So obviously a joint or cable connection which becomes hot will only ever become hotter until, if you are lucky, it is identified by thermal imaging, and if you are not so lucky, when the lights go out as the connection burns out and the protective device operates.But what if you can’t use thermal imaging because there is no direct line of site to the connections. These can cook away deep inside a panel and not be spotted until it’s too late.Critical supplies fail regularly because of overheating connections due to high resistance connections burning out. Because of their critical nature, this makes regular isolation and maintenance almost impossible. Think about hospitals and data centers. Health and data are probably two of the most critical but vulnerable installations but get the least downtime for maintenance of enclosed switchgear assemblies and panel busbar systems.Using the formula W=I2R we can estimate the power lost over a connection or connections.For a 10kA joint/s with a 0.1mΩ resistance, the power is 10kW. For a 10kA joint/s with a 1mΩ resistance, the power is 100kW.For a 6kA joint/s with a 0.1μΩ resistance, the power is 36W. For a 6kA joint/s with a 100mΩ resistance, the power is 3600kW.Simply, the power manifests itself as heat.Using a DLRO to check the contact resistance of switchgear, lapped joints on busbars and cable lug connections before the power is switched on is the only sure way to prevent poor connections becoming potentially catastrophic failures.Industries with significant resistance problemsIndustries 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 which you can not use. In addition, there are industries that 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 major electrical surge. A number of sub-assembly operations supply components to aircraft manufacturers that specify low resistance connections to the airframe. Strap connections between cells on a power back-up battery system also require very low resistance. A general list of industries include: ■Power generation and distribution companies ■Chemical plants ■Refineries ■Mines ■Railroads ■Telecommunications companies ■Automotive manufacturers ■Aircraft manufacturers ■Anyone with UPS battery back-up systems What equipment needs low resistance testingAs we have shown, low resistance ohmmeters have an application in a wide range of industries, and can help identify a number of problems that could lead to apparatus failure. In general manufacturing industries, motor windings, circuit breakers, bus bar connections, coils, ground bonds, switches, weld joints, lightning conductors, small transformers and resistive components all require to be tested for low resistance. The following are some of the more typical applications.Motor armatureArmature windings can be tested to identify shorting between adjacent coils or conductors. Squirrel cage bars in the rotor can separate from the end plates, resulting in loss of performance. If a motor seems to be losing power, a low resistance test should be done. Alternatively, tests can be made when bearings are being replaced at a periodic or annual shutdown. ■Motor bar to bar testsMotor bar to bar tests on d.c. motor rotors are done to identify open or shorted coils. These tests are done with spring loaded hand probes. This is a dynamic method to determine the conditions of the windings and the soldered connections to the riser on the commutator segments. When test data is reviewed periodically, the effects of overheating due to excessive temperature rise can be identified.For more detailed information, see 'motor bar to bar tests' section' in Appendices. Automotive assemblyCable leads in a 'robot' spot welder can work harder through continual flexing. Eventually fatigue can occur, causing strands to break. This condition results in a high lead resistance with loss of power to the weld, producing a poor spot weld (nugget) or even complete failure of the machine.Power generation and distributionHigh current joints, connections and bus barsBus bars in a power system consisting of lap joints and other connections, are used to deliver current to the elements in the system. These bolted connections can be degraded by vibration and corrosion (see Fig 2). The bolts are stressed to a specific tightness (torque), and the quickest and most economical way to determine the quality of the connection is to measure the resistance across the joint. The user should have historical data to make a determination on the suitability of the connection. If left uncorrected, loss of power and / or excessive heating could lead to a meltdown at the connection.Fig 2: Bus bar connectionsTransformersTransformer winding tests are done in the factory and then periodically in the field. The factory test is done at ambient temperature. A second factory test is a heat run to check that, at rated power, the resistance of the windings stays within its designed temperature rise characteristics.Large transformers have 'taps' on both the primary and secondary windings. The condition of the taps requires verification, since the secondary taps are operated daily and are exposed to excessive wear and vibration as the power distribution system balances the load carried on the various circuits. The taps on the primary side are critical to major adjustments in the power distribution and should be tested to ensure that a low resistance connection is available for the new power condition. Tap connections can corrode when not in use and can overheat due to the high current (which can result in a fire).For more detailed information, see 'testing of transformers' section in Appendices. Uninterruptible power supply - battery strapsOn series connected industrial batteries, straps (lead coated copper bars) are secured to the posts on adjacent batteries, (+) to (-), with stainless steel bolts. These surfaces are cleaned, greased and tightened to a preset torque value. As noted previously, they are subject to vibration, chemical corrosion and heat due to the charging and high current discharges associated with the application. The quickest and best way to determine the quality of the connections is to measure the resistance between the two adjacent battery terminals (see Figs 3 and 4). This is the only field application in which the user makes measurements on an energized system. For more detailed information, see 'battery strap test' section in Appendices.Please note that there are various levels of 'float current' in a battery system and the test procedure must account for this current flow. A test is done with the test current added to the float current and a second test is made with the test current opposed to the float current. These two measurements are averaged to determine the 'ohmic' value of the connection.Standard procedures require measurements on a regular schedule, as past experience has determined that battery straps are one of the weakest elements in the operation of a battery system. When not attended to on a regular test program, high resistance connections can develop. This situation can result in the battery being unable to deliver sufficient current when called for, or when combined with current surge and hydrogen gas evolved from the battery cells, can cause a fire in the battery system, destroying the UPS. Fig 3: Single strap with two contact surfaceswww.megger.com 76 A guide to low resistance testing www.megger.com
