Today Xiaobian to introduce the difference between the following four kinds of resistance:
1, insulation resistance
A DC voltage is applied to the medium. After a certain time of polarization, the corresponding leakage current through the dielectric is called insulation resistance. Insulation resistance is the most basic insulation index of electrical equipment and electrical circuit. Low voltage electrical equipment handover test, motor, distribution equipment and distribution line insulation resistance shall not be less than 0.5 mω (operating equipment and wiring insulation resistance shall not be less than 1 megohm/kV). The insulation resistance of low-voltage appliances and their connecting cables and secondary circuits is generally not less than 1mω; In a relatively humid environment, should not be less than 0.5 mω; The insulation resistance of the small circuit bus must be no less than 10 mω. The insulation resistance of class I hand-held power tools must be no less than 2Mω.
2. Grounding resistance
Grounding resistance IS THE RESISTANCE encountered when a current flows from a grounding device into the earth and then flows through the earth to another grounding body or propagates to a distance. The unit of measurement is ω. The ground resistance reflects the contact degree between the electrical device and the earth, and the size of the grounding network.
Grounding resistance measurement methods include two-wire method, three-wire method, four-wire method, single clamp method and double clamp method. Each of the five methods has its own characteristics. In the actual measurement, try to choose the correct measurement method and measuring instrument, in order to ensure the accuracy of the results.
3. Direct current group
This resistance is mainly used for DC resistance testing of inductive loads such as transformers, motors, CT/PT, and the measurement unit is Mω. It can reflect the quality of winding welding, winding inter-turn short circuit, winding disconnect or lead disconnect, tap changer and wire poor contact and other faults. In fact, it is also an effective means to judge whether the DC resistance of each phase winding is balanced and whether the tap of voltage regulating switch is correct.
Dc resistance measurement methods include: current voltage method, DC bridge method, three-phase winding compression method at the same time, now DC resistance tester generally use DC bridge method, this method requires the test current is not less than 1A, the measurement resistance value is very 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 that determines whether a conductive circuit is well connected. The unit of measurement is μω. Each type of high voltage circuit breaker is assigned a value range. If the loop resistance exceeds the specified value, the conducting circuit may be poorly connected. In the case of high current operation, the local temperature rise will increase when the contact fault, and even the vicious cycle will lead to oxidation and burn loss, so the circuit breaker used in high current operation should be more attention.