Support for the DLRO10HD and DLRO10HDX digital low resistance ohmmeters
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Troubleshooting
If the unit does not power up after a full battery charge, it could be due to battery and/or internal components damage. Unfortunately, you must send the instrument back to Megger or an authorised repair centre for evaluation and repair.
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.
| Temp ºC (ºF) | Resistance μΩ | % Change |
|---|---|---|
| -40 (-40) | 764.2 | -23.6 |
| 32 (0) | 921.5 | -7.8 |
| 68 (20) | 1000.0 | 0.0 |
| 104 (40) | 1078.6 | 7.9 |
| 140 (60) | 1157.2 | 15.7 |
| 176 (80) | 1235.8 | 23.6 |
| 212 (100) | 1314.3 | 31.4 |
| 221 (105) | 1334.0 | 33.4 |
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
FAQs
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.
Resistance measurements are dependent on temperature. If the original data was read at one temperature, but later tests are conducted at other temperatures, this temperature data is required to determine the suitability of the measurements. All materials do not react to temperature to the same degree. Aluminum, steel, copper, and graphite have specific temperature coefficients that will affect the degree of changes that can occur with varying temperatures at the measurement site.
Low resistance measurements rely on you conducting the tests within the operating temperature range of the instrument (you must be aware of field conditions). When you see out-of-tolerance measurements, one of the first steps is to check the instrument’s reading with a suitable calibration shunt.
The resistance of all pure metals increases with rising temperature. The proportional change in resistance for a specific material with a unit change in temperature is called the temperature coefficient of resistance for that material. Temperature coefficients are expressed as the relative increase in resistance for a one degree increase in temperature. While most materials have positive temperature coefficients (resistance increases as temperature rises), carbon graphite materials have negative temperature coefficients (resistance decreases as temperature rises).
When making a measurement on a specific material, you can calculate the change in resistance due to a change in temperature by multiplying the resistance at the reference temperature by the temperature coefficient of resistance and by the change in temperature:
- R2 - R1 = (R1 )(a)(T2 – T1 )
- R1 = resistance of the conductor at the reference temperature
- R2 = resistance of the conductor when the measurement is made
- T1 = reference temperature
- T2 = temperature at which the measurement is made
- a = temperature coefficient of resistance for the material being tested
You should also be aware of the operating and storage temperature specifications of the instrument they are using to ensure that it is suitable for the environment in which it will be used.
The relative humidity of the test specimen should only affect the resistance reading if the material is hygroscopic, in which case more moisture will be absorbed into the sample at higher humidities. This will change the measurement conditions and will affect the achieved result. However, most conductors are non-hygroscopic. Therefore, since instruments are typically designed with an operating range of 0 to 95 % RH, providing that moisture is not condensing on the instrument, a correct reading will be obtained.
Four-wire tests are the most accurate method when measuring circuits below 10 ohms, as this method eliminates errors due to lead and contact resistances. This is the test method associated with low resistance ohmmeters. Four-wire DC measurements use two current and two potential leads. The four-wire DC measurement negates the errors due to the probe lead wire and any contact resistance values in the final reading by moving the connection points of the high-impedance voltage measurement from within the instrument to the actual test piece. This results in much more accurate resistance measurements.
