1. Insulation type
To ensure the normal operation of the circuit and when there is a higher than dangerous voltage in the circuit, insulating materials and insulation protection must be used. UL60950 divides insulation into five categories: functional insulation, basic insulation, supplementary insulation, double insulation and reinforced insulation.
① Functional insulation: It is the insulation necessary for the correct operation of the equipment. It does not provide electric shock safety protection, but it can reduce fire and combustion. For example, the paint leather of enameled wire.
②Basic insulation: the most basic insulation to avoid electric shock. It is not safe to rely on basic insulation alone, and safety requirements must be met through additional insulation and protective grounding secondary protection, such as coil interlayer insulation.
③Supplementary insulation: The insulation added independently to the basic insulation ensures that the basic insulation fails occasionally and provides secondary protection against electric shock. The minimum thickness of the additional insulation single layer material must be greater than or equal to 0.4mm.
④Double insulation: It is composed of basic insulation and supplementary insulation, and is a secondary insulation system.
⑤Reinforced insulation: A single insulation system to prevent electric shock, equivalent to double insulation. The minimum thickness of the single layer used inside is greater than or equal to 0.4mm. There may be several layers, but each layer cannot be tested individually.
2. Equipment grade and insulation requirements
Different types of circuits require different types of insulation:
①Class I equipment. This type of equipment adopts protective grounding, that is, the metal chassis is connected to the protective ground, as a level 1 protection. Only basic insulation is required between any hazardous voltage parts and the chassis, such as station communication power supplies, etc.
②Class II equipment. Use double insulation and reinforced insulation, without metal chassis and grounding screws, such as portable chargers.
③Class III equipment. Powered by a SELV source and internally generated with no potentially hazardous voltages, so only basic insulation is required, such as circuit board power supplies. Figure 8.2.3 is a simple functional block diagram inside the power supply. From the block diagram, first distinguish whether the internal circuit function block is LCC, SELV, TNV, ELV or dangerous voltage, and then determine the insulation level and quantity between the functional blocks and between the internal components and the use. It can be seen from the figure that at least two poles of insulation are required between the dangerous voltage and the parts accessible to the user.
For example, the floating ELV path must have two-pole insulation, and at least one level between the ELV and the user, otherwise the ELV circuit will be unsafe once a single level fails. However, if the ELV circuit is grounded (providing level 1 protection), only adding level 1 is sufficient. In similar cases such as radiators and metal enclosures, Class 2 insulation must be used to isolate users from dangerous voltages. Class 1 can only be used unless the metal is grounded.
3. Insulation material
Select solid insulating materials according to working voltage, electrical strength, temperature requirements, mechanical strength and working environment, and these materials should be moisture-resistant and flame-retardant materials; if it is a wire insulating material, the material flexibility is also required.
Semiconductor devices and other components are molded into solid insulating materials, and their insulation is individually quality-assessed and tested during factory manufacturing. There are thickness requirements for films, strips, and sheets of solid insulating materials: if single-layer insulation, the minimum thickness is greater than or equal to 0.4mm; if it is two-layer insulation, there is no thickness requirement, but each layer must meet the electric strength requirements; if it is For three or more layers, there is no minimum thickness requirement, but every combination of two layers must meet the corresponding electrical strength requirements. There are also no thickness requirements for basic or functional insulation.
Post time: Apr-11-2023