The Megger LT310-FR delivers 2-wire and 3-wire loop testing at 500 V with 2 percent accuracy under CAT III 300 V, and is stocked for European 230 V final-circuit verification work [S3]. For the same family the LT320-FR adds the same voltage and accuracy envelope at CAT III 300 V, giving electricians a two-step choice inside one model line [S1].
Combined instruments such as the 6010A fold 3-wire and 4-wire loop tests with AC, A, and DC-selective RCD tests into a single 250 V enclosure, which trims box count on a service van and reduces calibration spread across instruments [S4]. On the power-machine side, AC impedance testers are built for generator-rotor impedance and transformer short-circuit characterisation, not LV installation loops, and that duty split should drive the first fork in any loop tester selection [S2].
Test-current class: LV installation loop vs power-machine AC impedance
Low-voltage installation loop testers inject a regulated current, typically in the 6 A to 25 A range, into a de-energised or partially energised circuit so that line-to-earth and line-to-neutral path impedance can be read against the disconnection time of the upstream protective device; CAT III 300 V is the typical installation-side safety category published for the Megger LT310-FR and LT320-FR [S3][S1]. The reading is the basis for proving that an MCB, fuse, or RCD will trip within the time the IET Wiring Regulations (BS 7671) demand, which is why the test is a sign-off step rather than a commissioning optional [S6].
Power-machine AC impedance testers operate the other way around: they sweep or apply 50 Hz/60 Hz AC at controlled current into generator rotors, transformer windings, or VTs/CTs to derive impedance, power, and phase angle; the Sogou instrument record lists synchronous voltage, current, active power, and frequency capture plus over-voltage and over-current protection, and 10-year-plus data retention as built-in features [S2]. Buyers comparing 2726NA-class hardware against an installation loop tester should treat them as separate SKUs because the cable sets, current limits, and result interpretation differ [S5].
Wire count: 2-wire versus 3-wire versus 4-wire
2-wire loop tests connect line and protective-earth conductors only, which is acceptable for a quick pass/fail on simple radial circuits but is sensitive to parallel earth paths. 3-wire testing adds the neutral conductor and removes the parallel-earth ambiguity, and that is the configuration published for both the LT310-FR and LT320-FR [S3][S1]. 4-wire (Kelvin) testing separates current injection from voltage sensing, which is why it appears on the 6010A's 3-wire/4-wire list and is preferred where the prospective fault current is high or where the loop resistance is small enough that lead resistance would otherwise dominate the reading [S4].
Decision rule: 2-wire for fast screening on domestic boards, 3-wire for commercial and TN-S systems where parallel earth paths exist, 4-wire for high-current industrial boards and EV charge-point final circuits where 0.01 ohm resolution on the loop path is the difference between a pass and a nuisance-trip investigation. RS Components' category page groups these alongside loop calibrator signal sources and loop power distributor supplies, which are 4-20 mA process-loop instruments, not mains loop testers, and confusing the two classes is a common procurement error [S6].
Voltage range, accuracy and safety category
Published loop testers in the LT300 series run at 500 V test voltage with 2 percent accuracy and CAT III 300 V, which is the rating band needed to verify 230 V and 400 V installations safely without over-voltage tripping the upstream protective device mid-test [S3][S1]. The 6010A uses a 250 V test loop envelope and adds RCD AC and DC-selective trip-time testing, so its accuracy and bandwidth spec has to cover both loop ohms and RCD half-cycle trip-current ramps [S4].
Where the installation includes SPDs, RCBOs, or smart meters with embedded electronics, a tester with selectable no-trip or low-current modes is the safer pick because the standard high-current injection can both trip the device and inject enough energy to damage sensitive MOV components. The Megger family datasheets list selectable ranges for this reason, and field engineers should confirm the low-current mode is selected before testing any circuit downstream of an SPD [S3]. Buyers should also check the IP rating for outdoor distribution board work, where IP54 or better keeps the instrument usable in light rain; CAT III 300 V on its own does not imply water resistance [S1].
Combined instruments versus single-function testers
Combined testers such as the 6010A bundle loop and RCD checks, and the category page adds insulation, continuity, and earth-fault loop impedance as adjacent functions, so a single instrument can cover periodic inspection and verification of commercial installations without a second box [S4][S6]. The trade-off is weight, calibration surface area, and firmware update path: a single 6010A calibrated as one asset versus three or four separate units, each with its own certificate, is usually cheaper to maintain, but a failed loop-test section on a combined unit takes the RCD function with it.
Single-function testers like the LT310-FR stay lighter, are easier to swap on a service round, and let a contractor standardise on one model across mixed residential and light-commercial work; this is the common reason RS lists them as a family rather than one SKU [S3]. For a three-person firm running two vans, the working pattern is usually one combined instrument per van plus a dedicated 3-wire loop tester as backup, because a combined unit in for calibration should not idle a whole crew. The same logic applies to comparison-shop against deadweight tester sourcing for pressure calibration, where a single vendor with a multi-function box often beats stocking four single-function gauges [S4].
Who should not pick a 2-wire only loop tester
Industrial plants with TN-S earthing, parallel earth paths via cable armour, and EV charge-point final circuits should not specify a 2-wire-only loop tester, because parallel earth impedance will mask the true line-to-earth path and produce a falsely low reading that fails sign-off under BS 7671. A 3-wire or 4-wire instrument is the minimum on those circuits, and the 6010A's 3-wire/4-wire list with RCD AC and DC-selective support is closer to that duty than the LT310-FR's 2-wire/3-wire combination [S3][S4].
Conversely, a four-wire industrial loop tester is overkill on a domestic consumer unit, where a 2-wire pass/fail screen at 230 V plus a 3-wire verification on the worst-case radial is the entire inspection scope. Picking above the duty band burns budget on a heavier instrument and a longer calibration cycle without lifting first-pass yield, which is why RS keeps both tiers in active stock [S6]. Where the cabinet feeds a smart meter with an integrated RCM, the 6010A's RCD section also lets the inspector cross-check the meter without breaking the seal.
Standards, sourcing, and calibration
Installation-side loop testing is governed by BS 7671 IET Wiring Regulations, with IEC 60364 as the parent international standard for low-voltage electrical installations; the LT300 series datasheet is the supplier-published source of CAT III 300 V and 2 percent accuracy, and the 6010A datasheet is the supplier-published source of the 250 V loop and RCD bundle [S1][S3][S4]. RCD trip-current and trip-time requirements flow from IEC 61008/61009 for RCCBs and RCBOs, and AC-selective plus DC-selective trip modes on the 6010A are designed against those classifications [S4][S6].
On the power-machine side, AC impedance tests on synchronous generator rotors, transformer windings, and VTs/CTs sit under IEC 60076 for power transformers and the relevant GB/T specifications for the Sogou-listed rotor instrument, which is sold with over-voltage, over-current protection, and 10-year-plus onboard data retention [S2]. Sourcing on Made-in-China lists the generator-rotor AC impedance class in the US$3,175 to US$3,714 band with a 1-set MOQ, which gives a cost benchmark for the rotor-side purchase but is not directly comparable to the LV installation class [S7]. Calibration intervals of 12 months are typical for both classes, and combined instruments should carry one calibration certificate covering all functions to avoid audit findings on partial coverage [S4].
For more on how 4-wire Kelvin techniques and 0.01 ohm loop resolution play into EV charge-point commissioning, the Power Quality Analyzer Price 2026 spec-driven cost band guide covers adjacent mains-quality instrumentation. Where loop testers are deployed in mining or heavy-industrial distribution networks, the Multi-Gas Detector Selection for Mining 2026 spec map outlines the same trade-off between single-function and combined ATEX-rated handhelds. A trackable next signal is whether combined loop-and-RCD units like the 6010A migrate to IP54 enclosures as standard on the 2027 RS refresh, given the 2026 listings still publish only the basic CAT and voltage band [S4].