Analysis of common safety problems of LED fluorescent lamps

This article will combine the contents of GB18774-2002 and GB7000.1-2007, from the aspects of structure, internal wiring, protection against electric shock, insulation resistance and electrical strength, creepage distance and clearance, heat resistance and fire resistance. Possible safety issues with LED fluorescent lamps.

First, the structural aspects

LED fluorescent lamps are prone to problems in three places: the lamp cap, the screw, the basic insulation and the contact with the accessible metal parts. The lamp cap needs to meet the torque test specified in GB18774-2002 and the dimensional requirements specified in GB2799-2001. Among them, the torque test specified in GB18774-2002 requires that the high temperature test must comply with the provisions of the standard 2.3.1, that is, when the torque test is applied, the rotation between the parts of the lamp head does not exceed 6 °. The high temperature conditions applied by the lamp cap are (125 ± 5) ° C and the heating time is (2000 ± 50) h. If the G13 base is used and the lamp power is greater than 40W, the high temperature condition is: (140 ± 5) °C. High temperature conditions and duration The evaluation of LED fluorescent lamps using plastic materials for the lamp head is harsh. Using plastic lamp caps with low heat resistance, after applying such high temperature conditions for a long time, it is likely that the plastic parts have softened. If a metal base is used, it is relatively easy to meet the test conditions, but it is also necessary to consider the connection between the metal base and the LED fluorescent lamp body and the creepage distance and clearance.

The screw at the lamp head of the LED fluorescent lamp mainly serves as a connection and fixing between the lamp cap and the lamp body. At present, the diameter of the screw at the common LED fluorescent lamp head is less than 3 mm, and according to the provisions of GB7000.1, such screws need to be screwed into the metal. It is now common practice to use an aluminum housing for the LED fluorescent lamp, and this screw is screwed directly into the metal housing, which also satisfies the requirements. However, a common problem that arises is that after the screw is screwed into the metal casing, the screw is too close to the charged metal piece of the lamp cap, which easily leads to unsatisfactory creepage distance and clearance.

The internal wiring of the LED fluorescent lamp is mainly used to connect the input and output of the lamp plug and the built-in power supply, and the built-in power supply and the aluminum casing are separated by a sleeve. According to the provisions of GB7000.1, the insulation between the internal live parts and the accessible metal parts shall satisfy double insulation or reinforced insulation. This requires that the insulation of the casing needs to meet the electrical strength requirements of the reinforced insulation grade.

Second, internal wiring

According to the requirements of GB7000.1 standard, the internal wiring used for LED fluorescent lamps needs to be evaluated mainly in terms of wire diameter and insulation thickness, mechanical damage, heating temperature of insulation layer, and whether insulation meets the requirements. In general, the internal line will not have problems in the protection of mechanical damage, and the main problem will be in the other three aspects. According to the requirements of GB7000.1 standard, when the normal current is lower than 2A (the working current of LED fluorescent lamp will not exceed 2A), the nominal cross-sectional area of ​​the internal wire is not less than 0.4mm2, and the thickness of the insulation layer is not less than 0.5mm. Moreover, from the perspective of insulation, since the aluminum shell is an accessible metal part, the internal basic insulation cannot be in direct contact with the aluminum shell, which requires the inner conductor to be a double insulated wire, unless there is a certificate to prove that the insulation layer of the wire can meet the reinforcement. Insulation requirements, it is also possible to use a single insulated wire for the internal wire. However, the internal wires used in LED fluorescent lamps on the market rarely take into consideration the cross-sectional area, the thickness of the insulating layer and the level of the insulated wire.

In addition, when the internal wires are being routed, care should be taken to avoid direct contact between the wires and components with large internal heating, such as transformers, filter inductors, bridge stacks, heat sinks, etc., because these components are very hot when operating LED fluorescent lamps. It is possible to exceed the heat resistance temperature value of the internal wire insulation material. When the internal wires are routed, they do not touch the components with large heat generation, which can avoid the damage of the insulation layer caused by local overheating of the insulation layer, and cause safety problems such as leakage or short circuit.

Third, anti-shock protection

LED fluorescent lamps in the protection against electric shock, there may be two cases of failure. First, the unreliable connection between the lamp cap and the lamp body allows the manual to directly remove the lamp cap, resulting in direct contact with the test and internal live parts; The internal insulation is not completed, resulting in leakage of the casing.

Fourth, insulation resistance and electrical strength

From the classification of electrical, LED fluorescent lamps belong to the type II electric shock prevention type, which requires the input of the LED fluorescent lamp between the accessible parts and the insulation resistance and electrical strength requirements of the reinforced insulation level to be input between the mounting surfaces. At present, many LED fluorescent lamps can pass through the insulation resistance test, but the electric strength test is not passed, mainly due to the selection of the transformer of the built-in power supply and the mounting position of the aluminum substrate of the LED module. Many enterprises choose to use non-isolated transformers for the purpose of cost saving or high power efficiency. This will result in the insufficiency of the insulation level of the insulation level between the input and output terminals of the built-in power supply. At the same time, since the aluminum substrate of the LED module is in direct contact with the metal casing during installation, the insulation level between the input end and the accessible member cannot meet the electrical strength requirement of the reinforced insulation grade.

To meet the electrical strength requirements of the reinforced insulation grade, an isolation transformer can be used to electrically isolate the input and output of the built-in power supply, or to use an outer casing of insulating material without using a metal casing. However, if an insulating material casing is used, it is also necessary to consider the problem that the screw having a diameter of less than 3 mm as described above needs to be screwed into the metal.