Both of these industries require not only low ground bed resistance but also low resistance 'a.c. / d.c. bonds' between the ground bed and the active circuits.The difference between continuity and low resistanceIn basic terms, continuity shows us that we are connected to both ends of the same cable. This is normally done as a 2-wire test with a resistance measurement of 10 mΩ or above. In many cases, this is acceptable for a value to be recorded on certification. But it is worth bearing in mind that continuity can also be proved with an indication such as a buzzer or test lamp. Low resistance measurements can start at 0.1μΩ, often revealing connection issues with joints and contacts which can prove to be points of failure in waiting. This test uses the 4-wire test method which is not susceptible to test lead or probe / clip connection resistance to the device under test as it can be on the continuity 2-wire method.Test modesDigital low resistance ohmmeters designed in the 1970s and 1980s tended to offer two modes of operation, each designed for specific applications. Recent microprocessor technology has allowed newer instruments to include additional modes, further extending the capabilities of these models. The following is a brief review of the types of test modes available on different vintage instruments:Models designed in the 1970s and 1980sContinuous Mode: Allows the test current to flow and a measurement taken when the current and potential probes contact the test specimen. This mode of operation is usually implemented when the helical spring point lead sets are used and is the normal method when conducting field tests. Battery life is extended, as the test current flows only when the tests are in progress.Momentary Mode: Requires both sets of test leads to be connected to the specimen. The measurement is done when the switch is toggled to the Momentary position. This mode of operation is used when separate current and potential leads are connected to the specimen.10 amp modelsNormal Mode: The user connects all four test leads and presses the test button on the instrument to start a test. The instrument checks the continuity of the test connections and then applies forward and reverse current. The reading is shown for a short period (10 seconds).Auto Mode: Allows forward and reverse current measurements to be made (the average value is shown) by making contact with all four probes. Each time the probes are removed and reconnected to the load, another test is done. This mode, which is similar to the Continuous Mode found on older instruments, is an excellent time saving method to use when battery straps are tested with hand-spikes. It has the added advantage, when hand-spikes are used, that the contact detection sensing ensures good contact before heavy currents are applied. This avoids arcing when contact is made, which erodes the probe tips as well as potentially damaging the surface of the item under test. Continuous Mode: Allows repeated measurements to be made on the same test sample. Once the test leads are connected and the test button pressed, a measurement is made every set number of seconds until the circuit is broken.Unidirectional Mode: Applies a current in one direction only. While this type of measurement does not negate standing emfs, it does speed up the measuring process. In many test conditions, such as battery straps tests, it is not necessary to do a reversed current test on the sample. This mode is also used when objects with inductive properties, such as motors and transformers, are tested.100 amp and above modelsNormal Mode: The user connects all four test leads and presses the test button on the instrument to start a test. The instrument checks the continuity of the test connections and then applies the test current.Continuous Mode: Used to monitor test conditions for a period of time. After the test leads are connected and the test button is pressed, tests will be recorded every set number of seconds until the test button is pressed again or contact is broken with any of the test probes. Auto Mode: Because of the heavy test currents used, the user connects the current leads, selects the desired test current and presses the test button. As soon as the potential leads are connected, a test will start. To make another test, the user breaks contact with the voltage probes and then remakes contact. This is an excellent mode for measuring individual joints in a bus bar.How does a low resistance ohmmeter operate?A low resistance ohmmeter uses two internal measuring circuits. The supply injects a current into the test sample through two leads, usually identified as C1 and C2, and the magnitude of the current is measured. Concurrently, two probes (normally referred to as P1 and P2) measure the potential across the sample. The instrument then does an internal calculation to determine the resistance of the test sample.Why does this approach result in a measurement that is independent of lead resistance and contact resistance?We have represented the complete measurement circuit in Fig 19. Current is injected into the item under test via leads C1 and C2. The current that flows will be dependent upon the total resistance of this loop and the power available to push the current through that resistance. Since this current is measured, and the measured value is used in subsequent calculations, the loop resistance, including the contact resistance of the C1 and C2 contacts and the lead resistance of C1 and C2, does not have an effect on the final result.Fig 19: Basic operation diagramFrom Ohm’s Law, if we pass a current through a resistance we will generate a voltage across the resistance. This voltage is detected by the P1 and P2 probes. The voltmeter to which these probes are connected internally has a high impedance, which prevents current flowing in this potential loop. Since no current flows, the contact resistance of the P1 and P2 contacts produces no voltage and thus has no effect on the potential difference (voltage) detected by the probes. Furthermore, since no current flows through the P leads their resistance has no effect. A high current output is one of the qualifying characteristics of a true low resistance ohmmeter. Generic multimeters do not supply enough current to give a reliable indication of the current carrying capabilities of joints, welds, bonds and the like under real operating conditions. At the same time, little voltage is required, as measurements are typically being made at the extreme low end of the resistance spectrum. Only the voltage drop across the measured resistance is critical, and it is measured at the millivolt level.Good instruments alert the user of open circuit conditions on the test leads while a few models have automatic range selection. SafetySafety is the responsibility of the field test engineer or technician, whoever will be in contact with the sample being tested. The majority of field tests are done on de-energized circuits. When magnetic components are tested, a state of winding saturation can occur. The user should connect a short circuit across the winding to neutralize the energy stored in the winding and then make a voltage test to check the neutral state of the sample. Some instruments have indication lamps on the test probes to alert the user to a live voltage condition.Battery strap tests represent a special condition, as the batteries must remain connected. The user is required to use insulated gloves, face mask and a body apron for protection when performing these tests. This is one of the few times when electrical resistance tests are done in the field on energized systems. Special probes, rated for 600 V operation, are available with the newer instruments to do these tests.Using instruments with the capacity to store measured values improves the safety as the user does not have to write down the readings between each test.Test on de-energized samplesAs a general safety measure, tests should always be done on de-energized samples. Special training and equipment are required to do tests on energized circuits. Internal fused input circuits are designed into a few instruments that will protect the instrument if inadvertently connected to an energized test sample. The low input impedance of the current supply internal to general instruments becomes a willing current sink when connected across a live circuit.www.megger.com 1514 A guide to low resistance testing www.megger.com
