Significance of various DC resistances in circuit breakers
2019-08-31 15:11:00
The DC resistance of each phase of the circuit breaker actually includes the resistance of the bushing conducting rod, the resistance at the connection between the conducting rod and the contact, and the contact resistance between the dynamic and static contacts. This is actually to measure the contact resistance of the dynamic and static contacts. When the contact resistance of the running circuit breaker increases, the contact will overheat under the normal working current, especially when passing the fault short-circuit current, it may overheat the contact locally, burn the insulation around and cause the contact to burn and bond, thus affecting the tripping time and breaking capacity of the circuit breaker, and even cause the situation of misoperation. Therefore, this test shall be carried out after the installation of the circuit breaker, after three times of major and minor repair and concealment of the fault current.
test method
The resistance of each phase conducting circuit of the circuit breaker shall be measured between the two bushing ends of each phase under DC voltage. The measurement method is the same as the general DC resistance measurement method, that is, the bridge method or voltage drop method can be used.
matters needing attention
In addition to the general precautions for DC resistance measurement, the following points shall be paid attention to according to the characteristics of the circuit breaker:
(1) Because the DC resistance of each phase of the circuit breaker's conducting circuit is very small, it is generally measured by micro ohm, so when using the bridge method, it is necessary to use a double arm bridge; when using the voltage drop method, it is necessary to use a DC power supply with sufficient capacity, and at the same time select a suitable millivoltmeter and ammeter.
(2) During the test, special attention shall be paid to eliminate the influence of measuring lead wire and connector resistance. The lead wire shall have sufficient cross-sectional area and be as short as possible. The lead connector of current and voltage must be strictly separated.
(3) Before the measurement, the circuit breaker shall be tripped and closed several times. If it is an electric closed circuit breaker, the measurement shall be conducted after the electric closing.
(4) In order to protect the measuring instrument, the tripping mechanism of the circuit breaker shall be blocked during the measurement to prevent sudden tripping and damage to the instrument during the test.
(5) Each phase shall be measured at least 3 times, and the average value shall be taken. If the measurement result is doubted again, it can be measured several times more.
Analysis and judgment of results
The measurement results of DC resistance of each phase of conducting circuit of the circuit breaker shall conform to the regulations of the manufacturer.
If the measured results are more than twice the previous results, the contact shall be inspected; if there is a large difference between the three phases, attention shall be paid and careful inspection shall be carried out. If the measurement result does not exceed the manufacturer's standard, the circuit breaker can be tripped once and then re measured. If it is still too large, the cause shall be found out and handled.
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