Principle and Instructions for Use of Leakage Current Tester

Leakage current tester, leakage current refers to the current formed between mutually insulated metal parts in electrical equipment, or between live parts and grounded parts, through the surrounding medium or insulating surface when there is no fault voltage applied. According to the US UL standard, leakage current is the current that can be conducted from the accessible part of household appliances, including capacitive coupling current. The leakage current consists of two parts, one is the conduction current I1 through the insulation resistance; the other is the displacement current I2 through the distributed capacitance. The latter capacitive reactance is Xc=1/2πfc, which is inversely proportional to the power supply frequency. The distributed capacitance current increases with the increase of frequency, so the leakage current increases with the increase of power supply frequency. For example: when powered by thyristor, its harmonic components increase the leakage current.

If the insulation performance of a circuit or a system is being assessed, the current should include not only all currents flowing into the earth (or the conductive parts outside the circuit) through the insulating material, but also the current flowing into the earth through the capacitive devices in the circuit or system (distributed capacitance can be regarded as capacitive devices). Longer wiring will form a larger distributed capacity and increase the leakage current, which should be paid special attention to in an ungrounded system.

The principle of measuring leakage current is basically the same as that of insulation resistance. Insulation resistance is actually a leakage current, but it is expressed in the form of resistance. However, the regular measurement of leakage current applies an AC voltage, so the leakage current contains the capacitive component current.

When conducting a withstand voltage test, in order to protect the test equipment and test according to the specified technical indicators, it is also necessary to determine the maximum current value allowed to flow through the device under test (insulating material) without destroying the device under test (insulating material) at the highest electric field strength. This current is usually also called leakage current, but this method is only used under the above-mentioned specific field. Please pay attention to the difference.

Leakage current is actually the current flowing through the insulation part of an electrical circuit or device when there is no fault and voltage applied. Therefore, it is one of the important indicators to measure the insulation quality of electrical appliances and is the main indicator of product safety performance. Limiting the leakage current to a very small value plays an important role in improving product safety performance. The leakage current of low-power electrical appliances (<2KW) can be measured using a leakage current tester with its own isolated power supply.

The leakage current tester is used to measure the leakage current that is not related to the work and is generated by the working power supply (or other power supply) of the electrical appliance through the insulation or distributed parameter impedance. Its input impedance simulates the human body impedance.

The leakage current tester is mainly composed of test power supply, impedance conversion, range conversion, AC/DC conversion, indication and sound and light alarm circuit.

How to operate the leakage current tester:
1. Plug in the power supply, turn on the power switch, and the power indicator light will light up;
2. Select the power range and press the required current button;
3. Select the leakage current alarm value;
4. Select the test time;

5. Connect the object to be tested to the measuring end, start the leakage current tester, raise the test voltage to 1.06 times (or 1.1 times) the rated working voltage of the object to be tested, switch the phase conversion switch, read the two readings respectively, and select the leakage current value with the larger value. When the conversion switch K is connected to the neutral line, the tester samples the leakage current between the neutral line and the shell; when K is connected to the phase line, the leakage current between the phase line and the shell is tested. It must be noted that: when K is connected to the neutral line or K is connected to the phase line, the leakage current is not necessarily the same. This is because the location of the insulation weakness of household appliances is random. Therefore, the leakage current test should be carried out by converting the polarity through K, and the larger value should be taken as the leakage current value of the electric heating appliance to be tested.

Test precautions
1. When measuring leakage current at working temperature, if the tested electrical appliance is not powered by an isolation transformer, the tested electrical appliance should be insulated from the ground by an insulating pad with reliable insulation performance. Otherwise, part of the leakage current will flow directly through the ground without passing through the leakage current tester, affecting the accuracy of the test data.
2. The leakage current measurement is carried out with power on, and the outer shell of the tested electrical appliance is charged. Therefore, the test personnel must pay attention to safety, and various laboratories should formulate safety operating procedures. The tested electrical appliance must not be touched before the current is cut off.
3. The impact of the environment on the test data should be minimized. The temperature, humidity and contamination of the insulation surface of the test environment have a great impact on the leakage current. The high temperature, high humidity, and serious contamination of the insulation surface will result in a large leakage current value.