where a considerable number of improvements could be made to the 1970s designs. Newly designed low resistance ohmmeters by Megger include data storage and downloading capability, additional test modes, reduced weight, extended battery life, etc. Why measure low resistance?Measuring low resistance helps identify resistance elements that have increased above acceptable values. The operation of electrical equipment depends on the controlled flow of current within the design parameters of the given piece of equipment. Ohm’s Law dictates that for a specified energy source, operating on V a.c. or V d.c., the amount of current drawn will be dependent upon the resistance of the circuit or component.In the modern age of electronics, increased demands are placed on all aspects of electrical circuitry. Years ago the ability to measure 0.01 ohms was acceptable, but, in the present industrial electronic environments, the field test engineer is now required to make measurements, which show repeatability within a few microhms or less. These types of measurements require the unique characteristics of a low resistance ohmmeter’s four wire test method, which is detailed in "Four wire measurements" on page 13.Low resistance measurements are required to prevent long term damage to existing equipment and to minimize energy wasted as heat. They show any restrictions in current flow that might prevent a machine from generating its full power or allow insufficient current to flow to activate protective devices in the case of a fault.Periodic tests are made to evaluate an initial condition or to identify unexpected changes in the measured values, and the trending of this data helps to indicate, and may forecast, possible failure conditions. Excessive changes in measured values point to the need for corrective action to prevent a major failure. When making field measurements, the user should have reference values that apply to the device being tested (the manufacturer should include this information in the literature or name plate supplied with the device). If the tests are a repeat of previous tests, then these records can also be used to observe the range of the anticipated measurements.If, when conducting tests, the user records the results and the conditions under which the tests were done, the information becomes the start of a database that can be used to identify any changes from fatigue, corrosion, vibration, temperature or other condition that can occur at the test site.What is a low resistance measurement? A low resistance measurement is typically a measurement below 1 Ohm. At this level it is important to use test instruments that will minimize errors introduced by the test lead resistance and / or 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, which need to be identified. To allow a measurement to compensate the errors, a four terminal measurement method is employed with a reversible test current and a suitable Kelvin Bridge meter. Low resistance ohmmeters are designed specifically for these applications. In addition the upper span on a number of these meters will range into kilohms, which covers the lower ranges of a Wheatstone bridge (see "Wheatstone and kelvin bridges" on page 32 for a discussion of each method). The lower range on many low resistance ohmmeters will resolve 0.1 microhm. This level of measurement is required to do a number of low range resistance tests.What does a low resistance measurement tell the user?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 in accordance with the formula W=I²R. The practical unit is the ohm. The low resistance measurement will show to the observant user when degradation has or is taking place within an electrical device. Changes in the value of a low resistance element are one of the best and quickest indications of degradation taking place between two contact points. Alternatively, readings can be compared 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, bus bar with cable joints and bond connections to ground beds. The measurement will alert the user to changes having taken place from the initial and / or subsequent measurements. These changes can occur from a number of influences including temperature, chemical corrosion, vibration, loss of torque between mating surfaces, fatigue and incorrect handling.These measurements are required on a regular timed cycle to chart any changes taking place. Seasonal changes may be evident when summer and winter data are reviewed.What problems create the need for a test?Assuming a device has been correctly installed in the first place, temperature, cycling, fatigue, vibration and corrosion all work to cause the gradual degradation of the resistance value of an electrical device. These influences build up over a period of time until a level is reached at which the device no longer operates correctly. The critical degrading factor will be determined by the application. Environmental and chemical attacks are relentless. Even air will oxidize 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 the area while increasing the resistance of the component. 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 rise. These conditions result in excessive heating at the location when 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 = 3,600 kWatts, which will result in excessive heating. If left unattended, these types of problems can lead to failure in the electrical system containing the affected components. Excessive heating will ultimately result in failure due to burnout, which can open an energized circuit.Backup battery power supplies provide a good practical example of how degradation can occur under normal operating conditions. Changes in current flow cause expansion and contraction of the terminal connections, causing them to loosen or corrode. Additionally, connections are exposed to acid vapors, causing further degradation. These conditions result in a decrease in the surface-to-surface contact area with an associated increase in surface-to-surface contact resistance, ultimately causing excessive heating at the junction.Saving money by low resistance testingIf you think about it, a joint that carries current will heat up over time. The amount of heat is dependent on the resistance of the connection and the amount of current it carries and also the amount of time!The alloy is modified for strips of material used in measuring shunts, which operate at a higher ambient, up to 50 ºC.The purpose of this booklet is to help the engineer, technician or user to understand: ■The rationale behind low resistance tests ■How to make a low resistance measurement ■How to select the correct instrument for the test application ■How to interpret and use the resultsBrief history of low resistance ohmmetersThe original DUCTER™ low resistance ohmmeteriv was developed by Evershed & Vignoles (one of the companies that evolved into Megger and the developer of the first insulation resistance tester) in 1908 and employed the cross-coils meter movement that was already used in the insulation resistance tester. This initial design evolved into field units in the 1920s that required a leveling procedure at the time of the test due to the sensitivity of the coil (to being level). These early models did not travel well and were sensitive to shock and vibration. For fifty years, field portable low resistance ohmmeters were analog units. In 1976, in response to numerous customer requests, the James G. Biddle Company (another one of the companies that ultimately became Megger) developed and introduced a digital low resistance ohmmeter. This unit was known by its trade name, the DLRO. Ultimately, the James G. Biddle Company released 10 A and 100 A versions of the DLRO, including a single box design for some versions that simplified the test process, and an extended range model.Through the acquisition of Programma Electric AB, Megger strengthened the program of high current low resistance ohmmeters's (LRO's). Back in the late seventies the MOM (Micro Ohm Meter) was one of the first products developed by Programma Electric AB, and in the decades that followed that series has been supplemented with MJÖLNER and MOM2. The MJÖLNER moved from transformer based technology to switched technology, which has the benefit of a much lighter test instrument. The latest innovation is the MOM2, which uses a patented ultra capacitor technology to generate the high current, which makes it possible to get over 200 A in a hand held product that weighs less than 1 kg.This style of instrument served the industry well for a number of years, and the various versions continue to help end users solve problems. However, electronics and battery technology advanced to the point iv Basic Electrical Measurements; Melvin B. Stout; 1950; page 61.www.megger.com 54 A guide to low resistance testing www.megger.com
