Carrier strips 'carry' the plates in a cell. The plates are suspended from the carrier strips into the liquid in the cell. If the resistance of the terminal to carrier strip welds is too high, the battery’s ability to carry current is limited. In addition to measuring strap resistance, a low resistance ohmmeter can also be used to measure the quality of these welds (see Fig 5).Fig 4: Parallel straps on a large battery complexFig 5: Measuring carrier strip resistanceCement plants and other raw material processing applicationsThe electrical system at a cement plant or other raw materials processing facility includes motors, relays, disconnect switches, etc. Tests of these power carrying elements, as part of a regular program or when major retrofits take place, is critical to the ongoing operation of the plant. The quality of the current connections can identify weak elements or connections in the system. Note: Cement dust is chemically active (corrosive) and will attack metallic connection.Circuit breakersDue to arcing at the pads of a circuit breaker, carbonized layers can build up and the live contact area will reduce or become pitted, leading to increased resistance and heating. This situation reduces the efficiency of the circuit breaker and can lead to failure on an active transmission system resulting in the loss of a substation. When planning a test, the user must be aware of IEC62271-100 (minimum 50 A) ANSI and ANSI C37.09 (minimum 100 A) for test current requirements. When tests are done on large oil circuit breakers, the best instrument is one that ramps up current, holds it for a period of time and then ramps down (see "Ramp testing" on page 31). When d.c. is run through a circuit with a Current Transformer (CT), the CT will be magnetized. The problem caused is that the positive flank in the d.c. can cause a transient that might trip the relay. A d.c. with a large ripple is particularly problematic. Care should be taken when making a measurement across a CT as high d.c. currents can saturate the CT, desensitizing it to potential faults. Also, a ripple on the test current can cause circuit breakers to trip. Careful positioning of the current probes should prevent this from happening, and the ripple present on the current waveform may be minimized by separating the test leads. Alternatively use a test set with a ramp feature and smoothed d.c.Aircraft assemblyBonding test of all main frame electrical and mechanical connections is required to ensure a stable 'ground plane' within an aircraft. These physical 'bond' connections provide a uniform path for static electricity to be discharged to the wicks on the trailing edge of the wings and tail assembly. This path reduces the chance of lightning damaging the avionics in the event of a lightning strike situation. Over time, the bonding of static wicks, antenna, control linkage and battery terminals must be inspected. The integrity of a welded exhaust system should also be checked and documented. In normal operations, excessive static electricity will not effect the operation of most navigation and communications systems. The best (lowest) resistance connections will improve the performance of such systems. Fig 7: Series of measurements across a weld seamWelding spot or seamThe quality of a spot weld can be determined by measuring the resistance across the joined materials. The quality of a seam weld can be determined by a series of tests along the weld seam. Readings should stay within a narrow band of values. An increase and then a drop in readings shows that the uniformity of the weld is out of specification. To make the measurement correctly, the user should fabricate a fixture to keep the probes in a fixed relationship. Readings are then taken at a number of points across the weld seam and plotted (see Fig 7). These measurements are normally in the microhm region and special care is required in the design of the test fixture.Strap and wire bonds between rail segments (railroad industry)In the railroad environment, bonds are exposed to vibration as the wheels pass over the rails (each click-clack causes vibration across the interface bonding the strap to the rail). These bonds are part of the control system which tells the user the location of different trains. Within the rail system, a telephone system uses the rail conductors to communicate. The resistance of these bonds is critical to the performance of the control system. In systems that use three rails, the third rail is the active source of power for the engine, and power lost across a high resistance bond (such as a poor Cadweld joint) reduces the efficiency of the transit system. The user can select a five foot section of track without a bond, make a measurement and then measure a five foot section with a bond to determine the quality of the connection. As a rule of thumb, these measurements should be within a few microhms (or ±5%).Graphite electrodesGraphite electrodes have a negative temperature characteristic (as the temperature of the element increases the resistance measurement will decrease). Graphite slugs are extruded as large diameter cylinders and can be up to six feet in length. One of the applications for these large slugs is in aluminum refineries where high currents (150,000 A) are used to reduce bauxite ore to high grade aluminum.Low resistance tests are done as a quality control step to check the density of the graphite extrusion. Due to the size of the electrodes, this test requires a special test fixture to introduce the test current across the surface of the ends, ensuring a uniform current density through the volume of the sample. The potential probes are then connected across a known length of the sample to determine the 'ohms per unit length' (see Fig 6). Fig 6: Test on graphite slugs for uniform density (ohms / inch)www.megger.com 98 A guide to low resistance testing www.megger.com
