Support for the DLRO100 series of digital low resistance micro-ohmmeters
When the test is in process, this LED remains lit until the DLRO no longer detects voltage on the lead set. If this is lit when the instrument is not conducting a test, then it is indicating there is a fault. DO NOT USE THE INSTRUMENT if this happens. Do not attempt to repair the instrument. Please return the instrument to the Megger Repair Department for repair.
If the internal temperature of the instrument exceeds a safe level, the test will be aborted and indicated on the screen. The temperature must drop before testing can be continued.
This indicates that there is noise on the system. If possible, earth or ground the asset under test to help reduce the noise.
Interpreting test results
Measuring low resistance helps identify resistive elements that have increased above acceptable values. Resistive elements, including weld joints, electrical crimps, terminations, and current-carrying contacts, are unavoidable in constructing an electrical asset or system. These are points in an electrical circuit where it is desired that the resistance is as low as possible. Low resistance measurements are required to prevent long-term damage to existing equipment and minimise energy wasted as heat. This testing reveals impeded current flow that may prevent a machine from generating its full power or protective devices from activating in the case of a fault.
When evaluating results, it is vital first to pay attention to repeatability. A good quality low resistance ohmmeter will provide repeatable readings within its accuracy specifications. 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 result must be compensated by an appropriate temperature coefficient if the ambient temperature deviates from the standard calibration temperature.
Spot readings can be instrumental in understanding the condition of an electrical system. You should have some idea of the level of the expected measurement based on the system’s data sheet or the supplier’s nameplate. Variances can then be identified and analysed using this information as a baseline. You can also make a comparison with data collected on similar equipment.
A device’s data sheet or nameplate should include electrical data relevant to its operation. You can use the voltage, current, and power requirements to estimate the resistance of a circuit. Meanwhile, the operating specification can be used to determine the allowed change in a device (for example, with battery straps, connection resistances will change with time).
The temperature of the device has a strong influence on the expected reading. For example, the data collected on a hot motor will differ from a cold reading taken at the time of its installation. As a motor warms up, its resistance readings will go up. The resistance of copper windings responds to temperature changes based on copper’s positive temperature coefficient. 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|
R(end of test)/R(start of test) = (234.5 + T(end of test))/(234.5 + T(start of test)
In addition to comparing low resistance measurements against some pre-set standard (spot test), the results should be tracked against previous measurements and saved for future trending. 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 other components will become unsafe so you can make the necessary repairs.
Remember that degradation can be a slow process. Electrical equipment is subject to 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. Various national standards provide guidance for test cycles.
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 resistance value of circuit breakers can vary widely from below 10 micro-ohms for generator circuit breakers to below 300 micro-ohms for transmission circuit breakers. The circuit breaker manual or commissioning checklist should provide reference, or maximum, limits for micro-ohm values. If no information is available, you can also use a comparison of the phases. The value of the resistance should not vary by more than 50 % between phases.
The conductive path of a circuit breaker can have different greases that are used as lubricants throughout the connection path. These connection points can have oxidation; it is best to ‘burn in’ a reading by letting the current flow through the circuit breaker for at least 30 to 45 seconds to eliminate this oxidation. The value will often decrease during this time.
The DLRO measures the resistance of the entire path between the connection points. Therefore any variation in the placement of the clamps may yield different resistance values. It is essential to be as consistent as possible when measuring to have comparable results. Small changes in measured resistance are expected with different connections, but you should investigate more significant changes.
It’s very easy to take a battery-powered micro-ohmmeter to a test site. There’s no messing around with power supplies and long leads or dragging heavy generators into inaccessible substations. So for years, many engineers have been using battery-powered 10 A micro-ohmmeters to measure circuit breaker contact resistance because these instruments are convenient and appear to do what’s needed. But do they?High voltage circuit breakers operate at currents much higher than the 10 A produced by one of these micro-ohmmeters. So, injecting 10 A into a breaker means that it may not react in the same way as it would if it were tested with a current closer to its normal operating level.For example, the carbonised layers may resist an injection of 10 A but pass a higher test current, leading to differences in the readings. A low test current might result in the circuit breaker unnecessarily failing the test. The stability and reliability of micro-ohmmeter readings increase at higher currents as the very small voltages being measured are captured more reliably.IEC and ANSI standards recognise the value of higher test currents for testing circuit breaker contact resistance. ANSI/IEEE standard C37.09 recommends 100 A as a minimum test current. Meanwhile, IEC62271-100 stipulates that any test current between 50 A and the rated current of the circuit breaker may be used. The common practice on circuit breakers is to test at 100 A, meeting both IEEE and IEC standards.
Yes, the battery is user-replaceable. You can order Megger part number 1005-973 and follow the install instructions accompanying the battery; a battery install guide is also linked above (dlro100battery--2007-461_ug).
No, to test transformer windings, you will need a device designed to handle the inductive loads that transformers present. These test instruments will have higher compliance voltages, enabling the transformer’s core to be saturated for stable readings. The Megger TRAX and MTO are designed to test transformer windings.
You can connect the leads to each end of a conductive pipe or metal wrench and run a standard test. Move the leads closer together and perform the test again. When the distance between the leads is halved, the micro-ohms value should also be approximately half.