1 Principles and general requirements
The insulation resistance and electrical strength test clauses seem to be very simple, and most people in the industry think that the clause can be understood, but there are often many errors in the operation.
First, the difference between high-voltage breakdown in electrical strength and insulation resistance and leakage current should be distinguished. In the use of electrical appliances, the insulation layer will be impacted by high-voltage pulses generated by the inductive electrical disconnection, and will also be affected by the trigger pulse and lightning induced pulse. The electrical strength index is to assess the ability of the insulation layer to resist electrical breakdown. There are two cases of breakdown. One is that there is not enough electrical clearance, and space breakdown discharge occurs under high voltage. Such breakdown generally does not cause permanent insulation damage. Another type of breakdown is the breakdown of the dielectric medium. Since the dielectric medium is usually organic, when the organic substance is broken down by high voltage, the insulation at the breakdown point is carbonized by the high voltage discharge, thus causing permanent electrical insulation to be destroyed. The high voltage breakdown of lighting appliances is mostly the latter. In the IEC standard, the general electrical breakdown of electric appliances is defined by the high voltage, when the current caused by high voltage is ≥100 mA, the electrical strength is not satisfactory, and the electrical appliance is generally unrecoverable after electrical breakdown. It is a direct threat to personal safety. The insulation resistance and leakage current are the leakage currents generated by the insulation under a certain voltage. This leakage current does not invalidate the insulation. When this leakage current is large (depending on the sensitivity of people's electrical perception, it is about 0.5 mA to 5 mA), it will cause people to feel the electricity. This leakage current will not directly harm the healthy people. However, if you are instinctively dodging because of the feeling of numbness when working at heights, it may cause damage from high altitude. In addition, certain heart disease people may have obvious harm, so restrictions are imposed. Most of them use a megohmmeter to measure the insulation resistance to limit the leakage current of the control device and some parts of the lamp. However, it is required to disconnect the capacitance and inductance that affect the measurement result when measuring the insulation resistance (see GB 7000. 1 Standard 10. 2), using a leakage current meter with a weighted network to measure the leakage current of the entire luminaire, the leakage current measurement of the entire luminaire not only includes the leakage current generated by the insulation under the voltage, but also includes the luminaire The leakage current generated by capacitive sensing (see GB 7000. 1 Standard 10. 3).
After understanding the connotation of the above electrical strength test, we should also pay attention to the requirements of the IEC standard for the electrical strength of lighting appliances. The operating voltage referred to in the standard means the maximum steady-state rms voltage (excluding the transient value) generated between the terminals or the terminals to the ground during operation of the lighting appliance, including normal and abnormal conditions. Due to the development of the lighting industry, there are many sources of light source control devices that are inspected according to the above requirements in the working state or in an abnormal state. The operating voltage may exceed the input voltage (the light source is also required to be marked in the marking project). Control device operating voltage value). Therefore, in the electrical strength test, U in the test voltage 2U + 1000V of each light source control device should be calculated according to the nominal operating voltage.
2 Electrical strength test requirements for lamps using LED light sources
The electrical strength test of luminaires using LED light sources is mostly more complicated than that of general luminaires. According to the different control devices used in the luminaires and the classification of anti-electric shock protection of lamps, they can be basically divided into the following cases:
(1) When using a luminaire with an output that is a safety extra low voltage (SELV) or an isolated control device, the LED control device's marking should be the first priority when conducting insulation resistance and electrical strength tests.
The output of the control unit shown in Figure 1 is isolated safety extra low voltage (SELV), so the internal input and output must also be isolated.
For luminaires using such LED control devices, the following tests shall be carried out in accordance with the requirements of Table 10.1 and Table 10.2 of Chapter 10 of GB 7000. 1 .
Figure 2 is a schematic diagram of each of the typical I-type lamps using LED light sources.
1) Because the output is SELV, the insulation resistance of the output surface to the mounting surface and the metal casing of the lamp (in most cases, the two potentials are connected together) should be ≥1MΩ, and the voltage applied by the electrical strength is 500V /1min. wear.
The test between the different polarities on the output side can only be carried out after the control device and the LED module wiring on the terminal B are removed, and the insulation resistance should also be ≥1MΩ. The voltage applied by the electrical strength test is also 500V / 1min.
2) Because the input terminal is non-SELV and is of type I structure, the input end should be evaluated according to the basic insulation for the mounting surface and the metal casing of the lamp (in most cases, the same potential), that is, the insulation resistance should be ≥ 2MΩ, electrical strength The applied voltage should be (2Uin +1 000V) / 1min.
If the switch S of the power supply is present in the luminaire, the insulation resistance and electrical strength test of the live parts with different polarities shall be performed for L and N with the switch in the open position. The power cord and control unit wires on terminal A should also be removed, and the terminal A should be tested for insulation resistance and electrical strength of live parts with different polarities. Since the latter two cases require only basic insulation to meet the requirements, the test limits and voltages are the same as in the first case of this article.
3) As can be seen from Figure 2, there are two possible channels for the input terminal electrode to the accessible conductive part:
a) person - lamp housing (ground) - (basic insulation) - input power terminal and related lines
b) Human-output terminal-LED control device (double or reinforced insulation) - input power terminal and related circuit
Note: Because when the output voltage is SELV and the load voltage is ≤25V, the output can be exposed and accessible. For the a) channel, it belongs to the class I structure, which is better understood and belongs to the form of basic insulation plus protective grounding. For b) channel, because the output voltage is SELV, and when the load voltage is ≤25V, the output terminal can be exposed, and the output terminal can not achieve protection grounding, so it is a local class II structure in the class I luminaire. The reinforced insulation in this channel is achieved by the enhanced isolation inside the LED control unit. Therefore, for such lamps, the insulation resistance and electrical strength test between input/output should be performed. The insulation resistance should be ≥ 4MΩ and 4UIN + 2750V reinforced insulation electrical strength test.
It should be noted here that even for the output voltage > 25V, it still belongs to the SELV output. Because the output terminal is accessible to humans, only 1 MΩ of insulation resistance is required and only 500V electrical strength test is performed, which does not have basic insulation. And the basic requirements of additional insulation, so the insulation resistance and electrical strength test should still be carried out between the input/output according to the requirements of reinforced insulation. If the electrical box is separated from the light source cavity (source cavity is Class III), the LED control unit must have enhanced isolation inside.
It is worth pointing out that in GB 7000. 1 Standard 10.2.2, “For Class II luminaires with both reinforced and double insulation, it should be noted that the voltage applied to the reinforced insulation should not be used for basic or additional insulation. Excessive voltage." We know that most of the high voltage meter's output has a high potential pole and a low potential pole to ground, and its low potential pole is generally grounded (or connected to the casing of the humid chamber). At this time, if the luminaire has been subjected to the tidal state treatment and is in the humidity chamber, the enhanced test voltage of the high-potential output of the high-voltage device will be directly applied to the reinforced voltage regardless of whether it is connected to the input terminal or the output terminal. Basic insulation. The correct way is to cut off the low-potential pole of the high-voltage device to the ground and confirm that there is no connection between the electrode and the ground inside the high-voltage device. The two output electrodes are directly led out to the input of the above-mentioned lamp. End and output.