DLRO10 and DLRO10X digital low resistance micro-ohmmeters

A low resistance measurement is typically a measurement below 1 ohm. At this level, it is essential to use test instruments that will minimise errors introduced by the test lead resistance and the contact resistance between the probe and the material under test. Also, at this level, standing voltages across the item being measured (e.g., thermal electromotive forces (EMFs) at junctions between different metals) can cause errors that need to be identified. A four-terminal measurement method should be employed with a reversible test current and a suitable Kelvin Bridge metre to eliminate or compensate for these measurement error sources. 

Low resistance ohmmeters are designed specifically for these applications. In addition, the upper span on a number of these metres will range into kilohms, which covers the lower ranges of a Wheatstone bridge. The lower range on many low resistance ohmmeters will resolve 0.1 microhm. This level of measurement is required to do several low-range resistance tests.

Resistance (R) is the property of a circuit or element that determines, for a given current, the rate at which electrical energy is converted to heat per the formula W=I²R. The unit of measure is the ohm. The low resistance measurement will show you whether degradation has or is occurring within an electrical device.

Changes in the value of a low resistance element are one of the best and quickest indications of degradation between two contact points. Alternatively, you can compare readings to ‘like’ test specimens. These elements include rail bonds, ground bonds, circuit breaker contacts, switches, transformer windings, battery strap connections, motor windings, squirrel cage bars, busbar with cable joints, and bond connections to ground beds.

The measurement will alert you to changes that have taken place from the initial and/or subsequent measurements. These changes can occur from many influences, including temperature, chemical corrosion, vibration, torque loss between mating surfaces, fatigue, and incorrect handling. These measurements are required on a regular cycle to chart any changes. Seasonal changes may be evident when you review summer and winter data.

Low resistance ohmmeters have an application in a wide range of industries. They can help identify various problems that could lead to apparatus failure. In general manufacturing industries, motor windings, circuit breakers, busbar connections, coils, ground bonds, switches, weld joints, lightning conductors, small transformers, and resistive components all require testing for low resistance.

The following are some of the more typical applications:

• Motor armature - motor bar-to-bar tests
• Automotive assembly - cable leads in welding robots
• Power generation and distribution - high current joints, connections, and busbars
• Transformers - primary and secondary taps
• UPS - battery straps
• Wind turbine - the weather mast, roof cooler, control panels, nacelle-tower junction, nacelle-hub junction, and machine support

Assuming its correct installation, factors such as temperature, cycling, fatigue, vibration, and corrosion all work to cause the gradual degradation and increase in resistance of an electrical device. These influences build up over some time until a level is reached at which the device no longer operates correctly. The application will determine the critical degrading factor.

Environmental and chemical attacks are relentless. Even air will oxidise organic materials, while the ingress of moisture, oil, and salt will degrade connections even more rapidly. Chemical corrosion can attack the cross-sectional area of an element, reducing its effective size while increasing the component’s resistance. Electrical stresses, particularly sustained overvoltages or impulses, can cause welds to loosen. Mechanical stress from vibration during operation can also degrade connections, causing resistance to increase. These conditions result in excessive heating at the location where the component is carrying the rated current, based on the formula W=I²R. For example:

• 6000 A across a 1 μΩ bus = 36 Watts
• 6000 A across a 100 mΩ bus = 3600 kW

If left unattended, these problems can lead to failure in the electrical system containing the affected components. Excessive heating will ultimately cause failure due to burnout, which can open an energised circuit. Backup battery power supplies provide a good practical example of how degradation can occur under normal operating conditions. Changes in current flow cause the terminal connections to expand and contract, causing them to loosen or corrode. Additionally, connections are exposed to acid vapours, causing further degradation. These conditions cause a decrease in the surface-to-surface contact area with an associated increase in surface-to-surface contact resistance, ultimately resulting in excessive heating at the junction.

Industries that consume vast amounts of electrical power must include low resistance ohmmeter measurements in their maintenance operations. Not only does abnormally high resistance cause unwanted heating, possibly leading to danger, but it also causes energy losses, which increase operating costs. In effect, you are paying for energy you cannot use.

In addition, some industries have critical specifications on bond connections to ensure solid connections to ‘ground beds.’ Poor connections reduce the effectiveness of the ground bed and can cause significant ‘power quality’ related problems and/or catastrophic failure in the event of a major electrical surge. Several sub-assembly operations provide components to aircraft manufacturers that specify low resistance connections to the airframe. Strap connections between cells on a power backup battery system also require very low resistance.

A general list of industries includes:

• Power generation and distribution companies
• Chemical plants
• Refineries
• Mines
• Railroads
• Wind turbines
• Telecommunications companies
• Automotive manufacturers
• Aircraft manufacturers
• Anyone with UPS battery backup systems