Many industries use contact resistance meters to measure the contact resistance in power systems, circuit breakers, switches, etc. High resistance can damage equipment and power systems due to overheating. Therefore, it’s essential to measure contact resistance in different industries to protect their systems.
What is a Contact Resistance Meter and Its Operating Principle
This meter can measure low resistances, ranging from micro-ohms to milliohms. It identifies problems in electrical joints that may not be visible with the naked eye. The meter is used during installation, maintenance, or inspection of power systems.
The product works on the principle of Ohm’s law to measure the resistance at the interface of two conducting surfaces. It detects the voltage drop across the interface when a known current flows through it.
To find contact resistance, the meter works by doing the following:
- A constant DC current, usually ranging from 10A to 100A (sometimes more), is passed through the contact point or joint that needs testing. Using a high current helps to overcome the effect of oxide layers or surface contaminants on the contact surfaces.
- As the current flows through the contact, a voltage drop is created across it. This voltage is very small (in the millivolt range), so the meter uses special Kelvin probes or four-wire connections to measure it accurately.
- In a four-wire method, two wires carry the current, and two separate wires measure the voltage. This helps avoid errors caused by resistance in the wires themselves.
- Once the current and voltage drops are known, the meter calculates the resistance using Ohm’s Law. The result is usually displayed on a screen in micro-ohms or milliohms.
Drawbacks of Conventional Contact Resistance Meter
The contact resistance meter designed according to the conventional design principle is found in the field test, and there is a common problem: when the meter’s voltage wiring circuit has poor contact or an open circuit, the meter will also display a value, and the following types of conditions will appear:
(1) The voltage contact is open, and there is no significant electrical field interference at the test site. In this scenario, because the differential mode voltage input by the amplifier is essentially zero, the test value reported by the instrument is close to 0.
If the tester has enough field test experience, it can be determined that the instrument’s voltage contact test line is abnormal, and the final correct test result can be obtained after the abnormal voltage contact test line of the instrument is eliminated. Test, replace, or repair the instrument, delay the power outage time, and add unnecessary trouble to the test work.
(2) The voltage circuit has a weak contact. After long-term use, the circuit breaker terminals typically form an oxide or oil coating on the outside surface of the terminal block. When the voltage test clamp of the contact resistance meter is applied to such a terminal block, it can if there is poor contact, the voltage test wire clamp will have a particular contact resistance. When the contact resistance value equals the internal resistance value of the voltage sampling circuit, the test results will be significantly impacted.
Conclusion
Conventional contact resistance meters are essential for maintaining safe and reliable electrical connections. They work on a simple principle based on Ohm’s law. But they have some practical drawbacks like temperature sensitivity, limited portability, and offline-only testing.
S.B.ELECTROTECH
S.B. Electrotech is a company that specializes in manufacturing of electrical and electronic testing and measuring instruments. With years of expertise in this field, we ensure to provide high quality, safe, and sustainable products.
