Purpose and principle of measuring insulation resistance
According to the daily electrical testing experience, Jiangxi Electric to talk about the reason for insulation testing, the most important reason is to ensure public safety and personal safety.
The possibility of fire caused by short circuits can be eliminated by performing insulation tests in disconnected current-carrying conductors, ground conductors, and wires to be grounded.
Extend equipment life
Insulation testing is also important for protecting and extending the service life of electrical systems and motors.
Periodic maintenance testing provides data that can be analyzed while predicting possible system failures.
In addition, when a fault occurs, it is also necessary to determine the cause of the fault through insulation testing.
National standard provision
Whether it is production materials or electrical equipment, insulation preventive tests need to be carried out according to the corresponding national standard requirements to verify the quality of the electrical equipment produced and ensure that the equipment meets the regulations and safety standards.
Principle of insulation test
Insulation testing is similar to finding leaks in water pipes. The pipe is usually filled with high pressure water to find leakage.
The water flow after pressurization can make the leakage point easier to be insulated resistance tester is to measure the leakage current during pressurization, and calculate the insulation resistance value through Ohm's law.
These instruments are designed to apply and control the test voltage in a "non-destructive" manner.
Although the voltage provided is very high, the current is very limited. This avoids secondary damage to the equipment due to poor insulation and also ensures the safety of the operator.
Why can't a multimeter be used to measure insulation resistance?
Although a multimeter can measure resistance, it cannot accurately indicate the working condition of the insulator.
Because the multimeter uses a 9V DC power supply when making measurements, the high voltage required for testing cannot be obtained. (You can choose a multimeter with insulation test function, such as [Fluke 1587C FC insulation multimeter])
Method of measuring insulation resistance
According to GB50150-2006 electrical equipment installation engineering electrical equipment handover test standard:
For electrical equipment or circuits with operating voltage below 100V, use 250V test voltage;
Electrical equipment or circuits with operating voltage of 100V~500V, use 500V test voltage;
Electrical equipment or circuit with operating voltage of 500V~3000V, use 1000V test voltage;
Electrical equipment or circuit with operating voltage from 3000V to 10000V, use 2500V test voltage;
For electrical equipment or circuits with an operating voltage of more than 10000V, use 5000V or 10000V test voltage;
Insulation resistance test procedure
(Taking insulation resistance tester as an example)
a. Turn the device or system off and remain disconnected from all other circuits, switches, capacitors, carbon brushes, arresters, and circuit breakers.
b. The measured system is fully discharged to the ground.
c. Select an appropriate test voltage.
d. Wiring, if the measured insulation resistance value is large, it is recommended to use a shielded wire, while adding a ground wire to prevent breakdown. The test line should be avoided to reduce the test error.
e. Start the test, after a period of time (usually one minute) read the value of the meter, record the data and the ambient temperature at this time.
f. After the test, if the tested object is a capacitive device, the device should be fully discharged. Finally remove the connecting wire
An instrument for measuring insulation
Insulation resistance testing is done with special testing instruments. The most common test instrument is a dial or insulation resistance tester, but other types of instruments can also be used to check the integrity of different insulation types.
The hand-powered megohm meter is the swing meter, which was born in the 1950s and 1960s and is the earliest insulation resistance testing instrument. There are different specifications, such as 250V, 500V, 1000V and so on. Use by shaking the crank to generate DC voltage, pointer dial, generally need two people to operate at the same time, one person to operate the dial, one person to time and record data.
Battery-powered megohm meters, the test voltage has a variety of adjustable gear, electronic display to make the reading more accurate, and usually designed with safety protection features, such as automatic discharge, leakage current monitoring. In the test project is also more diverse, multimeter test function, polarization index, media absorption ratio, etc., make its application range is wider. Compact design and simple operation, one engineer can complete all the test steps.
Leakage current clamp meters can be used to measure the insulation of equipment that cannot be disconnected, and the magnetic fields generated by the load current cancel each other out. Any unbalanced current comes from the conductor leaking into the ground or elsewhere. To measure this current, the leakage current clamp meter should be able to detect a current of less than 0.1mA.