residual charge after each measurement. This makes touching grounding wire to just a measured wire. For safety, this wire can be fixed on a dry wooden hold.
Measuring process
Expose the voltage that will issue a megaometer. It is not selected arbitrarily, but from the table. There are megaommeters that work only with one voltage, there are working with several. The second, understandable thing is more convenient, since they can be used to test various devices and chains. Switching test voltage is made by a handle or button on the front panel of the device.
| Name of element | Megaommeter voltage | Minimum allowable insulation resistance | Notes |
|---|---|---|---|
| Electricity and apparatus with voltage up to 50 V | 100 B. | Must match the passport, but not less than 0.5 moments | During measurements, semiconductor devices must be checked |
| also, but voltage from 50 V to 100 V | 250 B. | ||
| also but voltage from 100 V to 380 V | 500-1000 B. | ||
| Over 380 V, but not more than 1000 in | 1000-2500 B. | ||
| Distributing devices, shields, conductor | 1000-2500 B. | At least 1 mΩ | Measure each section of the distribution device |
| Wiring, including the lighting network | 1000 B. | Not less than 0.5 mom | In dangerous measurement premises are held once a year, in Druich - once every 3 years |
| Stationary electric stoves | 1000 B. | At least 1 mΩ | Measurement is carried out on a heated disconnected plate at least 1 time per year. |
Before using the megaommeter, we are convinced of the absence of voltage on the line - a tester or indicator screwdriver. Then, having prepared the device (set the voltage and on the shooter to display the measurement scale) and connecting the probe, remove the ground from the cable checked (if you remember, it connects before starting work).
The next stage - we include a megaommeter: on the electronic you press the TEST button, in the direction of the dynamo handle in the shooter. In the shooter, we turn around until the lamp is on the housing - it means the required voltage in the chain is created. In digital at some point, the value does not stabilize the value. The numbers on the screen are insulation resistance. If it is not less than the norm (the average are specified in the table, and the exact is in the passport to the product), it means everything is normal.
How to measure megaommeter
After the measurement is over, cease to twist the megohmmeter knob or click on the measurement end button to the electronic model. After that, you can disconnect the probe, remove the residual voltage.
Briefly - this is all rules for using the megaommeter. Some measurement options will look at more.
Cable insulation resistance measurement
Often you need to measure the insulation resistance of the cable or wire. If you know how to use the megaommeter, when checking a single-core cable it will take no more than a minute, it will have to mild longer. The exact time depends on the number of lived - you will have to check each.
Test voltage choose depending on whether the wire will operate in which voltage. If you plan to use it for wiring for 250 or 380 V, you can set 1000 V (see the table).
Checking the three-core cable - you can not twist, and move all the pairs
To check the insulation resistance of a single-core cable, one probe cling to the core, the second - on the armor, supply voltage. If there is no armor, the second probe is secure to the "earth" terminal and also supply test tension. If the readings are more than 0.5 MΩ, everything is normal, the wire can be used. If less is insulation, it is impossible to apply it.
If you need to check the stranded cable, testing is carried out for each vein separately. At the same time, all other conductors twisted into one harness. If it is also necessary to test the breakdown on the "Earth", the wire connected to the corresponding bus is added to the overall harness.
If there lived a lot, before using the megaommeter, the veins are cleaned of isolation and twisted in the harness
If the cable has a screen, a metal sheath or armor, they are also added to the harness. When forming a harness, it is important to ensure good contact.
Approximately the measurement of the insulation resistance of the socket groups occurs. Out of the outlets turn off all the devices, turn off the power on the shield. One probe is installed on the ground terminal, the second one is in one of the phases. Test voltage - 1000 V (on the table). Turn on, check. If the measured resistance is greater than 0.5 mΩ, the wiring is normal. We repeat with the second residential.
If the electrical wiring of the old sample is only phase and zero, testing is carried out between two conductors. Parameters are similar.
Check the insulation resistance of the electric motor
To carry out the measurement, the engine is disconnected from power. It is necessary to get to the conclusions of the winding. Asynchronous engines operating at voltage up to 1000 are tested with voltage of 500 V.
To check their insulation, one probe connect to the engine body, the second alternately apply to each of the conclusions. You can also check the integrity of the connection of the windings among themselves. For this check, it is necessary to establish a pair of windings.
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