Support for the DLRO10 and DLRO10X digital low resistance micro-ohmmeters
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Make sure both C1 and C2 leads are making proper contact with your test specimen. Additionally, you can check for the continuity of those two leads using a multimeter to rule out any potential damage. If these two suggestions fail, it is likely because the current terminals C1 and C2 have become disconnected from the power board, in which case, you will need to send in the instrument for repair.
This is usually the result of the power battery losing charge due to normal ageing or terminal wires loosening. You can replace the battery on-site following the instructions in the User Guide. If that does not fix the issue, wiring issues may necessitate returning to Megger’s repair department.
Non-volatile memory batteries lose charge over time due to natural ageing. Replacing the battery is insufficient, as all calibration settings will have been lost. As such, you need to return the DLRO10 to Megger for recalibration.
The calibration constants have been lost. The DLRO will continue to work, but we can no longer guarantee its accuracy. You need to return the DLRO10X for recalibration.
An error has occurred during the measurement; for example, contact has been lost on one of the probes. Rectify the error and repeat the measurement.
Interpreting test results
Measuring low resistance helps identify resistance elements that have increased above acceptable values. Low resistance measurements prevent long-term damage to existing equipment and minimise energy wasted as heat. This testing reveals any restrictions in current flow that might prevent a machine from generating its full power or allow insufficient current to flow to activate protective devices in the case of a fault.
When evaluating results, it is crucial to pay attention first to repeatability. A good quality low resistance ohmmeter will provide repeatable readings within the accuracy specifications for the instrument. A typical accuracy specification is ±0.2 % of reading, ±2 LSD (least significant digit). For a reading of 1500.0, this accuracy specification allows a variance of ±3.2 (0.2 % x 1500 = 3; 2 LSD = 0.2). Additionally, the temperature coefficient must be factored into the reading if the ambient temperature deviates from the standard calibration temperature.
Spot readings can be critical in understanding the condition of an electrical system. You can get some idea of the level of the expected measurement based on the system’s data sheet or the supplier’s nameplate. Using this information as a baseline, you can identify and analyse variances. You can also make a comparison with data collected on similar equipment. The data sheet or nameplate on a device should include electrical data relevant to its operation. You can use the voltage, current, and power requirements to estimate the resistance of a circuit and the operating specification to determine the allowed change in a device (for example, with battery straps, connection resistances will change with time). Various national standards provide guidance for periodic test cycles. The temperature of the device will have a strong influence on the expected reading. For example, the data collected on a hot motor will differ from that of a cold reading taken at the time of the motor’s installation. As the motor warms up, the resistance readings will go up. The resistance of copper windings responds to changes in temperature based on the fundamental nature of copper as a material. Using the nameplate data for a motor, you can estimate the expected percentage change in resistance due to temperature using Table 1 for copper windings or the equation on which it is based. Different materials will have different temperature coefficients. As a result, the temperature correction equation will vary depending on the material being tested.
|Resistance μΩ||% Change|
R(end of test)/R(start of test) = (234.5 + T(end of test))/(234.5 + T(start of test)
In addition to comparing measurements made with a low resistance ohmmeter against some preset standard (i.e., a spot test), the results should be saved and tracked against past and future measurements. Logging measurements on standard forms with the data registered in a central database will improve the efficiency of the test operation. You can review previous test data and then determine on-site conditions. Developing a trend of readings helps you better predict when a joint, weld, connection, or another component will become unsafe and make the necessary repairs. Remember that degradation can be a slow process. Electrical equipment faces mechanical operations or thermal cycles that can fatigue the leads, contacts, and bond connections. These components can also be exposed to chemical attacks from either the atmosphere or man-made situations. Periodic tests and recording of the results will provide a database of values that can be used to develop resistance trends.
Note: When taking periodic measurements, you should always connect the probes in the same place on the test sample to ensure similar test conditions.
User guides and documents
The “unidirectional mode” applies a current in one direction only. While this measurement type does not negate standing EMFs, it does speed up the measuring process. In many test conditions, such as battery strap testing, doing a reversed current test on the sample is unnecessary.
The “inductive mode” continuously applies a current in one direction until the test is stopped. This mode allows the instrument to charge the inductive element of the load and, therefore, measure just the resistive part.
The “continuous mode” allows repeated measurements to be made on the same test sample. Once you connect the test leads and press the test button, the instrument completes a measurement every set number of seconds until the circuit is broken.
Connect all four test leads and press 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 currents. The reading is shown for a short period (10 seconds).
The “auto mode” allows forward and reverse current measurements to be made (the average value is shown) by making contact with all four probes. Another test is done each time the probes are removed and reconnected to the load. This mode, similar to the continuous mode found on older instruments, is an excellent time-saving method to use when testing battery straps with hand spikes. Moreover, when hand spikes are used, this mode has the advantage that ‘contact detection sensing’ ensures that good contact is made before heavy currents are applied. This avoids arcing when contact is made, which erodes the probe tips and potentially damages the surface of the item under test.
These problems can be overcome relatively easily by making a measurement, then reversing the polarity of the test leads and making a second measurement. The required resistance value is the arithmetic average of the measurements. Some instruments, such as those in the Megger DLRO10 range of digital low resistance ohmmeters, feature automatic current reversal so that the correct result is displayed without operator intervention, even if there is a standing EMF on the circuit under test.