What Is a Multifunction Tester (MFT)? Functions, Standards, Cost

A multifunction tester, or MFT, is the hand-held instrument that carries out the dead and live tests BS 7671 requires before an EIC, Minor Works Certificate or EICR can be signed. One unit built to BS EN 61557-10 combines low-resistance continuity, insulation resistance, earth fault loop impedance, prospective fault current and RCD testing, and most add earth electrode resistance and phase sequence. Every figure on a schedule of test results comes out of it, which is why IET Guidance Note 3 expects the instrument to be kept accurate and the scheme assessor will ask how you prove it. Prices checked on 24 October 2026 ran from £663 to £1,319 ex VAT for current UK models.

What does a multifunction tester actually measure?

Before MFTs existed an electrician carried a low-ohm continuity meter, a hand-cranked insulation tester, a loop impedance tester and an RCD tester, each with its own leads and its own battery habit. The MFT brings those together behind one rotary switch or touchscreen, with a display that shows the measured value and, on most current models, the relevant limit beside it.

The functions are not arbitrary. Each one corresponds to a test that BS 7671:2018+A4:2026 Chapter 64 (initial verification) or Chapter 65 (periodic inspection) calls for, and each is defined by a part of BS EN 61557, the product standard for test equipment used on low-voltage distribution systems up to 1,000 V a.c. and 1,500 V d.c. A combined instrument has to meet every part that applies to it, and Part 10 covers the combination itself.

BS EN 61557 also sets the performance envelope. For most functions it permits an overall operating uncertainty of up to 30% across the stated measuring range, which is one reason GN3 tells you to compare Zs readings against 80% of the Table 41 maximum, or to correct for conductor temperature under Regulation 643.7.3, rather than against the raw tabulated value. An instrument reading 0.01 Ω on its display is not accurate to 0.01 Ω unless the specification says so for that range; the Megger MFT-X1's high-resolution loop mode (0.001 Ω) exists precisely because ordinary two-wire readings near the origin of a supply are too coarse to be useful.

Which BS 7671 tests does an MFT perform, and how?

Continuity to BS EN 61557-4

A continuity tester for installation work must deliver at least 200 mA with an open-circuit voltage between 4 V and 24 V. That current is what separates an MFT from a multimeter: a 200 mA test will show up a loose terminal or a corroded crimp that a multimeter's microamp test current sails through. On a ring final circuit the end-to-end r1, rn and r2 values are taken first, then the cross-connected test gives R1+R2 at every socket; the method is set out in Ring Final Circuit Testing: r1, rn, r2 and R1+R2 Step by Step and the radial version in Continuity Testing: R1+R2 Method, Wander Lead and Values. Null the leads before you start. A forgotten null adds 0.2 to 0.5 Ω to every reading, which is enough to turn a healthy 0.4 Ω R1+R2 into something that looks like a long run of 1.0 mm².

Insulation resistance to BS EN 61557-2

The insulation function must be able to supply the rated test voltage into a load that draws 1 mA, so a 500 V instrument has to hold 500 V into 0.5 MΩ. BS 7671 Table 64 sets the test voltages and minimum values: 250 V d.c. and 0.5 MΩ for SELV and PELV circuits, 500 V d.c. and 1 MΩ for circuits up to and including 500 V, 1,000 V d.c. and 1 MΩ above that. Regulation 643.3.1 lets you join line and neutral and test to earth where there is a risk to electronics, and 643.3.2 and 643.3.3 cover SPDs and connected equipment. GN3's practical trigger is lower than the regulation: anything under 2 MΩ should be investigated, because ten circuits at 20 MΩ each read 2 MΩ at the board, and a single poor circuit hides inside a good aggregate. The full method, including what to disconnect and in what order, is in Insulation Resistance Testing: Voltages, Minimums and Method.

Loop impedance and prospective fault current to BS EN 61557-3

Loop testing is done live. Ze is taken at the origin with the main earthing conductor disconnected and the installation isolated; Zs is taken at the furthest point of each circuit and compared with the corrected maximum from Tables 41.2 to 41.5. Modern instruments give two or three ways of getting the number. A high-current two-wire test passes several amps for a few cycles and gives the steadiest reading, but will trip any upstream RCD. A non-trip test uses a lower current and clever timing to stay under the RCD's tripping threshold; it is what you use on every RCBO-protected final circuit, at the cost of a little more scatter. The MFT-X1 offers Mode 1 non-trip at 0.01 Ω resolution, Mode 2 high current at 0.01 Ω and Mode 3 high resolution at 0.001 Ω. Prospective fault current is derived from the same measurement: the instrument measures L-PE and L-N loop and divides the supply voltage by each, reporting the higher as Ipf. Near a transformer, where loop values sit in the milliohm range, hand-held instruments struggle to repeat and the Ohm's-law calculation ignores reactance, so treat very high kA figures as indicative. Limits and the 80% rule are in Maximum Zs Values BS 7671: Tables 41.2 to 41.5 and the 80% Rule and the practical method in Earth Fault Loop Impedance Testing: Ze, Zs and Ipf Explained.

RCD testing to BS EN 61557-6

This is where the old textbook procedure has changed most, and where a lot of MFTs are still set up for a regime that no longer applies to certificates. Under BS 7671:2018+A2:2022, Regulation 643.8 asks for the RCD to be tested at its rated residual operating current, IΔn, and the Amendment 4 model forms carry a single "RCD disconnection time" column rather than the old ½×, 1× and 5× set. A general (non-delayed) RCD to BS EN 61008, 61009 or 62423 must trip within 300 ms at IΔn; an S-type sits between 130 ms and 500 ms. The 40 ms at 5× IΔn figure survives as a Note on verifying additional protection, and the ramp and 5× tests remain useful for fault-finding and for proving a device that trips at 1× but seems sluggish. Set the instrument to the device type before you press the button: a Type A RCD tested on an a.c. waveform, or a Type B tested on a Type A setting, can return a false fail. The sequence and what to record are in RCD Testing Explained: The One Test BS 7671 Now Requires, and the device types in What is an RCD? Types AC, A, F, B and RCBOs explained.

Earth electrode resistance to BS EN 61557-5

On a TT installation the electrode resistance RA is measured either with the fall-of-potential method using two spikes and three or four leads, or, where the installation is small and the electrode can be isolated, with the instrument's loop function. Regulation 411.5.3 requires RA × IΔn ≤ 50 V, so a 30 mA RCD needs RA under 1,667 Ω, and GN3 regards anything over 200 Ω as unstable. The MFT-X1 and the Kewtech KT66DL both do the spike method with an optional lead kit; the method and the coding of poor electrodes are in Earth Electrode Testing for TT Systems: Methods and Limits.

Tests the MFT does not do

Polarity is a visual and continuity check, not a separate instrument function, though the continuity range does the work (/guides/polarity-testing-explained). Functional testing of switchgear, interlocks and the RCD test button is done by hand. Verification of voltage drop under Regulation 643.11 is usually by calculation, though several instruments now estimate it from loop and current figures. AFDDs have a test button and nothing else that a site instrument can exercise.

What does an MFT need for EV charge points and Type B RCDs?

Section 722 brought two instrument requirements that older MFTs lack. Regulation 722.531.3.101 requires protection against d.c. fault current above 6 mA at a charge point, either by a Type B RCD or by a Type A RCD with a residual direct current detecting device (RDC-DD) to BS EN IEC 62955. Testing that device means injecting a smooth 6 mA d.c. residual current and timing the trip, which is a different waveform from anything a Type AC or Type A test produces. Current instruments advertise it as a "6 mA DC" or "EV RCD" test: the MFT-X1 has an RDC-DD test and an EV non-trip loop mode, and the Kewtech KT66DL lists EV 6 mA ramp, auto and single tests. Three-phase chargers also need the instrument to handle 400 V L-L loop and RCD testing, and a charge-point adaptor such as the Megger EVCA210 or EVX100 to bring the charging state through to the tester. The certificate side, including the G98/G99 and OZEV paperwork, is covered in EV Charger Installation Certificate: Section 722 and the EIC.

How do you keep an MFT accurate, and what will the assessor ask?

BS 7671 does not specify a calibration interval and neither does GN3. GN3's position, in the chapter on test instruments, is that instruments should be regularly recalibrated against traceable standards or have their accuracy cross-checked against known references, at a frequency that follows the manufacturer's guidance and takes account of use and environment. The IET's free note "Ongoing accuracy of test instruments" turns that into a routine: start from a formal calibration so you know the instrument and its leads are right, then check monthly against fixed references and keep a tabulated record. If a reading drifts by more than 5% from the baseline the instrument goes back to the calibration house; if it holds, the IET suggests it can run for some years between formal calibrations.

NICEIC's guidance "Safety through accuracy of test equipment" (reference SAFETY-TESTINSTRUMENTS/29-01-25) gives certified businesses three acceptable approaches: formal calibration at manufacturer-recommended intervals with traceable certificates, UKAS-accredited for preference; recorded cross-checks against other instruments the business uses; or recorded measurements of designated reference circuits or devices. NICEIC recommends every system starts from a formal calibration and warns that new or repaired instruments may arrive without a certificate unless you ask. Each instrument must be uniquely identified, and if the business cannot show an effective control system the assessor records a non-compliance. The EAS, which NICEIC, NAPIT and the other schemes assess against, carries the same requirement for registered firms; the Qualified Supervisor's responsibilities around results being accurately recorded are in Qualified Supervisor Sign-Off Explained: EAS Rules for 2026.

In practice the overwhelming majority of UK sole traders still send the instrument away once a year. On 24 October 2026 tester.co.uk listed a 12-month calibration certificate at £35 ex VAT alongside the Kewtech KT66DL, and a proprietary checkbox for the monthly reference checks is a one-off purchase that pays for itself the first time it catches a drifting instrument. Whichever route you take, write it down; a calibration certificate alone proves the instrument was right on one day in the past, and an assessor knows it.

What do GS38 test leads require?

HSE Guidance Note GS38, fourth edition (2015), covers test probes, leads, lamps and voltage indicators for use above 50 V a.c. or 100 V d.c., which is every live test an MFT performs. Probes should have finger barriers and no more than 4 mm of exposed metal tip; 2 mm or spring-loaded retractable shrouds are recommended. Leads should be flexible, adequately insulated, colour-coded and long enough that you are not stretching across live terminals. Leads should include a fuse of low rating and high breaking capacity, unless the manufacturer has built the protection into the instrument and shown compliance with BS EN 61010 and BS EN 61557, which is why some makers supply unfused leads with a current MFT and the practice is still compliant. The instrument itself carries a measurement category: CAT III 300 V or CAT IV 300 V is typical for a UK MFT, with the MFT-X1 rated CAT IV 300 V. Inspect the leads before each job; a cracked probe shroud is the commonest GS38 failure an assessor spots.

How much does a multifunction tester cost in 2026?

Prices below were checked on 24 October 2026 and exclude VAT. Kits and promotions change monthly; the free-gift bundles common in autumn (leads, fuse finders, software licences) are worth checking before you buy.

A few things the table hides. The step from the MFT1711 to the MFT1721 buys the two-wire non-trip loop test and Type B RCD testing, both of which you will want on a modern RCBO board or an EV job. The X1's battery is a modular design that takes the Li-ion pack, AA NiMH or alkaline cells, which matters on day three of a commercial periodic. The KT66DL price includes the earth spike kit; the bare instrument is cheaper. And the Fluke figure is a distributor list price, so compare trade quotes before ruling it out. For most sole traders doing domestic and light commercial certification, the £900 to £1,350 band buys everything the A4 forms ask for, including EV and Type B RCD testing.

What has changed for MFTs under Amendment 4?

Amendment 4 to BS 7671:2018 was published on 15 April 2026, and BS 7671:2018+A2:2022 and A3:2024 were withdrawn on 15 October 2026. Nothing in A4 changes what an MFT must measure, but three things change how you use the readings. First, the model schedule of test results now has a single RCD disconnection time column, so the instrument's auto-sequence should be set to record the IΔn result and not a ½×/1×/5× set. Second, GN3 10th Edition (£46, updated to A4) revises the maximum permissible measured earth fault loop impedance guidance and the initial inspection frequencies, so the limits your instrument displays as "max Zs" may be the uncorrected Table 41 values and need the 80% rule or the 643.7.3 correction applied by you. Third, the A4 EICR adds inspection items for SPD indication (item 4.19) and AFDD presence (4.23); neither is an instrument test, but both sit next to the readings on the form. The full A4 schedule walk-through is in How to Do an EICR Step by Step: Inspection, Testing and Sign-Off and the column-by-column guide in How to Fill In a Schedule of Test Results, Column by Column.

Common mistakes with multifunction testers

• Not nulling the leads before continuity testing, which adds 0.2 to 0.5 Ω to every reading on the schedule.
• Testing insulation resistance with equipment connected, giving low readings and sometimes destroying dimmers, SPDs, smart switches and USB sockets. Disconnect, or use 250 V L+N to earth after reconnection as 643.3.3 permits.
• Using the non-trip loop test on a circuit with no RCD, where the high-current test would give a more accurate Zs.
• Testing an RCD on the wrong type setting and recording a false fail, or recording a 5× result in the single A4 disconnection time column.
• Reading the wrong figure off the display. Instruments show Zs and Ipf on the same screen, and several show the limit beside the measurement. Copying the limit instead of the reading is the commonest transcription error on an EICR.
• Lapsed calibration with no record of cross-checks. The assessor wants the system, not just the sticker.
• Trusting a 0.01 Ω loop reading at the origin as a precise figure. BS EN 61557 allows up to 30% operating uncertainty; use the high-resolution mode or a corrected calculation where the number matters.

How Certio helps

Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is £4.99 a month, charged when you sign up.

Written by the Certio team. Last checked 24 October 2026.