To understand why and how low resistance testers are used, we first need to look at the famous Ohm’s law on electric current. It states that for a specified source of energy that operates on V (ac) or V (dc), the amount of current that will be drawn depends on the resistance of the component’s circuit. Basically, the entire thing revolves around the resistance, which is defined as a ratio of voltage to current, and if that ratio is constant over time, it’s said that the material is “ohmic”.
This leads us to the second important point of this issue, the importance and use of resistance measuring instruments. Depending on the degree of information you need from the measurement, you can find resistance measuring instruments with 2, 3 or even 4 leads, that can cover a wide range of values from micro-ohms to mega ohms. The way these instruments operate is very simple – a low resistance ohmmeter uses two internal measuring circuits. The current is injected into the two leads, marked as C1 and C2 and the magnitude the current creates is measured. An internal calculation is then performed inside the instrument and the resistance is measured.
There are different types of low resistance testers that operate on the above explained principle. The important thing to remember is that measurement should be performed on de-energized samples so that safety is ensured.
10- A Micro Ohmmeters
This is the most commonly used microohmmeter in many different industries, as it covers the most fields. It’s considered to be the ‘work-horse’ instrument for most operators with its 10-amp output. As such, it provides a test current that is comfortable and suitable through the test sample. Also, it allows reduced weight and improved battery operation.
100+ A Micro Ohmmeters
Among the frequently used micro ohmmeters are the 100+ A type. This means that the test current should be a minimum of 100 A (amperes). Of course, most electrical utilities prefer testing at a lot higher currents, since they believe that this minimum cannot represent actual working conditions where the electric current is a lot higher. There are field portable micro-ohmmeters which range of testing is between 100 and 600 A to perfectly serve that purpose.
Transformer winding and tap changers are best measured with transformer ohmmeters. Some of these instruments include dual meters with range control from high resistance being the primary winding and low resistance being the secondary winding. These meters allow for both windings to be measured at the same time. The transformer ohmmeter is a multi-current meter with a measurement resolution to 1 micro-ohm and as such is used for performing factory tests and field perating verification.
These devices use low pulsed currents from 0.01A -1A and have ranges from 199.9 micro-ohms to 1999 ohms. The meter has pulses + and – , which allow it to eliminate any errors that may occur due to thermal emfs (emf is a very small voltage in the microvolt range). What you should remember about these devices is that they’re not made for measuring inductive circuits.