Significance of Direct Current Resistance of each Phase Conductive Circuit of Circuit Breaker

The DC resistance of each phase conductive circuit of a circuit breaker actually includes the resistance of the sleeve conductive pole, the resistance at the connection between the conductive pole and the contact, and the contact resistance between the dynamic and static contacts. This is essentially a measurement of the contact resistance of the dynamic and static contacts.

Increasing contact resistance of a circuit breaker in operation will overheat the contacts under normal working current, especially when short circuit current is passing through a fault, it may overheat the contacts locally. In severe cases, it may burn the insulation around the contacts and cause burning and bonding of the contacts, which will affect the tripping time and breaking ability of the circuit breaker, and even cause rejection. Therefore, this test is required after the breaker is installed, after size repair and after covering up the fault current three times.

test method

The resistance of each phase conductive circuit of a circuit breaker should be measured between the ends of two bushes in each phase under DC voltage. The method of measurement is the same as that of general DC resistance measurement, that is, 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 should be noted according to the characteristics of the circuit breaker:

(1) Because the direct current resistance of each phase conductive circuit of the circuit breaker is very small and is usually measured in micro-Euclidean, it is necessary to use double-arm bridge when using bridge method to measure. When measuring with voltage drop method, a DC power supply with sufficient capacity must be used, and a suitable millivolt meter and a current meter must be selected.

(2) During the test, special attention should be paid to eliminating the influence of measurement lead and connector resistance. Leads should have sufficient cross-section area and be as short as possible. Lead connectors of current and voltage must be strictly separated.

(3) The breaker should be tripped several times before the measurement. If the breaker is electrically closed, the measurement should be made after the electrically closed.

(4) In order to protect the measuring instrument, the tripping mechanism of the circuit breaker should be jammed during the measuring process to prevent sudden tripping and damage to the instrument during the test.

(5) Measure each phase at least three times, take its average value, and if the measurement result is doubtful again, measure more times.

Analysis and Judgment of Results

The measurement results of DC resistance of each phase conductive circuit of the circuit breaker shall conform to the requirements of the manufacturer.

When the measured result is more than twice that of the previous one, the contact should be examined. When the difference between three phases is large, attention should be paid and the treatment must be carefully checked. If the measurement results do not exceed the manufacturer's standard for a long time, the breaker can be jumped once and then re-measured, if it is still too large, the cause should be identified and handled.