The electrical contact resistance is the resistance at the electrical connection point that oppose the flow of electric current. The high electrical contact resistance leads more opposition to current flow. The contact resistance of the different contacting surfaces may vary. But, for a reliable electrical system, the contact resistance must be as minimum as possible.
Electrical devices, such as breakers, contactors, relays, switches, and other switching devices, are used for closing and opening the electrical circuit. Thus, all these devices have the contact resistance when in operation. The resistance between the contact points depends on the tightness of the connections and the metallurgy of contacts.
The cables have contact resistance when we join two cables through straight-through joints. Also, the cable conductor has contact resistance at the cable termination point at the connector in the feeder. The cable jointing and the termination point must have minimum contact resistance.
Why Contact Resistance should be low?
The contact resistance creates impedance in the path of current flow. This impediment causes a voltage drop in the circuit. Thus, the receiving end voltage reduces with an increase in contact resistance. The more adverse effect of contact resistance is the heating on the contact surfaces. More resistance cause more heating.
From the above formula, it is clear that the heating increases with an increase in the contact resistance.
The heating may cause disconnection of an electrical circuit. Also, this heating may cause a fire. Therefore, it is important and necessary to keep contact resistance low.
Reason for an increase in Contact Resistance
The following reasons contribute to increasing contact resistance.
- Corrosion – The corrosion of contact points leads to an increase in contact resistance.
- Looseness – The contacts of the switching devices must fit into one another. Any looseness in the connection cause increase in contact resistance.
- Bur/Pitting – The bur/pitting may form when the switching equipment makes or break the fault current. Closing or interruption under fault conditions may cause pitting on the contacts, resulting in increased contact resistance.
- Oxidation – The contact area oxidizes after some time. The oxides formation increases the contact resistance. therefore regular cleaning is a must for keeping the contact resistance minimum.
Symptoms of Increased contact Resistance
The following parameters show the contact resistance is above the acceptable limit.
- Loose connectors
- High-voltage drop in the system
- Low tightness in bolted joints
- Corroded surfaces of bus bars
- Increased electrical panel temperature
How to Monitor Contact Resistance?
The increase in contact resistance causes serious problems in the electrical circuit. Therefore, the contact resistance measurement is a must to ensure the healthiness of electrical connections.
The electrical contact resistance test is carried out for switching devices, cable joints, and other electrical connections. The contact resistance measures two parameters.
- The voltage drop between the contacts
- The contact resistance of the joint or connections
The test measures the contact resistance in the range of milli-ohm level or micro-ohm level.
Measurement of the contact resistance helps in the identification of fretting corrosion of contacts and allows contact corrosion to be diagnosed and prevented. An 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 the resistance of the joints or connections.
Consequences of Higher Contact Resistance
The high contact resistance leads to the followings.
- Higher Power loss
- More Heat generation
- Loss of useful life of the electrical system
- Loss of switchgear capacity