IntroductionThe quantitative study of electrical circuits originated in 1827, when Georg Simon Ohm published his famous book 'Die galvanische Kette, mathematisch bearbeitet' in which he gave his complete theory of electricity. In this seminal work, he introduced the relationship or 'Law' that carries his name:Resistance (R) = Voltage (E) / Current (I)At that time, the standards for Voltage, Current and Resistance had not been developed. Ohm’s Law expressed the fact that the magnitude of the current flowing in a circuit depended directly on the electrical forces or pressure and inversely on a property of the circuit known as the resistance. Obviously, however, he did not have units of the size of our present Volt, Ampere, and Ohm to measure these quantities.At this time, laboratories developed resistance elements, constructed of iron, copper or other available alloy materials. The laboratories needed stable alloys that could be moved from place to place to certify the measurements under review. The standard for the ohm had to be temperature stable and with minimum effects due to the material connected to the ohm standard. In 1861, a committee was established to develop a resistance standard. This committee included a number of famous men with whom we are now familiar, including James Clerk Maxwell, James Prescott Joule, Lord William Thomson Kelvin and Sir Charles Wheatstonei. In 1864, a coil of platinum-silver alloy wire sealed in a container filled with paraffin was used as a standard. This was used for 20 years while studies were made for a more reliable standard. These studies continued as the old National Bureau of Standards (NBS), now known as the National Institute of Standards and Technology (NIST), controlled the standard for the 'Ohm'. Today the industry uses Manganin alloy because it has a low temperature coefficient so that its resistance changes very little with temperature. Melvin B. Stout’s 'Basic Electrical Measurements' highlights the key properties of Manganin.Table 1: Key properties of ManganinComposition %ResistivityTemperature Coefficient per ºCThermal emf Against Copper μv/ ºCMicrohms for cm CubeOhms for Cir. mil FootCu 84%Mn 12%Ni 4%44 μΩ264 Ω*±0.00001º1.7*Manganin shows zero effect from 20º to 30º C.i Swoope’s Lessons in Practical Electricity; Eighteenth Edition; Erich Hausmann, E.E., ScD.; page 111.The thermal emf against copper shows the thermocouple activity of the material whereby a voltage is generated simply by connecting two different metals together. The goal is to minimize thermocouple activity as it introduces error into the measurement.With the metric system, the measurements are in meters and the resistivity is determined for a one meter cube of the material. However, more practical units are based on a centimeter cube. With the USA system, the resistivity is defined in ohms per mil foot. The wire diameter is measured in circular mils (0.001)ii and the length in feet. Fig 1 shows the temperature resistance curve for Manganin wire at 20 ºC (68 ºF). For Manganin shunts, the 20 °C curve shifts to 50 ºC (122 ºF), as this material will be operating at a higher temperature due to the application. The Manganin alloy was designed for use in coils used to do stable measuring conditions at 20 ºC ambient room conditions. Fig 1: Qualitative Resistance Temperature Curve for Manganiniiiii Swoope’s Lessons in Practical Electricity; Eighteenth Edition; Erich Hausmann, E.E., ScD.; page 118.iii Basic Electrical Measurements; Melvin B. Stout; 1950; page 61.FIGURESFig 1: Qualitative Resistance Temperature Curve for Manganini 4Fig 2: Bus bar connections 7Fig 3: Single strap with two contact surfaces 7Fig 4: Parallel straps on a large battery complex 8Fig 5: Measuring carrier strip resistance 8Fig 6: Test on graphite slugs for uniform density (ohms / inch) 9Fig 7: Series of measurements across a weld seam 9Fig 8: Determining the remaining length of cable on a reel 10Fig 9: Conventional test, one kelvin at either end of a multi-core cable 10Fig 10: The C2 and P2 shown as separate cables from a meter to one of the cores 11Fig 11: C1 connected to an adjacent core on the same end of the multi-core cable 11Fig 12: P1 connected to another core on the same end of the multi-core cable 11Fig 13: The other end of the cable showing the unmarked core 11Fig 14: Contact area reduced due to overtightening 12Fig 15: Typical joints that should be tested 12Fig 16: Typical faults that can be prevented by low resistance testing 12Fig 17: Selection of optimum measuring technique 12Fig 18: Simplified example of a 4 wire measurement 13Fig 19: Basic operation diagram 15Fig 20: ASTM standard B193-65 17Fig 21: Probe / lead configurations 17Fig 22: Trending analysis of low resistance readings 22Fig 23: C1 clip being connected to end of circuit being tested 22Fig 24: Duplex hand spike being used to perform same test as shown in Fig 23 22Fig 25: Correct and incorrect probe placements 24Fig 26: Basic styles of probes 24Fig 27: Temperature resistance curves for iron, copper and carbon 26Fig 28: Circuit breaker corrosion 27Fig 29: Noise 27Fig 30: Hot spots 28Fig 31: Bar to bar test on d.c. motor rotor 29Fig 32: Lap winding test data 30Fig 33: Commutator with 24 coils in series 30Fig 34: Wave winding test data 30Fig 35: Wave winding coil arrangement 31Fig 36: Single strap resistance target 31Fig 37: Parallel strap resistance target 31Fig 38: Wheatstone bridge circuit 32Fig 39: Kelvin bridge circuit 32Fig 40: DLRO100 Series 35Fig 41: DLRO10 / DLRO10X 35Fig 42: DLRO10HD 36Fig 43: DLRO600 36Fig 44: DLRO200 37Fig 45: MOM2 37Fig 46: MJÖLNER200 37Fig 47: MJÖLNER600 37Fig 48: MOM690A 38Fig 49: MOM200A / MOM600A 38Fig 50: BT51 38Fig 51: DLRO247000 39Fig 52: Duplex connect test leads 39www.megger.com 32 A guide to low resistance testing www.megger.com
