Contact Resistance Testing Service - UAE

Carelabs is authorized provider of Electrical Installation’s Study, Analysis, Inspection, and Certification services in UAE, and provide Contact Resistance Testing Service.

Contact resistance testing also known as Ductor testing, measures the resistance of electrical connections – terminations, joints, connectors, busbar sections or cable connections and so on. These can be connections between any two conductors, for example, cable connections or busbar sections. The instrument which is used to perform the ductor test is called an Ohmmeter, and since its function is to perform the ductor test, the ohmmeter is also known as a ductor tester.

The ductor tester can be found in many variations such as Micro, Mega and Milli- Ohmmeters, static resistance tester or DLRO which stands for Digital Low Resistance Ohm Meter. Is used to measure resistance in different applications of electrical testing. This tester consists of a DC ammeter and a few other components. The test measures the resistance at the micro- or milli-ohm level and is used primarily to verify that electrical connections are made properly, and can detect the following problems:

• Loose connections
• Adequate tension on bolted joints
• Eroded contact surfaces
• Contaminated or corroded contacts

Why You Need Contact Resistance Test?

The contacts in the circuit breaker needs to checked periodically to ensure that the breaker is healthy and functional. Poorly maintained or damaged contacts can cause arcing, loosing phase, and even fire.

This test is especially important for contacts that carry large amounts of current (e.g. switchgear busbars) because higher contact resistance can lead to lower current carrying capacity and higher losses. Ductor testing is usually performed using a micro/milli-ohmmeter or low ohmmeter.

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.

What is Done During Contact Resistance Testing?

The two common checks conducted on the contacts of a circuit breaker are the visual inspection check and the contact resistance check.

1. The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts.
2. The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 100A, 200A and 300 A through the contacts and measuring the voltage drop across it. This test is done with a special contact resistance measuring instrument. Then, using Ohm’s law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records.

Both these tests need to be done together. As there are cases of contacts having good contact resistance yet being in a damaged conditions. Thus, for a contact to be certified healthy, it needs to have a good contact resistance and should clear the visual inspection test.

Ductor Tester

There are two types of ductor testers in general:

1. Series Type Ohmmeter has 4 resistors, internal battery voltage – E, and output terminals, A and B. When connected the A and B terminals with the R1 and R2 resistors, the battery forms a simple series circuit.
2. Shunt Type Ohmmeter, used for measuring small values of current resistance. When the A and B terminals are closed, the needle reads zero because the current flows only through the resistor RX. When these two terminals are opened, there is no current flowing through the RX resistor, thus the reading on the ductor tester is marked as infinite.

How to Conduct Contact Resistance Test?

Test Criteria

The criteria for evaluating the contact resistance of electrical connections largely depends on the type of connection (e.g. bolted, soldered, clamped, welded, etc.), the metallic contact surface area, the contact pressure, etc. These will differ by equipment and manufacturer and there is no code or standard that mandates minimum contact resistances. As such, manufacturer recommendations need to be consulted. For example, manufacturers sometimes quote a maximum contact resistance of 10 micro-ohms for large bolted busbar joints.

Contact resistance measurement and its application domain are fairly extensive.

Electrical Connections

The electric connections of circuits have various ways and means, such as connected by welding, by pressing, by plug in and blot tightly and so on. To know the quality of a connector and its conduction characteristic, we just need to measure its contact resistance. The contact resistance is often applied in quality testing of switches, relays and PCB pads.

Machinery Assembly

At the aspect of the machinery assembly, the contact resistance of metals contact surface is used in.