A micro-ohmmeter is a dedicated low resistance meter that is superior to general purpose and high end bench multimeters when measuring low resistance values. This meter is ideal for on-site testing of circuit breakers, switches and power cables. This meter can be built on a perf board for under $10 (not counting the battery). It uses standard one percent resistors.
Accuracy
A micro ohmmeter can measure very low resistance values, down to a millionth of an ohm. This measurement capability makes these instruments valuable tools for many electrical and industrial testing applications. These meters use a four-terminal Kelvin probe system to eliminate contact resistance errors and reduce the effects of lead and weld joint resistance. They can be used to test a variety of materials and connections, including bonding joints, circuit breakers, power cable connectors and weld points in busbars.
The OM 21 is fully programmable by the user directly through the instrument interface or using the software delivered with LOG OM data management system. Up to 1,000 sample measurements can be stored inside the instrument and recalled either on display or via computer.
The ULTRA PROTECTION OM 21 is a highly reliable digital Microohmmeter designed for continuous operation with an internal battery. It is the ideal tool for measuring inductive resistances (coils, transformers), contact resistances of switches and relays, metallisation and ground continuity measurements.
Range
The first type used a battery to apply a fixed voltage to the galvanometer, which was connected in series with the resistance under test. This ensured that the deflection of the scale remained correct over the full range. However this design was inherently unreliable and required a zero adjustment before each measurement (the battery would eventually run down). Furthermore, this type of ohmmeter formed a circuit by itself and could not easily be incorporated into a multimeter design.
In contrast the shunt type of ohmmeter uses electronic circuits to pass a constant current through the measurement component, and another set of circuits to measure the voltage across the measured resistance using Ohm’s law. This information is digitized and displayed on the display.
Our best-selling RMO-A micro ohmmeters feature 600 A of ripple-free DC test current and weigh only 8 kg. This allows them to be operated in very harsh environments, e.g. for contact resistance measurements on high voltage circuit breakers, disconnecting switches, knife-contact fuses and bus bars.
Display
The MPK-253 digital very low resistance ohmmeter features optimized filters and protection, making it suitable for measurements in electrical substations. It also offers rapid and accurate measurement of contact resistances in circuit breakers, switches, isolators and busbar joints with test currents up to 10A.
The microprocessor-controlled instrument employs the four-terminals measurement method, eliminating errors caused by lead and contact resistances. The results of the measured resistance are shown on an alphanumeric display with up to 4 1/2 digits-resolution.
The microprocessor-based design uses an Arduino Nano to control a MCP3424 analog-to-digital converter (ADC) and a 128×64 OLED display. Its channel 3 input runs through a six ohm 1% resistor and a 0.1 ohm 1% resistor to measure the voltage across both of these components. Resistors R2 and R3 along with zener diodes ZD1 and ZD2 protect the ADC input from potential voltage surges that may be generated when measuring motor winding resistances. Another set of resistors allows the ADC to display internal battery voltage on the OLED.
Power
In electrical substations, low contact resistance on circuit breakers can lead to arcing between contacts which can cause carbonised layers that reduce the live contact area and lead to failure. To protect against this, a standard procedure is to test contact resistance on a regular basis with a micro-ohmmeter. These are typically handheld, battery powered units that use a four-wire Kelvin measurement connection to minimize lead resistance errors and provide rock-steady readings.
Megger has a hand-held 200A digital micro-ohmmeter that is rugged and compact for on-site testing. It features a unique supercapacitor design that injects high current for a short duration and then allows a quick response time for a highly accurate result. It also has a number of options including a data logger that can store up to 1,000 readings.
Another option is a handheld, portable micro-ohmmeter from Amptec. This battery powered unit can measure a wide range of resistances with 4 1/2 digits and 0.02% basic accuracy, providing very good resolution. It also offers an 80 dB ac noise rejection to give excellent readings in noisy locations and is compatible with a selection of optional probes, clips and fixtures.
More Words
A microohmmeter, also known as a digital low resistance ohmmeter (DLRO), measures very low resistance with high accuracy at specified test currents. It can measure circuit breaker contacts and switches, fuses, wires and cables, busbar joints, and transformer windings. Please note that this kit requires a battery to conduct resistance measurements. Never use an alkaline cell, as it can damage the Nano’s five volt regulator.
