Ductor testing, also known as contact resistance test, measures the resistance of electrical connections - terminations, joints, connectors 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, used to measure resistance in different applications of electrical testing. This tester consists of a DC ammeter and a few other components including:
- A 3V battery which is a DC source of potential;
- Resistors (one or more of which at least one is variable)
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.
Ductor testing is used to verify that electrical connections are made properly and it has been designed to detect problems such as:
- Loose connections
- Eroded contact surfaces
- Corroded contacts
- Adequate tension on bolted joints
This test is especially important for contacts that carry large amounts of current 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.
The criteria for evaluating the contact resistance greatly depends on the metallic surface area, the contact pressure, the type of connection (ex. soldered, bolted, clamped, welded, etc.) and so on. These differ by manufacturer and equipment and there is no standard or code that mandates minimum contact resistance. Therefore, the manufacturer needs to be consulted for recommendations. For instance, sometimes manufacturers quote a maximum contact resistance of 10 micro-ohms for large bolted busbar joints.
If required, the ductor test can be backed up by Thermal Imaging in order to detect any Hot-Joint on the Bus-Bar-System.