Loop impedance tester

Data records

• At the finest resolution offered by two-wire loop testers the contact resistance of the probe tips, which changes with the pressure the operator applies, becomes a noticeable part of a very low loop impedance; four-wire voltage sensing applies the load current through one pair of connections and senses the voltage through a separate pair, so that the probe contact resistance is kept out of the result. Source: Megger Electrical Tester article, 2018-07-13 edition. Verified 2026-10-27. Record instr.loop-impedance-tester.four-wire-and-resolution
• In the two-wire high-current method the tester connects a sizeable load between the line and protective conductors, or line to line or line to neutral, and derives the impedance from the voltage drop; it gives the most stable readings but cannot be used on a circuit protected by a residual current device. Source: Megger Electrical Tester article, 2018-07-13 edition. Verified 2026-10-27. Record instr.loop-impedance-tester.high-current-method
• Instruments of this class measure both line impedance (line to neutral or line to line) and loop impedance (line to protective conductor) together with the supply voltage and frequency, and some evaluate the result against built-in protective-device tables to show a pass or fail. Source: Metrel product page, web page edition. Verified 2026-10-27. Record instr.loop-impedance-tester.line-and-loop
• A loop impedance tester measures the earth fault loop impedance of a live circuit: the impedance of the path from the supply source through the line conductor, the fault and the protective conductor back to the source. Source: Megger Electrical Tester article, 2018-07-13 edition. Verified 2026-10-27. Record instr.loop-impedance-tester.measurand
• No test technique can guarantee that a residual current device will not trip during a low-current loop test, so the consequences of an unexpected trip, such as loss of power to IT equipment, are weighed before the test is made. Source: Megger Electrical Tester article, 2018-07-13 edition. Verified 2026-10-27. Record instr.loop-impedance-tester.no-trip-caveat
• The three-wire low-current (no-trip) method keeps the current through the protective conductor below the tripping level of a residual current device, measuring the line and neutral impedances at high current and only the protective-conductor part at low current; the instrument combines the stages and displays the line-to-protective-conductor value. Source: Megger Electrical Tester article, 2018-07-13 edition. Verified 2026-10-27. Record instr.loop-impedance-tester.no-trip-method
• From the measured loop impedance and the measured line voltage the instrument calculates the prospective fault current, the current that would flow in a short circuit at the test point, which is compared with the breaking capacity and the operating characteristic of the protective device. Source: Megger Electrical Tester article, 2018-07-13 edition. Verified 2026-10-27. Record instr.loop-impedance-tester.prospective-fault-current
• During a no-trip test the low test current can partly saturate the magnetic core of a residual current device, adding an impedance that is not part of the circuit; this RCD uplift inflates the reading and lowers the calculated fault current, and some instruments apply a measuring technique that removes it from the displayed value. Source: Megger knowledge hub article, 2026-02-03 edition. Verified 2026-10-27. Record instr.loop-impedance-tester.rcd-uplift