Contact resistance is the resistance to current flow, due to surface conditions and other causes, when contacts are touching one another (in the closed condition of the device). This can occur between contacts of:
- Other switching devices
Measurement of the contact resistance helps in identification of fretting corrosion of contacts, and allows contact corrosion to be diagnosed and prevented. Increase in contact resistance can cause a high-voltage drop in the system, which needs to be controlled.
The electrical contact resistance test is used for switching devices, cable joints as well as other connections. It can measure either the voltage drop or the resistance offered by the contacts or resistance of the joints or connections.
This test detects the resistance at the milli-ohm level or micro-ohm level.
The measurement of contact resistance is done to to ensure that contactor make-break contacts are in good condition. In the case of connections and joints of current carrying parts, the test can:
- Identify loose connectors
- Show areas of high-voltage drop in the system
- Indicate low tightness in bolted joints
- Indicate corroded surfaces of bus bars
Contact corrosion leads to:
- Power loss
- Heat generation
- Loss of contactor capacity
- Failure to interrupt current when required
These problems can lead to major failure in connected motors.
Fretting corrosion at high temperatures is one of the causes of contact failure resulting in high contact resistance. Different surface coatings and other treatments are used to improve corrosion resistance of contact surfaces and contact tips.