How often does an MFT need calibrating?

There is no fixed legal interval, but IET Guidance Note 3 requires test instruments to be accurate and recommends formal calibration, typically every 12 months, plus regular checks against a known reference such as a check box. NICEIC and NAPIT assessors will ask for the calibration certificate and evidence of ongoing accuracy checks.

Can an MFT test an RCD?

Yes. An MFT applies a controlled residual current at half, one and five times the rated tripping current (for a 30 mA device that is 15, 30 and 150 mA) and measures the time to trip. It tests at both 0 and 180 degrees, and most models include a ramp test and an auto sequence that runs all steps in one go.

Do I need a separate loop tester if I have an MFT?

No. Loop impedance (Ze and Zs) and prospective fault current are standard MFT functions. Most instruments offer a no-trip loop test so you can measure Zs on an RCD-protected circuit without tripping the device, and a high-current two-wire test for a more accurate reading on non-RCD circuits.

Which tests on an MFT are needed for an EICR?

For an EICR you typically use continuity (R1+R2 or R2), insulation resistance at 500 V, earth fault loop impedance (Ze at the origin, Zs at each circuit), prospective fault current at the origin and RCD operating time for every RCD. The results go on the schedule of test results for each distribution board inspected.

What is a multifunction tester (MFT)? Explained

Q: What does MFT stand for in electrical testing?

MFT stands for multifunction tester. It is a single instrument that combines the tests BS 7671 requires for certification: low-resistance continuity, insulation resistance, earth fault loop impedance, prospective fault current and RCD operation. Most models also test earth electrodes and phase rotation. Without one you would need four or five separate instruments.

A multifunction tester (MFT) is the hand-held instrument UK electricians use to carry out the tests BS 7671 requires for an EIC, Minor Works Certificate or EICR, combining low-resistance continuity, insulation resistance, earth fault loop impedance, prospective fault current and RCD testing in one unit built to BS EN 61557. Every reading on a schedule of test results comes from an MFT, which is why IET Guidance Note 3 requires the instrument to be calibrated and its accuracy checked regularly.

Key takeaways

An MFT replaces the separate continuity, insulation, loop and RCD testers that used to fill a toolbag.
The instrument must meet BS EN 61557 and be kept accurate; annual calibration is the norm.
The five core functions map directly onto the columns of the schedule of test results.
A no-trip loop test allows Zs to be measured on RCD-protected circuits.

What is a multifunction tester in practice?

Before MFTs, an electrician carried a low-ohm continuity meter, an insulation resistance tester, a loop impedance tester and an RCD tester. The MFT brings those together behind one rotary switch, with a display that shows the measured value alongside the limit for the test selected. Instruments from Megger, Fluke, Kewtech, Metrel, Di-Log and Martindale dominate the UK market; features differ, but the core measurements are defined by BS EN 61557 and by BS 7671 Chapters 64 and 65.

When is an MFT used?

An MFT is needed for initial verification under Chapter 64 and periodic inspection under Chapter 65, which covers every EIC, Minor Works Certificate and EICR. It is also used for fault-finding and for checking earthing before connecting an EV charger or PV system.

MFT function	What it measures	Where it appears on the certificate
Continuity (low ohms, 200 mA test current)	r1, rn, r2 on ring finals; R1+R2 or R2 on radials; bonding conductors	Continuity columns of the schedule of test results
Insulation resistance (250, 500 or 1000 V DC)	Resistance between live conductors and to earth, in MΩ	Insulation resistance columns
Loop impedance (Ze, Zs)	Impedance of the earth fault path, in Ω	Ze on the supply characteristics page; Zs per circuit
Prospective fault current (PSCC, PEFC)	Highest current that could flow on a short circuit or earth fault, in kA	PFC at the origin
RCD test	Trip time in ms at ½×, 1× and 5× rated residual current	RCD columns per circuit

How are MFT readings taken and measured?

Continuity uses a DC test current of at least 200 mA, as GN3 specifies. The leads must be nulled first, otherwise their resistance is added to every reading. On a ring final circuit, r1, rn and r2 are measured end-to-end, then the cross-connected test gives R1+R2 at each socket.

Insulation resistance is tested at 500 V DC on 230 V circuits. BS 7671 sets a minimum of 1 MΩ; GN3 advises investigating anything below 2 MΩ. Electronic equipment and surge protective devices must be disconnected first.

Loop impedance is measured live. Ze is taken at the origin with the main earthing conductor disconnected and the installation isolated. Zs is measured at the furthest point of each circuit and compared to the maximum values in BS 7671 Chapter 41.

RCD testing applies a residual current and times the trip. For a 30 mA device providing additional protection, the 5× test at 150 mA must trip within 40 ms.

Common mistakes with MFTs

Not nulling the leads before continuity testing, which adds 0.2 to 0.5 Ω to every result.
Testing insulation resistance with equipment connected, giving low readings and sometimes destroying dimmers, SPDs and smart switches.
Using the no-trip loop test on a non-RCD circuit where the high-current test would give a more accurate Zs.
Lapsed calibration. Scheme assessors check the certificate date, and GN3 expects routine accuracy checks too.
Reading the wrong value off the display. Instruments show Zs and PFC on the same screen, and some show the limit beside the measurement. Copying the wrong one is a common transcription error.

Related terms

How Certio helps

Certio records this on the BS 7671 model forms without the typing: photograph the board to draft the circuit schedule, point the camera at a Megger MFT-X1 to capture readings, or dictate them. Starter is free for 7 days.

Written by the Certio team. Last checked 14 September 2026.