Contact Resistance Testing (Ductor Testing) Services – ETC

ETC is an authorized provider of Electrical Installation Study, Analysis, Inspection, and Certification services across the UAE, offering professional Contact Resistance Testing Services.

Contact resistance testing, also known as Ductor Testing, measures the resistance of electrical connections such as terminations, joints, connectors, busbar sections, or cable connections. These connections may exist between any two conductors, for example, cable joints or busbar sections. The instrument used for this testing is called a Ductor Tester (a specialized Ohmmeter).

Types of Ductor Testers

Ductor testers are available in various forms, including Micro, Milli, and Mega Ohmmeters, Static Resistance Testers, or DLRO (Digital Low Resistance Ohmmeters). These testers measure resistance at the micro- or milli-ohm level to verify proper electrical connections and detect issues such as:

Loose connections

Insufficient tension on bolted joints

Eroded or worn contact surfaces

Contaminated or corroded contacts

Both visual inspection and ductor testing are required for a complete assessment. A contact is considered healthy only if it passes both the resistance measurement and visual inspection.

Types of Ductor Testers

Series Type Ohmmeter – Consists of resistors and a battery voltage source. When connected, it forms a simple series circuit to measure resistance.

Shunt Type Ohmmeter – Measures small resistances by allowing current to bypass the resistor. When the circuit is open, it reads infinite resistance.

How Contact Resistance Testing is Performed

Test Criteria

The acceptable contact resistance depends on factors such as connection type (bolted, soldered, clamped, or welded), metallic contact surface area, and contact pressure. Manufacturers often specify maximum resistance values—for example, a large bolted busbar joint may have a maximum resistance of 10 micro-ohms.

Applications of Contact Resistance Testing

Electrical Connections – Ensures quality and conductivity of circuits, including switches, relays, and PCB pads.

Machinery Assembly – Assesses the reliability and tightness of metallic contacts in machinery, with lower resistance indicating better conductivity.

Measurement Methods

The most common method is the Four-Wire (Kelvin) DC Voltage Drop, which provides accurate readings by eliminating errors from the tester’s leads and connections. This method uses:

Two current injection leads

Two voltage measurement leads, placed close to the tested connection

Special care is taken to eliminate errors caused by thermal EMFs (small voltages generated when different metals are joined). Testing may be repeated at higher currents to overcome oxidation or surface issues for more accurate readings.

Importance of Contact Resistance Testing

Periodic contact resistance testing ensures circuit breakers and electrical connections are healthy and fully functional. Poor contacts can lead to:

Arcing

Phase loss

Fires

Reduced current carrying capacity and higher losses

Measuring contact resistance also helps detect fretting corrosion and other degradation, enabling preventative maintenance before serious damage occurs.

Steps Performed During Contact Resistance Testing

Visual Inspection – Examine contacts for wear, pitting, or deformation.

Resistance Measurement – Inject a fixed current (typically 100A, 200A, or 300A) and measure the voltage drop. The resistance is calculated using Ohm’s Law.

Comparison & Trending – Compare measured values with manufacturer specifications and historical data to identify trends in connection health.

Testing Conditions:

Always use calibrated test leads

Maintain consistent testing positions for accurate trending

Follow manufacturer-recommended DC test currents (typically 10% of rated current)

IEC and ANSI recommended test currents: 50A (IEC 60694), 100A (ANSI)

Conclusion

Contact resistance testing provides critical information about the condition of electrical contacts and their ability to safely handle rated current. Combined with visual inspection, it ensures system reliability, prevents failures, and extends the life of electrical equipment.