DLRO100 series of digital low resistance micro-ohmmeters
Small, lightweight, and portable
Weighing only 7.9 kg, the battery-operated DLRO100 series can be used in almost any location
Advanced design for safe operation
Rated at CAT IV 600 V, and IP54 (lid open) for protection against dust and moisture whilst testing, the DualGround™ option for testing circuit breakers adds additional safety and convenience
High noise immunity
For stable readings in electrically noisy environments, which aids in comparing or trending results over time. From testing in industrial applications all the way to 765 kV substations, the DLRO100 will provide accurate consistent results day in and day out
Smooth DC output for circuit breaker testing
Reduce circuit breakers unexpectedly tripping during low resistance measurements with negligible ripple in the DC output
About the product
The DLRO100 series of digital low resistance micro-ohmmeters are portable, lightweight instruments that can deliver a test current of 100 A. They can be supplied with internal rechargeable Li-ion batteries that provide sufficient power for up to 200 manual/automatic tests on a single charge. This degree of autonomy makes it easy for you to perform high current low resistance testing in almost any location, including areas without access to a mains power supply.
To ensure dependable operation even in the most demanding environments, the DLRO100 instruments use novel circuitry that gives high noise immunity and guarantees stable readings. For physical protection, they feature enclosures with an IP54 ingress protection rating even when the lid is open and testing is in progress.
Operator safety in adverse conditions is assured by a CAT IV 600 V safety rating in line with IEC 61010. With an optional DC clamp, dual ground operation is also supported. DualGround™ greatly enhances safety when working in substations and similar environments by allowing tests to be carried out with both sides of the equipment under test grounded.
DLRO100 instruments have a wide range of applications, including checking the resistance of busbar and cable joints, measuring wire and cable resistance, and verifying lightning conductor bonding. They are also well suited for testing switchgear and circuit breakers during manufacture and in the field, and they offer a smooth DC output that is particularly valuable for circuit breaker tests.
Instruments in the DLRO100 series have a measuring range of 0.1 µΩ to 1.999 Ω with a resolution of 0.1 µΩ. Results are shown on a large LCD panel and, depending on the model, can also be stored in a large capacity internal memory for later access via the display or downloading via a USB drive. Versions are also available with support for remote operation, Bluetooth connectivity, and asset/result tagging.
There are three main models in the series, each have battery and mains power supply:
- DLRO100EB is a battery and mains operated unit.
- DLRO100XB is a battery and mains operated unit. DualGround capability with optimal clamp and internal memory storage with USB drive download.
- DLRO100HB is a battery and mains operated unit. DualGround capability with optimal clamp and internal memory storage with USB drive download. Remote control, asset tagging and Bluetooth®.
- Data storage and communication
- Data storage and communication
- Max output current (DC)
- 110 A
- Output type
- Smooth DC
- Power source
- Power source
- Safety features
- Safety features
- LED indicators
Product documentsAdditional documentation can be found on the support tab
FAQ / Frequently Asked Questions
The need for accurate low resistance measurement is very diverse. Typical applications include:Utilities and service contractors - commissioning new installations and regular maintenance testing of:
- Switchgear and circuit breaker contact resistance
- Busbar and cable joints
- Wire and cable resistance
- Lightning conductor bonding
Original equipment manufacturer - detecting manufacturing defects
- Production test of circuit breakers - contacts and joints
- Production test of high voltage switchgear
- Production test of welded joints and connections
Transport manufacturing and maintenance
- Railway rolling stock ground connections and manufactured joints
- Railway lines and conductor rails - commissioning and maintenance
- Aircraft frame bonds and static control
For the required values in the tens to hundreds of micro-ohms, a general-purpose ohmmeter will not give satisfactory results, even if it can nominally measure the low values of resistance involved. A kelvin type, four-wire measurement is required to calculate the low contact resistance accurately. It’s essential for the instrument to deliver a high test current; IEC requires 50 A or greater, and IEEE requires 100 A or more. A smooth DC output, including ramp up and ramp down of the current, will significantly reduce the potential of an accidental relay trip while testing. Ideally, the instrument should be suitable for use with DualGroundTM test techniques for added safety.
Megger has a wide variety of micro-ohm test equipment. The general practice for standard circuit breaker applications is to test at 100 A. By designing the unit around this current, the DLRO100 can be compact and lightweight while providing the necessary current to meet the standards. If more current is needed, Megger offers 200 A and 600 A units in the DLRO and MOM series. If you need an even smaller test instrument with test probes, Megger offers several 10 A versions of the DLRO to meet all your testing needs.
All DLRO models offer a choice of test modes. Manual mode allows you to initiate the test once the probes are in contact with the object under test. When you press the test button, the instrument performs a single test. In “Auto” mode, as soon as you connect the potential leads, the test starts automatically. To repeat a test, you must break and remake contact with the potential leads. In “Continuous” mode, you connect the test leads and press the test button. The instrument will continue to test, updating the display after each new test cycle until you press the test button again. With the simple keypad and rotary switches, you can operate all the DLRO100s with work gloves on.
If the DLRO100 has the battery option, it can still be operated off of mains power if the battery is dead or low on charge. Note: the charge time is only 2.5 hours if completely dead. So, unless you need to test all day continuously, the DLRO100 can be charged while setting up to provide enough battery to complete your work.
During its design, we had circuit breaker testing directly in mind. Therefore, the DLRO100 provides an authentic, smooth DC output with ramp-up and ramp-down current at the beginning and end of the test to minimise unwanted tripping on parallel equipment that is still in service.
You can order the DLRO100 with 5, 10, or 15 metre lead lengths that are CAT IV 600 V rated. When other lengths or clamps are required, the DLRO100 has a terminal adapter plug in which a spade-type connector can be attached for custom current cables. The voltage sensing leads use standard banana jacks.
The DLRO100 is equipped with a user-replaceable, high-power Lithium-ion battery that is less than 100 Wh. This battery meets federal aviation standards for travel, so you can carry the unit on the plane with you if travelling.
Further reading and webinars
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)
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
FAQ / Frequently Asked Questions
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.
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.
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.
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.