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Why contact resistance testing?

time:2020/5/28   source:华天电力  reading:328 time

The contacts in the circuit breaker need to be checked periodically to ensure normal operation of the circuit breaker. Improper maintenance or damage of contacts can lead to arc, loose phase and even fire.


This test is especially important for contacts that carry a large amount of current (such as switchgear busbars), as higher contact resistance leads to lower current carrying capacity and higher losses.


The measurement of contact resistance is helpful to identify the fretting corrosion of the contact, and can diagnose and prevent the contact corrosion. An increase in contact resistance can lead to a drop in the system's high voltage, which needs to be controlled.


Micro Ohmmeter.png


What was done during the contact resistance test?


Two common inspections of circuit breaker contacts are appearance inspection and contact resistance inspection.


1. Appearance inspection shall include checking whether the circuit breaker contacts are dented due to arc and contact wear or deformation.


2. The second check is the contact resistance measurement. This involves injecting a fixed current through a contact, usually around 100A, 200A, and 300A, and measuring the voltage drop at both ends. The test was performed using a special contact resistance measuring instrument. Ohm's law is then used to calculate the resistance. The resistance value needs to be compared with the value given by the manufacturer. This value should also be compared with previous records.


These two tests need to be done together. Due to the presence of contact appliances with good contact resistance but still in a damaged state. Therefore, for the contactor to be shown to be healthy, it needs to have good contact resistance and should be cleared for appearance inspection tests.


How do we do contact resistance testing?


Test standard


The criteria for evaluating the contact resistance of an electrical connection depends largely on the type of connection (e.g., bolted connection, welding, clamping, welding, etc.), metal contact surface area, contact pressure, etc. These vary from device to device. Manufacturer, no code or standard specifies minimum contact resistance. Therefore, we need to refer to the manufacturer's advice. For example, manufacturers sometimes cite large bolt busbar joints with a maximum contact resistance of 10 microohms.


Contact resistance measurement and its application are quite extensive.


Electrical connections


The electrical connection of the circuit can be made by various means, such as welding, pressing, insertion and close bonding. If you want to know the quality of the connector and its conductive properties, simply measure its contact resistance. Contact resistance is usually used in quality testing of switches, relays and PCB pads.


In the case of mechanical components, the contact resistance of metal contact surfaces can be used to estimate the reliability and tightness of mechanical components. The contact resistance is related to the electrical properties of the contact surface. The larger the surface area of the paired metal and the fewer impurities, the better the conductivity and the lower the resistance, and vice versa.


By measuring the contact resistance, we can qualitatively analyze the reliability and tightness of the mechanical components. This technique has been used in the quality testing of EMC shielding components. The measurement methods vary from application to application. For example, in the case of measuring the contact resistance of a high-power switch and relay, a large current, a pair of contacts should be used, just as what actually happens in the working state. For dry circuit connectors, the test current should be low to prevent the connection from melting due to heat (current less than 100mA).


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