Single bolted connections have always had issues with the relationship between tightness and optimal surface area contact. Fig 14: Contact area reduced due to overtighteningFor this reason, and to increase the surface area contact, many panel and busbar systems use clamped, lapped or sandwich type joints (see Fib 14). In assemblies that are subject to excesses of heat and vibration, the issues discussed can become dramatic very quickly, which is why we see more use of elaborate locking mechanisms to maintain the contact resistance once set. Fig 15: Typical joints that should be testedUsing a DLRO to measure the effectiveness of these types of connections (see Fib 15), the resulting data can be collected and using predictive maintenance techniques, trended over time to identify potential failures, in a joint or an assembly of connected parts by the early identification of a rise in resistance levels (see Fig 16).Fig 16: Typical faults that can be prevented by low resistance testingHow is low resistance measured?Two, three and four wire d.c. measurements Why do we have resistance measuring instruments, some with only two test leads, some with three and even some with four test leads? The answer depends on the degree of information required from the measurement, and the magnitude of the resistance being measured. Resistance readings cover a wide range of values from microhms into the thousands of megohms region. Fig 17 shows the measurement range in which each type of instrument works best.Fig 17: Selection of optimum measuring techniqueTwo wire measurementsTwo wire tests are the simplest method and are used to make a general assessment of a circuit element, conductor or the routing of a conductor in a circuit. The two wire lead configuration is most familiar to many users as it is the configuration used on most multimeters. It is generally used when the probe’s contact resistance, series lead resistance or parallel leakage resistances do not degrade the quality of the measurement beyond a point acceptable to the user.The measured value will include the test lead wire resistance and contact probe resistance values, which will affect the measurement by adding some tens of milliohms to the actual resistance. In most instances this will make little practical difference to the measured value, but when the measurement is below 1 ohm the two wire method can easily introduce an error, which could be several percent, into the measured resistance value. The specifications on some hand held meters show a 200 milliohm range with one milliohm sensitivity. The lead resistance can be zeroed out, but that leaves the uncertainty of the contact resistances, which can change with each measurement. Contact resistance values can be in the 35 milliohm range at each probe and can vary with the temperature of the material under investigation.The two wire test method is best used for readings above 10.00 ohm up to 1.0 to 10.0 megohm.Three wire measurementsThree wire d.c. tests are reserved for very high resistance and is typically used for measurements above 10 megohms. We normally associate these types of tests with diagnostic insulation resistance. The test method uses a third test lead as a guard, and allows for resistances, in parallel with the test circuit, to be eliminated from the measurement. This parallel resistance is usually considerably lower than the insulation resistance being measured. In fact it can, in severe cases, effectively short out the insulation resistance such that a meaningful measurement cannot be carried out without the use of a guarding circuit.This test method is described and illustrated in the Megger booklets 'A Stitch in Time' and 'A Guide To Diagnostic Insulation Testing Above 1 kV'.Four wire measurementsFour wire tests are the most accurate method when measuring circuits below 10 ohms as this method eliminates errors due to lead and contact resistances. This is the test method associated with low resistance ohmmeters. Four wire d.c. measurements uses two current and two potential leads (see Fig 18). The four wire d.c. measurement negates the errors due to the probe lead wire and any contact resistance values in the final reading, ensuring more accurate measurements. Fig 18: Simplified example of a 4 wire measurementD.C. vs. A.C. The issue here is the selection of the correct type of test current. A d.c. instrument should be used when trying to measure the pure resistance of a circuit or device. An a.c. instrument is used for applications such as ground bed tests or impedance tests.A special impedance meter is used to do tests on industrial batteries. The word impedance is used to show that a measurement comprised of a resistance and reactance, which can be either an inductive or capacitive component. These measurements are conducted as part of a battery maintenance program; typically a low resistance ohmmeter is used to do strap connection verification tests.Three or four wire a.c. measuring systems are used to do tests on 'ground beds' with special frequencies that exclude measurement errors from 50 / 60 Hz ground currents. The use of a.c. prevents the test current polarizing ions in the soil, thereby changing the conditions and thus the measured values. This is an area of interest to the electrical power distribution and telecommunication fields. The low ground resistance path is required for maintaining the potential of the ground wire to the 'earth' potential. Electrical performance of the power system minimizes shock hazards as a path to ground is made available for the energy from lightning and other static voltages that can affect the power control system. The same conditions pertain to the telephone systems, as high resistance grounds can cause excessive noise on the voice and data links (see the Megger booklet 'Getting Down to Earth' for more information on ground resistance tests). www.megger.com 1312 A guide to low resistance testing www.megger.com
