Low Resistance Testing: The Whats and Hows of Micro-Ohm Meters

Volts, amps, ohms, and watts. Not exactly everyday topics of discussion, unless you’re an electrician. A short refresher course tells us that volts measure the electrical potential difference between two points or voltage. Consider it the ‘pressure’ in a circuit, that pushes electrons to where they’re needed. For instance, the pressure that runs your lights. Amps or amperes measure electrical current, or put plainly, the strength of a current of electricity between the two points we mentioned. Ohms measure electrical resistance or the opposition to the flow of current in a circuit. And this can happen because of dozens of reasons, and some are potentially fatal. Lastly, watts can be seen as ‘power’ or more precisely, volts times amps.  

Electricians measure volts with a voltmeter, amps with an ammeter, ohms with an ohmmeter, and watts with a wattmeter. Some instruments can measure all units, of which a simple multimeter is a fine example and a useful tool around the house.  But where particularly precise measurements are needed, for instance in measuring resistance levels in circuit breakers, motors and transformers, cabling, and in ground connections, other test devices are used. These ensure that there are no faulty components in the tested equipment or machines and that they won’t suffer from power loss or cause dangerous situations for operators. One such piece of testing equipment used here is the reliable Micro Ohm Meter.  

What is Electrical Resistance and Why It’s Measured? 

Resistance is an electrical quantity that measures how a device or material reduces the current flow through it. During resistance testing, measurements are compared to preset values for the devices and materials to establish possible deviation. Even slight differences, measured in micro-ohms can be a sign of degradation. This can occur from factors that act on the measured samples, for instance, exposure to higher temperatures or corrosion for a specified period. Metal fatigue and vibrations are also factors that increase the likelihood of degradation.  

Such cases need to be discovered early on to avoid complete equipment or material failure. Hence, regular maintenance with the use of a micro–ohm meter is required across dozens of industries and settings. This reduces the probability of power loss between connections that can lead to unnecessary downtime or more extreme cases where due to excessive heat conductors and connections have completely melted away.  

Where is Low Resistance Testing Used? 

Testing for low resistance levels is regularly done by manufacturers of electrical components and cabling. Resistors, inductors, and chokes, for instance, have to meet specified resistance levels. Contact resistance is tested in switches, relays and connectors to verify that it is below a defined limit. Cables and conductors are tested for current flow relative to resistance and thickness. Motor windings and transformers are tested both when manufactured and, in the field, to assess performance and changes due to external factors. Industrial batteries and Uninterruptible Power Supplies (UPS) are tested for vibration, corrosion, or heat damage, with higher-than-normal resistance measurements between terminals. Fuses and circuit breakers are also tested for corrupted connectors and their ability to break a circuit in power surges and shorts. In the automotive industry, robot spot welders are tested for vibration damage due to continual flexing and the ability to produce consistent welds. The list goes on.

 In each case, a certified electrician will be called up with an appropriate low-resistance tester.  

Industries, where routine resistance testing is carried out, include power generation and distribution; automotive and aircraft manufacturers; mines, refiners, and chemical plants working with high loads; telecom companies, railways, and where UPS devices are a common occurrence. General low resistance testing is carried out on coils, windings, motors, circuit breakers, fuses, bus bar connections, ground bonds, switches, weld joints, lightning conductors, small transformers, and resistive components.  

How is a Low Resistance Test Done? 

Resistance values vary across different materials and electrical equipment. Readings can cover a range from a few micro-ohms to several thousand mega-ohms. Two-wire measurements with multimeters are used to get a general assessment of a circuit and are commonly used for readings between 10 Ohms and up to 1 to 10 megaohms. Three-wire measurements are often associated with insulation testing and high voltages. But where values below 10 Ohms (or less), four-wire measurements (also called the Kelvin Method) conducted with a dedicated micro–ohm meter rules out errors in readings due to lead and contact resistance, as happens in 2 and 3-wire testing. 

In the four-wire or Kelvin method, two current wires carry the current in the test object, and two potential wires or leads that measure the voltage drop. To get precise results, a digital micro-ohm meter (or ductor tester/Digital Low Resistance Ohmmeter), generates a fair bit of current. Even smaller lightweight units start with 2A, whereas heavy-duty industrial meters will produce 600A to get extremely precise resistance deviations of down to 1 micro-ohm, or a millionth of an Ohm. (Resistance is calculated using Ohm’s Law, whereby resistance is Voltage divided by Amperage)

Measurements are used to determine resistance changes and avoid any potential equipment failures. Most micro-ohm meters also feature continuity testing to ascertain whether there are breaks in the circuit. The feature list on all units is rich, with simplified one-touch testing, selectable amperage modes, digital noise detection, rechargeable NiMH batteries (and optional mains supply), included Kelvin leads, and more. Results are displayed on legible LCD screens that work in all environments and the tester is packaged in a protective carry case.  

These are specialised testing instruments out of reach for most residential uses because of the steep price, but necessary pieces of kit in any pro’s toolbox.