Fig 8: Determining the remaining length of cable on a reelCable reelsA reel of insulated copper wire may have a tag, which shows the wire gauge along with the ohms per unit length. When wire remains on the reel after partial utilization, the remaining length can be calculated by measuring the resistance of the wire and making a calculation using the ohms per length specification (see Fig 8).Alternatively, if the tag has been destroyed, the user can cut off a known length of wire, measure that sample and determine the ohms per length. This value can then be used with the reading taken when measuring the balance of wire on the reel to calculate the remaining length. The temperature of the reel of cable will be approximately the same as the temperature of the sample. Though the internal temperature of the reel can be slightly different, a reasonable estimate of the remaining length of cable can be calculated. If the user reviews the temperature charts in "Effects of temperature" on page 26, an estimate of the inaccuracy can be determined. This method also applies to aluminum and steel wires as long as the wire has an insulating coating to prevent shorting between adjacent loops of wire.Measuring cable resistance of multicore cables of at least 3 coresWhen measuring cable resistance, the standard method is to connect the current and potential lead at each end of the cable core to be tested (see Fig 9).Fig 9: Conventional test, one kelvin at either end of a core of a multi-core cable When the cable is too long to use extension test leads or passes through the floors of a building, the above method cannot be used. However, there is a way to configure the test leads to accurately measure the resistance of each core of the cable with the DLRO positioned at one end of the cable to be tested. The current and potential test leads must be connected individually and not as a single kelvin type connection.Step 1: Connect the current and potental leads C2 and P2 to the core under test. In Fig 10 it is the core with the blue marker.Step 2: Connect the current lead C to an adjacent core. In Fig 11 it is the unmarked core.Step 3: Connect the potential lead P1 to the other core. In Fig 12 it is the core with the red marker. Fig 10: The C2 and P2 shown as separate cables from a meter to one of the coresFig 11: C1 connected to an adjacent core on the same end of the multi-core cableFig 12: P1 connected to another core on the same end of the multi-core cableStep 4: At the other end of the cable, connect core C1 to core 1 and core 3 to core 1 using short jumper cable ensuring that the core carrying the P1 connection is on the inner side of the cable. Fig 13: The other end of the cable shows the unmarked core carrying C1 connected to core with the blue marker (the core to be tested) and the core with the red marker carrying P1 connected to core with the blue marker (the core to be tested) the connections with short jumper cablesUsing the simple configuration (see Fig 13) shows that the resistance of long multi-core cables can be measured by using 2 cores of the cable as part of the measuring circuit.Using low resistance measurements to set torqueOne application for the DLRO which is infrequently used is the use of low resistance measurements in the assembly of bolted components to a set torque.When bus bar lapped joints or terminal lugs are overtightened, the material of the joint becomes dished and instead of becoming a better connection the resistance starts to increase as the surface area contact becomes distorted. This is why each joint and connection in a system normally has a manufacturer’s torque setting. But that’s not the whole story. If the joint has some contamination when it is tightened to its torque setting, the higher resistance may go undiscovered and the connection begins a journey on the downward spiral to overheating, arcing and eventual failure.But what if the connection does not have a manufacturer’s torque setting? The DLRO can be used during tightening to ensure the resistance of the joint is at its optimal value before being made live and put to work.www.megger.com 1110 A guide to low resistance testing www.megger.com
