When procurement personnel receive quotations for switch loop resistance testers, they often see significant differences among manufacturers in test line configurations, current specifications, and accuracy indicators. A specific question arises: why do instruments that are all nominally based on the four-terminal measurement method show scattered measurement results after being used in the field for a period of time? To answer this question, it is necessary to compare the measurement scheme and daily maintenance at two levels.
Differences between the four-terminal measurement method and the two-terminal measurement method. The two-terminal measurement method shares one pair of test lines for current output and voltage acquisition, so the measurement results include the resistance of the test lines themselves and the contact points. It is suitable for rough judgment but difficult to distinguish micro-ohm-level changes. The four-terminal measurement method separates the current loop from the voltage loop, and the voltage measurement line is connected inside the current output line, so that the voltage acquisition point avoids the voltage drop of the current line and the clamping contact resistance, and the measured value is closer to the loop resistance of the device under test itself. For contact resistance measurement of switch control equipment, the four-terminal connection is more sensitive to the connection position and clamping state, and therefore places higher requirements on the maintenance of test lines.
Two easily overlooked points in maintenance. The first is the condition of the test lines. If oxidation or looseness occurs at the alligator clips and plugs, additional contact resistance will be introduced, causing measurement deviation, and this deviation is relatively more obvious in the low-resistance range. During daily maintenance, the jaws and plugs should be checked regularly, cleaned if necessary, and confirmed to be firmly connected. The second is wiring layout. Current lines and voltage lines should be connected separately to avoid mutual interference, and the voltage measurement line should be connected inside the current output line to avoid coupling effects caused by crossing routing. The instrument allows a test duration of up to 300 min. During long-term testing, more attention should be paid to cable temperature rise and clamping stability rather than only looking at a single reading.
Verification sequence when comparing with similar schemes. When comparing different products, first confirm whether the measurement principle is the four-terminal method, then check whether the test accuracy is marked as 0.5% and whether the power supply voltage adaptation is 220V plus or minus the fluctuation range, and finally check the accompanying configuration and jaw type of the dedicated test lines. Maintenance convenience is often reflected in whether the test lines can be easily replaced individually and whether the jaws are easy to inspect, which has a relatively large impact on the long-term use cost after procurement.
Incorporating maintenance conditions into procurement verification reflects the actual use cost of the instrument better than simply comparing ranges and current levels.
