By hvhipot 
1、 Measurement principle of clamp resistance meter, grounding resistance tester
Clamp on resistance meter is a novel measuring tool that is convenient, fast, and has a shape similar to a clamp on ammeter. During testing, no auxiliary testing pile is required, only a clamp on the measured ground wire is needed, and the measurement result can be obtained in a few seconds, greatly facilitating the work of resistance measurement. Another major advantage of a clamp type ground resistance meter is that it can measure the ground resistance of in use equipment online without cutting off the equipment power or disconnecting the ground wire.
Rx is the ground resistance of the measured ground pole, and R1, R2… Rn are the ground resistances of other grounding points in the distributed grounding system. This figure can be further equivalent to Figure 3. When measuring, the clamp type ground resistance meter uses the principle of electromagnetic induction to send a constant voltage E to the tested cable through the loop formed by its front end slot (with an electromagnetic coil inside). This voltage is applied to the circuit shown in Figure 3. The ground resistance meter can simultaneously measure the current I in the circuit through its front end slot. Based on E and I, the total resistance in the circuit can be calculated, that is: E/I=Rx+1/(1/R1+1/R2+…+1/Rn)
1/(1/R1+1/R2+…+1/Rn) is the total resistance of R1, R2… Rn connected in parallel. In a distributed multi-point grounding system, Rx is usually greater than 1/(1/R1+1/R2+…+1/Rn), where “>>” means “much greater than”. Assuming the above conditions are met, the measured ground resistance Rx=E/I.
In fact, the clamp type ground resistance meter sends a 1.7kHz AC constant voltage to the cable through its front-end snap ring, a special electromagnetic converter. In the current detection circuit, after filtering, amplification, and A/D conversion, only the current generated by the 1.7kHz voltage is detected. Because of this, the clamp type ground resistance meter eliminates the small current on the ground caused by commercial AC power and high-frequency noise generated by the equipment itself, in order to obtain accurate measurement results. It is also because of this that the clamp type ground resistance meter has the advantage of online measurement. In fact, the meter measures the impedance of the entire circuit, not the resistance, but in normal circumstances they differ very little. The clamp type resistance meter can immediately display the result on the LCD screen. When the card slot is not properly engaged, it can display “openjaw” or similar symbols on the LCD screen.
Due to the special structure of the clamp resistance meter, it can be easily used as an ammeter, and many of these instruments also have the function of a clamp ammeter. On the other hand, although a certain frequency signal is used to eliminate interference during the clamp resistance meter test, the measurement will also be affected by interference in the presence of a large current on the tested cable, resulting in inaccurate results. So, according to the requirements, the current on the cable should be measured first before use, and the ground resistance can only be further measured when the current is not very high. Some instruments automatically perform noise interference detection when measuring ground resistance, and give a prompt when the interference is too large to perform the measurement.
2、 Precautions for measuring clamp resistance meter
From the above introduction, it can be seen that the measurement principles of clamp type and hand cranked resistance meters are completely different. When using a hand cranked ground resistance meter, the grounding stake should be disconnected from the equipment to avoid the equipment’s own grounding body affecting the accuracy of the measurement. Hand cranked ground resistance meters can achieve high accuracy, regardless of whether it is a single point grounding or multi-point grounding system; The most ideal application for clamp type grounding meters is in distributed multi-point grounding systems. At this time, the grounding piles used in the grounding system should be measured sequentially, and the measurement results should be recorded and compared. For grounding piles with significantly larger measurement results than other points, special attention should be paid to inspection. If necessary, the grounding pile should be disconnected from the equipment and retested with a hand cranked grounding meter to expose poor grounding piles.
In a single point grounding system, the clamp type ground resistance meter should be used with caution. From its working principle, it can be seen that the resistance value measured by the clamp type ground resistance meter is the total resistance in the circuit. Only when Rx>>1/(1/R1+1/R2+…+1/Rn), this resistance value is close to the grounding pile resistance we want to measure. However, this condition is often not met, especially in single point grounding systems. For open circuit grounding piles that have been buried but not yet connected to the equipment, their ground resistance cannot be measured using this instrument.
HV Hipot Electric Co., Ltd. specializes in producing ETCR2000 clamp type grounding resistance tester and ET3000 dual clamp multifunctional clamp type grounding resistance tester, which are suitable for various shapes of grounding leads. Non contact grounding resistance is fast and safe, with double protection insulation, strong anti-interference ability, built-in testing self checking function, and high measurement accuracy.