What is the difference between the four common resistors?

Today, Xiaobian will introduce the difference between the following four kinds of resistance:
1. Insulation resistance

Apply a DC voltage to the medium.After a certain period of polarization, the corresponding leakage current flowing through the dielectric is called insulation resistance.Insulation resistance is the most basic insulation index for electrical equipment and electrical lines.The insulation resistance of low-voltage electrical installations, motors, power distribution equipment and distribution lines shall not be less than 0.5 MΩ (the insulation resistance of operating equipment and wiring shall not be less than 1 MΩ/kv).The insulation resistance of low-voltage electrical appliances and their connecting cables and secondary circuits is generally not less than 1MΩ; in a relatively humid environment, it should not be lower than 0.5MΩ; the insulation resistance of small circuit and small busbars should not be less than 10MΩ.The insulation resistance of Class I hand-held power tools shall not be less than 2MΩ.

2. Grounding resistance

Ground resistance is the resistance encountered when current flows from a grounded device into the ground and then through the ground to another grounded body or travels to a distance.The unit of measurement is Ω.The size of the grounding resistance can reflect the degree of contact between the electrical device and the earth and the size of the grounding grid.

Ground resistance measurement methods include two-wire method, three-wire method, four-wire method, single-clamp method and double-clamp method.Each of these five methods has its own characteristics.In the actual measurement, try to choose the correct measurement method and measurement instrument to ensure the accuracy of the results.

3. DC group

This type of resistance is mainly used for DC resistance testing of inductive loads such as transformers, motors, CT/PT, etc. The measurement unit is mΩ.It can reflect the welding quality of the winding, the short circuit between turns of the winding, the disconnection of the winding or the disconnection of the lead wire, the poor contact of the tap changer and the wire and so on.In fact, it is also an effective means to judge whether the DC resistance of each phase winding is balanced and whether the voltage regulator switch is correct.

DC resistance measurement methods include: current-voltage method, DC bridge method, and three-phase winding simultaneous compression method. Now DC resistance testers generally use DC bridge method. This method requires that the test current is not less than 1A.The measured resistance value is small.High test currents are common: 1A, 3A, 10A, 20A, 40A, 50A, 100A, these are only used for single-phase measurements.

4. Contact resistance

Contact resistance is a parameter for the good connection of a conductive circuit.The unit of measurement is μΩ.Each type of high voltage circuit breaker is assigned a range of values.If the loop resistance exceeds the specified value, the conductive circuit may be poorly connected.In the case of high-current operation, the local temperature rise will increase when the contact fault occurs, and even a vicious cycle will lead to oxidation and burnout. Therefore, the circuit breaker used for high-current operation should pay more attention.