Helpful video reference. Thomas Nagy is a UK-based electrician whose channel covers test instruments and domestic installations in practical detail. His Megger MFT1741 video walks through each test mode in turn and explains what the instrument is actually measuring - worth watching before you pick up the leads for the first time.
1. Understand what an MFT does
A multimeter measures AC and DC voltage, current and resistance. An MFT does all of that and adds the tests BS 7671 Chapter 61 actually requires:
- Continuity - checks that every protective conductor (CPC) and every neutral has a complete path end to end.
- Insulation resistance (IR) - checks that the insulation between conductors, and between conductors and earth, has not broken down.
- Earth fault loop impedance (Zs and Ze) - checks that the earth fault path is low enough in resistance to trip the MCB within the required disconnection time.
- Prospective fault current (PFC) - the worst-case fault current at the origin, used to verify the consumer unit's breaking capacity.
- RCD trip time - measures how quickly the RCD responds to a fault current at 0.5×, 1× and 5× its rated tripping current.
The Megger MFT1741 and similar instruments from Fluke, Kewtech and Robin combine all these modes in one handheld unit with a single socket set for the probes.
2. Inspect and set up the instrument
Check the calibration label before you start. Most MFTs require annual calibration to remain valid for EICR and EIC work - a reading from an out-of-calibration instrument cannot be used on a signed certificate. Inspect test leads and probes for cracked insulation or damaged banana plugs and replace any damaged lead before use.
Null the continuity leads: connect the probe tips together, select the low-ohms continuity mode and press the null or zero button. This removes the resistance of the leads themselves from every reading. Without nulling, every continuity result will be slightly high.
3. Test continuity - R1+R2 and Rn
With the circuit isolated and dead at the consumer unit, use the link method to measure R1+R2 (the line conductor plus the CPC) and Rn (neutral continuity).
For R1+R2: temporarily link the line to the CPC at the furthest accessory, then measure the loop resistance from the consumer unit end. Each result should be consistent with the cable length and cross-section - a 10-metre run of 2.5/1.5 mm² twin and earth typically gives around 0.3 to 0.5 Ω. An open-circuit reading (OL or several megaohms) means a broken or disconnected conductor and must be traced before you proceed.
Record all values on the Schedule of Test Results. Continuity tests must be completed and recorded before insulation resistance testing begins.
4. Test insulation resistance
With the circuit still de-energised and all equipment disconnected, select 500 V IR mode on the MFT. Test between:
- Line and neutral (with neutrals bridged to lines at the board if you prefer the combined L-N-E method)
- Line and earth
- Neutral and earth
BS 7671 Table 6A requires the result to be at least 1 MΩ. A healthy new circuit on new cable reads well above 200 MΩ. A reading below 2 MΩ suggests moisture or damaged insulation and needs investigation before energising.
Use 1000 V DC for circuits rated above 500 V. Disconnect any low-voltage electronics - dimmers, smart switches - before the IR test, as the test voltage will damage them.
5. Restore power and re-energise safely
Reconnect any equipment removed for the IR test. Confirm neutrals are on the neutral bar, CPCs on the earth bar and the circuit breaker is in place. Switch on the MCB or RCBO at the consumer unit.
Confirm the circuit is live with your voltage indicator before moving to live tests. Check L-N, L-E and N-E voltages at the first accessory to confirm polarity is correct.
6. Test earth fault loop impedance
With the circuit energised, measure Ze at the origin (the incoming supply impedance at the consumer unit) and Zs at representative points - typically the furthest socket on a ring or the furthest luminaire point on a radial.
Ze is measured at the consumer unit with the main switch open, or use the network operator's declared value if live Ze testing is not practical at the origin. Zs is measured by inserting the MFT test plug into a socket or using probe leads at a fixed accessory.
Compare each Zs reading against the maximum Zs in BS 7671 Table 41.2 or 41.3 for the protective device. For a 32 A Type B MCB protecting a ring, maximum Zs is 1.44 Ω. If your reading exceeds the table value, the fault path is too resistive for the MCB to disconnect within the required time.
7. Test RCD trip time
Select the RCD test mode and set the rated tripping current to match the installed device (30 mA for most domestic circuits). Test at:
- Half rated current (15 mA for a 30 mA RCD) - the RCD must not trip.
- Full rated current (30 mA) - must trip within 300 ms.
- Five times rated current (150 mA) - must trip within 40 ms.
Run the test at both 0° and 180° phase angles and record all six results. An RCD that fails the 40 ms test at 5× but passes the 300 ms test at 1× is still compliant for Type AC devices on standard circuits. A failure at 1× means the RCD is faulty and must be replaced.
8. Record and check your results
Every value measured above goes into the Schedule of Test Results on the Electrical Installation Certificate (for new work) or the EICR (for an inspection). The form requires instrument serial numbers, calibration dates, continuity values, IR values, Zs values and RCD trip times for every circuit.
Check every result against the relevant limit in BS 7671 before signing. If any circuit has a reading outside the acceptable range, do not sign the certificate until the fault is found and corrected.
When to call us
EICR inspections and Electrical Installation Certificates require a qualified, insured electrician to sign them. If you are buying or selling in Sandwich, Deal or Dover and need an EICR, or if you have completed new work and need the testing and paperwork, call Richard. Small local test jobs at the standard £10 per 10-minute rate.
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