The Micro-Ohmmeter is used to measure extremely low resistances with high accuracy at specified test currents and is used for contact and bonding applications. The Micro-Ohmmeter is also referred to as a DLRO, which is an acronym widely used in industry for "Digital Low Resistance Ohmmeter" and is registered trademark by Megger Corporation.
What are Kelvin Probes and why are they important for measuring Low Resistance?
Micro-Ohmmeters and Milliohmmeters are commonly used with a 4-wire measurement system referred to as the Kelvin System.
A Kelvin System allows low resistance measurements to be taken without the influence of the measuring probes, leads, contact points, and wiring. This is a way of measuring continuity and bonding resistances ensuring all contact and lead resistances are compensated, which allows a much greater accuracy in final measurements. Four wire measurements using Kelvin Probes reduces the voltage drop in test leads that cause measurement inaccuracies, making them excellent for resistance measurements below 1 ohm.
Typical Applications for Micro-Ohmmeters
- Bonding Resistance Verification on the Grounding System
- Circuit Breaker contacts
- Bus-Bar Joints and Connections
- High-Current Links
- Switchgear Contact Resistance
- Electrical Substations
- Air Frame Bonding
- Rail Bonding
- Pipeline Bonding
- Cable Resistance
- Electric Motors
- Transformer Windings
- Printed Circuit Boards
- Battery Strap Resistance Checks
- Residential Electrical Integrity
Since an increase in contact resistance may lead to power loss and an increase in temperature, many problems in equipment or in the system may occur. The most common uses would be to measure the contact resistance of electrical connections either at the breaker, incoming service, or at other bus-bar or high voltage connections. It is suggested to measure contact resistance (bonding) at regular intervals.
Here is an example of the difference in power loss in the same circuit with different resistance values. At 10K amps with a contact resistance of 0.1 milli-ohm, the power loss would be 10 KW. If the contact resistance for the same connection was 1 milli-ohm, the power loss becomes 100 KW.
Features to Consider When Selecting Milli-ohm Meters
- Maximum Test Current
- Resistance Resolution
- Temperature Compensation
- Resistive or Inductive Measured System
- Display: Analog, Digital, Bar-Graph, Graphic
- Interface Capability
- Memory and Storage Capacity
- Power Supply
- Electrical Safety Category Rating
- Kelvin Probes Supplied and Current Rating
- Enclosure IP Rating
- Cable joint and bus bar connection checks
- Choice of Metal Characteristics
Testing of electrical connections and bonding may be one of the easiest tests performed by technicians. Performed on a regular basis, the safety of the installation connections may be confirmed, thus eliminating some possible problems and dangers.
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