A Comprehensive Guide to Low Resistance Testing

1. Introduction

The fundamental understanding of electrical circuits is rooted in Ohm’s Law, published by Georg Simon Ohm in 1827, which establishes the relationship between resistance (R), voltage (E), and current (I): R = E/I. While the initial standards for these units were not yet defined, this law provided the theoretical basis for quantifying the property of a circuit that opposes the flow of current.

The development of a stable, standardized unit of resistance was a critical step, leading to the adoption of alloys such as Manganin due to its exceptionally low temperature coefficient. This property ensures its resistance remains stable across a practical range of ambient temperatures, making it ideal for precision measuring shunts and standards.

The practical instrument for measuring low resistance, the Low-Resistance Ohmmeter (LRO), has a long history. The original DUCTER low resistance ohmmeter was developed in 1908 by Evershed & Vignoles. This analog design evolved over decades until the introduction of the first digital low resistance ohmmeter, the DLRO (Digital Low Resistance Ohmmeter), in 1976. Modern LROs, such as those in the DLRO, MOM, and MJÖLNER series, have advanced significantly, incorporating features such as data storage, microprocessor control, and lightweight designs, including the use of ultracapacitor technology for high-current output.

2. The Rationale for Low Resistance Testing

Low-resistance testing is a crucial component of electrical maintenance and quality control, primarily focused on identifying resistance elements that have degraded or increased beyond acceptable design values. The operation of electrical equipment relies on the controlled flow of current within specified parameters. An increase in resistance, even by a small amount, can lead to significant problems due to the power dissipation formula W = I²R.

Consequences of High Resistance

Degradation in electrical connections is a relentless process driven by factors such as temperature cycling, fatigue, vibration, and corrosion. These influences gradually increase the resistance of a component, leading to several critical issues:

• Excessive Heating: Increased resistance leads to excessive power dissipation, resulting in higher heat. For instance, a 6,000 A current across a bus bar joint with a resistance of 100 muOmega dissipates 3,600 Watts of power, which can lead to localized overheating and eventual failure.
• Energy Loss: The power dissipated as heat is wasted energy, increasing operating costs for high-consumption industries.
• Equipment Failure: Unattended high-resistance connections can lead to burnout, arcing, and catastrophic failure, potentially causing unexpected outages and safety hazards.
• Reduced Performance: High resistance can restrict current flow, preventing machinery from generating its full power or impeding the activation of protective devices during a fault.

The Role of Trending

Periodic low-resistance measurements are vital for establishing a baseline and tracking changes over time. Trending the data allows engineers to predict when a joint, weld, or connection will become unsafe, enabling proactive, condition-based maintenance rather than reactive repairs. Recording test results alongside environmental conditions (e.g., temperature, humidity) creates a valuable database for identifying changes caused by fatigue, corrosion, or torque loss.

3. Principles of Low Resistance Measurement

A low resistance measurement is typically defined as any measurement below $1 \Omega$. At this level, the inherent resistance of the test leads and the contact resistance between the probe and the test object can introduce significant errors if not properly compensated.

The Four-Terminal (Kelvin) Method

To achieve the high accuracy required for micro-ohm and milli-ohm measurements, LROs employ the four-terminal measurement technique, also known as the Kelvin sensing method. This method uses four separate connections to the test object:

By separating the current injection and voltage measurement circuits, the LRO ensures that the voltage drop caused by the resistance of the current leads and the contact resistance at the probe tips is excluded from the final calculation. This elimination of lead resistance error is essential for obtaining reliable, repeatable measurements in the (muOmega) range.

DC vs. AC Testing

The choice between direct current (DC) and alternating current (AC) for low resistance testing depends on the nature of the test object:

• DC Measurement: A DC instrument measures the pure resistance of a circuit or device. This is the standard for most LRO applications, such as contacts, bus bars, and windings.
• AC Measurement: An AC instrument is used for applications where impedance (resistance plus reactance) is required, such as ground bed tests or battery impedance tests. For ground resistance testing, AC is preferred because it prevents the test current from polarizing soil ions, which would otherwise alter the measured values.

4. Key Applications of Low Resistance Testing

Low resistance testing is indispensable across numerous industries, ensuring the safety and efficiency of critical electrical infrastructure.

5. Instrumentation and Test Methodology

Types of Low-Resistance Ohmmeters

LROs are generally categorized by their maximum test current, which is a key factor in determining their application:

The use of high test currents (100 A+) is often preferred by utilities because it better represents working conditions and can sometimes break through surface contamination on contacts, yielding a more accurate reading of the underlying connection.

Measurement Modes

Modern LROs offer various test modes to optimize the measurement process for different applications:

• Normal Mode: The standard operation where the user connects all four leads and initiates the test, which typically applies forward and reverse current to average out thermal EMFs.
• Auto Mode: Allows for rapid, repeated measurements, ideal for testing multiple joints in a bus bar or numerous battery straps. The test is initiated automatically upon contact with all four probes.
• Continuous Mode: Used to monitor test conditions over a period of time, recording measurements at set intervals.
• Unidirectional Mode: Applies current in one direction only. This is used to speed up testing when thermal EMF compensation is not critical, or when testing inductive loads like transformers, where a reversed-current test would significantly increase test time due to the need to saturate the core.

•Ramp Testing: Delivers a controlled, slow ramp of the output current. This is crucial when testing equipment protected by differential relays, as the slow current rise prevents the relay from incorrectly detecting a fault and tripping the circuit breaker.

6. Factors Affecting Measurement Accuracy

Achieving accurate low-resistance measurements requires careful consideration of several potential sources of error.

Environmental and Material Effects

• Temperature: Resistance is highly temperature-dependent. The resistance of materials like copper increases with increasing temperature. For example, the resistance of a copper winding at 100 ^oC is approximately 31.4% higher than at 20 ^oC. Measurements must be corrected to a reference temperature (e.g., 20 ^oC to allow for meaningful comparison and trending.

•Ingress Protection (IP) Rating: The IP rating (e.g., IP54) indicates the instrument’s resistance to ingress of solids (first digit) and liquids (second digit). Using an instrument in an environment that exceeds its IP rating can compromise its accuracy and longevity.

Noise and Thermal EMF

• Thermal Electromotive Force (EMF): Also known as the Seebeck voltage, thermal EMF is a standing voltage generated when different conducting materials are joined (in a thermocouple) and are at different temperatures. Since LROs measure at the millivolt level, even small thermal EMFs can introduce significant error. LROs compensate for this by performing a test with current flowing in both the forward and reverse directions and averaging the results.
• Background Noise and Induced Currents: In power environments, electrical noise can interfere with the low-level voltage measurement. The magnitude of the test current affects the instrument’s noise rejection capability; a higher test current (e.g., 10 A) provides a better signal-to-noise ratio than a lower current (e.g., 0.1 A).

Contact Resistance Contamination

Contamination on contact surfaces (e.g., oxidation, corrosion) can artificially inflate the measured resistance. In some cases, using a higher test current (e.g., 100 A) can physically break through or melt the contamination, allowing a more accurate measurement of the underlying bulk resistance. However, this practice must be balanced against the risk of excessive heating or damage to the test object.