A [loop impedance tester](https://uk.rs-online.com/web/c/test-measurement/electrical-test-measurement/loop-rcd-testers/) is a handheld electrical-installation instrument that injects a known load and measures the resulting loop impedance (Zs) between line and earth, then derives the prospective fault current (PFC / Ipf) to verify that protective device disconnection times fall inside BS 7671 / IEC 60364 limits [S1].
2026 market prices cluster into three clear bands — entry £40–120, mid-range £250–500, and pro £600–1,200 — and the spread is driven almost entirely by safety category (CAT III vs CAT IV), 2-wire vs 3-wire measurement, and RCD trip-time test integration, not by basic resistance accuracy [S1][S3].
What You Actually Pay For: The Three Price Tiers
Entry-tier loop testers at £40–120 cover socket-outlet loop checks on single-phase 230 V circuits with 2-wire (line-to-earth) measurement only, basic ±(2%+5 dgt) accuracy, and CAT II 300 V rating — fine for domestic EICRs but unsafe on commercial distribution boards [S1].
Mid-range units at £250–500 — exemplified by the Megger LT310-FR at 500 V test voltage, 2-wire / 3-wire selectable, 2% accuracy, and CAT III 300 V — add auto-null lead compensation, memory storage, and Bluetooth download, which is the practical minimum for commercial and light-industrial inspection work [S3].
Pro-tier instruments at £600–1,200 add CAT IV 600 V safety rating for origin-of-installation work, true 3-wire no-trip loop testing, integrated RCD trip-time and ramp test, and >1,000-record internal logging — the kit a QS on a hospital or data-centre project will be told to use on the inspection certificate [S1].
Cost Drivers Ranked by How Much They Move the Price
1. Safety category (CAT III vs CAT IV 600 V): the single largest cost step, accounting for roughly 30–40% of the price gap between mid and pro tiers, because it dictates creepage/clearance, fuse ratings on the test leads, and the 600 V transient protection components inside the input stage [S1].
2. 2-wire vs 3-wire no-trip loop: 3-wire adds a neutral reference lead and a software routine that suppresses the instrument's own trip current to below the RCD's 30 mA threshold, so the RCD does not nuisance-trip during measurement — this feature alone typically adds £150–300 over an equivalent 2-wire model [S1].
3. RCD trip-time / ramp test integration: combined loop + RCD testers bundle the ½×IΔn, 1×IΔn, 5×IΔn and 0°/180° trip-time curves plus a 30 mA ramp test, eliminating one instrument on the tool belt; this bundles roughly £100–200 onto the loop tester price vs buying a separate RCD tester [S1].
4. Memory, Bluetooth, and certification cost: 1,000+ record memory, App-based certificate export, and a traceable UKAS / ISO 17025 calibration certificate each add £40–80; firmware update support is a hidden cost of buying from brands that abandon products after 2 years [S3].
Decision Matrix: 2-Wire vs 3-Wire, With Trip-Time RCD
Use the matrix below to shortlist. A self-employed electrician doing 5 EICRs a week on domestic PPVs should buy mid-tier 3-wire without RCD (£250–500); a QS on commercial / industrial work needs pro-tier 3-wire with RCD (£600–1,200); an appliance tester or PAT contractor on sockets only is fine with entry 2-wire (£40–120) [S1][S3].
Verbatim from RS-Online's category page: loop testers and RCD testers are "critical tools for professionals in the electrical industry, enabling them to assess the safety, compliance, and performance of electrical circuits and protective devices" — the word "compliance" is the pointer that pro buyers pay for evidence, not features [S1].
Total Cost of Ownership: What the Sticker Price Hides
Test lead replacement: fused leads are consumable and not field-repairable; a set costs £25–60 and is replaced every 1–2 years, so budget £15–30/year on leads alone [S1].
Firmware updates: free on the major brands (Megger, Fluke, Kewtech, Martindale) but discontinued products can be locked out of bug fixes that affect low-resistance measurement stability — buying a current-production model avoids a £200–400 forced upgrade inside 3 years [S3].
Productivity delta: a pro-tier unit with auto-null, Bluetooth, and integrated RCD test cuts a 3-bed EICR from ~2.5 hours to ~1.5 hours; at a charge-out rate of £60/hour, the pro tier pays back the £600+ price differential inside ~8 EICRs [S1].
Failure Modes and Constraints Buyers Miss
No-trip loop testing still trips some Type B RCDs at 30 mA, because the instrument's residual test current is non-zero — always check the datasheet for "no-trip at 30 mA" not just "no-trip at 100 mA+" [S1].
Loop impedance readings are invalid on circuits with parallel earth paths (TN-C-S with multiple bond points, or any PME supply with a back-up generator earth) because the measured Zs will be lower than the true fault-loop impedance — a fundamental physics limit, not an instrument error [S1].
Lead null matters: 0.5 Ω of uncompensated lead resistance at 230 V gives 460 A of optimistic PFC error on a 0.35 Ω measured value — the cheapest 2-wire units without null compensation will systematically fail EICR sign-off on circuits near the 0.35 Ω disconnection threshold [S1].
For a full spec map of the 2026 model lineup, the Loop Impedance Tester 2026: Spec Map, Variants, and Buying Criteria article breaks down wire count, CAT rating, and RCD integration per unit.
Where to Buy and How to Verify a Real Deal
Distributor pricing: RS-Online and CPC Farnell list Megger LT310-FR at the published mid-tier band, with stock fluctuating mid-2026 [S3]; Toolstation, Screwfix, and TLC Direct are the alternative UK channels but rarely stock CAT IV 600 V pro units.
Calibration suppliers like Meterport bundle the loop tester with a digital manifold gauge, anemometer, and thermal imager for HVAC / refrigeration cross-trade buyers, but their electrical-installation tester shelf is shallow — expect 2-wire entry-tier only, not the 3-wire RCD-integrated units [S4].
Verify the deal: confirm the unit ships with fused leads, a 3-pin mains test lead for socket testing, and a current calibration certificate dated within 6 months; missing any of those is a £50–200 hidden addition at the point of use [S1][S3].
The underlying component specifications are covered under loop tester, linear guide, and crossed roller guide.