Ohmmeters

Voltage measurement up to 750V and up to 250V with resolution 0,1V ; Measurements using Uni-Schuko adapter or test wires 1,2; 5; 10 and 20m ; Loop impedance measurement with resolution 0,01Ω and much more

Loop Impedance Meter AMZC-306

Loop impedance measurement with resolution 0,01Ω ; Loop impedance measurement without tripping the circuit breaker > 30mA with resolution 0,01Ω (100...440V) ; Measurements possible in installations 110/190V, 115/200V, 127/220V, 220/380V, 230/400V, 240/415V, 290/500V and 400/690V (range 100...750V)

Loop Impedance Meter AMZC-305

Short circuit loop impedance measurement in networks with nominal voltage: 220/380V, 230/400V, 240/415V, with frequency 45…65Hz ; Short circuit loop impedance measurement with 15mA current, without triggering RCD. ; Detection of the replacement L and N in the socket and automatic swap in the meter

Loop Impedance Meter AMZC-304

The PDRM-1A ohmmeter is very accurate for measuring low resistances. it can measure values ranging from 1µΩ to 200Ω. For this reason, it allows for performing variety of measurements with a single equipment.

Ohmmeter PDRM-1A

With a range from 0.01µΩ to 200Ω, it is currently the most accurate micro-ohmmeter and has the highest resolution on the market, weighing only 860 g, which also makes it the most lightweight and compact. All these have made it the standard low resistance ohmmeter in many electrical utilities, industries and assembly companies. It has a resolution of 0.01µΩ.

Ohmmeters and meters PDRM-10A

The AMZC-310S is a professional portable meter for testing electrical installations with overcurrent protection. The equipment measures L-PE, L-N and L-L and the expected short-circuit loop current. The measurement can be performed with the 2-pole method and under test current (up to 42A) or with the 4-pole method and high test current (up to 280A), which allows measurements with high accuracy and resolution. The results can be stored in internal memory and sent to a computer via the serial interface.

High Current Loop Impedance Meter AMZC-310S

Principles of the ohmmeter. Measurement of four-wire resistance (Kelvin Method)

Suppose we want to measure the resistance of a component located a significant distance from the ohmmeter. This is a complicated situation because the ohmmeter / Ohmmeter measure all resistance in the circuit, which includes the resistance of the cables (Rwire) connection and the resistance object (Rsubject):

ohmmeter fig.1

Normally the wire resistance is very low (only a few ohms per hundreds of feet of cable, depending on the section of cable) but if the cables are too long or (Rsubject) have a low value, the error introduce the cables will be substantial.

An ingenious method of measurement of resistance value in cases such as the former involves the use of both a voltmeter and an ammeter. We know from Ohm's Law that resistance is the ratio between voltage and current (R=V/I). Thus we should be able to determine the resistance if we measure the current through and voltage drop:

Ohmmeters fig. 2

The current is the same at all points of the circuit since all elements are in series. Since we are only measuring the voltage drop in the measured object (and not the resistance of the cables) the calculated resistance is indicative of the actual value of the resistance (Rsubject). Our goal, however, is to measure the resistance at a distance, so our voltmeter must be somehow located near the ammeter, it is connected to (Rsubject) via cables which have a resistance.

Ohmmeters fig. 3

Apparently we have introduced a systematic error because now the voltmeter should measure a voltage drop across a long wire resistive torque, which introduces an external resistor in the circuit. However, if we make a detailed study we will not lose any precision at all, this is because the current through the voltmeter has a negligible value. The voltage drop in the wires of the voltmeter is negligible, an indication of the voltmeter being practically the same as if it were directly connected to (Rsubject).

Ohmmeters fig. 4

Any voltage drop in power cables will not be measured by the voltmeter. The measurement accuracy can be improved if the current of the voltmeter is minimized. This measurement method avoids the systematic error would introduce the lead resistance method is called the four-wire or Kelvin. There are special alligator clips (called Kelvin clips) to facilitate this kind of connection resistance.

Kelvin method
Ohmmeters fig. 5 Ohmmeters fig. 6
[Kelvin Clips] [Metrology]

The same principle of using different points of contact for current and voltage to drive current and measuring the voltage drop is used to measure high currents, in this case is calibrated resistance (shunt). As mentioned above, the shunt resistor works as a measure of current to reduce a certain amount of voltage per ampere of current passing through it, the voltage drop is measured by a voltmeter then. In this way calibrated shunt resistor "converts" a current value in a voltage proportional. Hence the current can be accurately measured by measuring the voltage drop in the shunt:

Ohmmeters fig. 7

The measurement of current through a shunt resistance is particularly suitable for measuring very high currents. For such applications, the shunt resistor must be of the order of milliohms or micro ohms, thus the voltage drop will be low relative to the current value. As low resistance values ​​are comparable to the resistance of the connections to the power cables, this means that the measure of the potential drop should avoid measuring the voltage drop in connections with the resistance wires. To ensure that the voltmeter measure only the voltage drop across the shunt itself, uninfluenced by the potential drops due to connections, shunts are usually equipped with four connections:

Ohmmeters fig. 8