MFT-X1 Multifunction tester

This is most likely caused by a flat battery, battery insertion incomplete, or contamination of the battery pack terminals.

Connect the battery to the charger and ensure the charger light is green. The charger will show a red light if the battery needs charging. Ensure the battery is properly inserted and both the catches are engaged.

Check there is no contamination on the battery pack terminals. Is present, clean with isopropyl alcohol (IPA).

If the battery pack is a secondary pack (not originally bought with the instrument) ensure there is only one o-ring seal. Additional battery packs are sold with a seal but if one is already fitted to the instrument there are too many seals and the battery will not insert into the case fully.

Live circuit measurements such as loop impedance and RCD place a load on the live circuit . Lack of a live voltage when the test button is pressed will prevent the test from running

A PE Warning appears if the instrument is connected to a live circuit but it cannot detect a safe earth connection. The warning will always appear in these circumstances, but the inhibit of testing can be disabled in settings.

The warning is activated when the user touches the test button.

Resistance of the test leads – These should have their resistance nulled in the instrument so that measuring a short circuit displays zero Ωs.  Typical test lead resistance will be around 0.035 Ω per lead. Failure to null this resistance can add significant errors at low resistances, especially below 1 Ω.

Fused Leads – These can also add additional fuse resistance. A 500mA fuse (as recommended by GS38) can add an additional 0.75 Ωs of resistance per lead.

Contact resistance – This will depend on the condition of the probe tip and that of the material to which they are applied. 0.04 Ωs in not uncommon and significantly higher values can be present. This can also apply to crocodile clips.

Crocodile clips – Due to the hinge mechanism within a crocodile clip, one side of a clip has a lower resistance than the other. The moving half having a higher resistance than the fixed half. Lead sets that are nulled prior to use may still induce errors if the croc clips were not nulled with their fixed halves joined together. Typical error can be around 0.03 Ω

Ambient temperature – Although this does influence the values reported, the changes in temperature are less significant than those detailed above. Compensation for temperature is well defined in electrical installation guidance documents and standards. 

Presence of supply voltage – On continuity measurement a small voltage across the measurement terminals can significantly affect the measurement. A continuity test typically uses 4 V DC to 5 V DC to make its measurement. A voltage on the circuit of only 2 V AC or DC can significantly affect the measurement. 

Electrical noise – Changes in the mains voltage or shape of the AC waveform during the test can create significant variation in the reported loop impedance. The more electrical noise on the circuit, the more variation in the results.

Load switching, harmonics, higher frequency noise all affect the measurement.

Micro-generation

Micro-generation, especially domestic solar power is a significant cause for variation in loop impedance, especially where load management is employed to divert power to internal loads rather than exporting to the grid.

RCD uplift

The RCD or RCBO can itself significantly affect the loop result during a non-trip test. The cause is the coils in the Phase and Neutral RCD providing the leakage sense. Uplift can be as high as 1 Ω but is more typically around 0.3 Ω to 0.5 Ω.

Proximity of a transformer – Continuity testers and Loop impedance testers tend to measure only the loop resistance of a circuit, rather than including the reactance (principally inductance) and calculating the true impedance of a circuit. Where the measurement is reasonably far away from a transformer, the most significant part of the measurement will be resistive, and errors are very small. Eg  R = 0.3 Ω  Xl = 0.01 Ω

Close to a transformer, the reactive component could be much higher compared to the resistance of the circuit, eg   R = 0.006 Ω   Xl = 0.025 Ω.

Most loop impedance testers struggle to measure this Reactance. Consequently the loop impedance and calculated fault current is based on the resistance of 0.003 Ωs and not on reactance of 0.025 Ωs. Loop impedance testers frequently measure <0.01 Ωs when the impedance is higher.

Using those figures above, at 230Vac the fault currant would be displayed on an instrument as:

Resistance of 0.006 Ωs      230 / 0.006 = 38.2KVA

Reactance of 0.025 Ωs      230 / 0.025 = 9.2KVA 

The range of an instrument is simply the minimum and maximum values the instrument is capable of measuring in any particular measurement mode.  

Overall range is constrained by the electrical capabilities of the instrument. Displayed range is defined by the display readout limitations, or digits.

An instrument that has '8888' as the digital readout may display 0.001 V on its minimum range, right up to 9999 V on its maximum range. Typically the range is adjusted to best fit the value being measured.

For the example above, the instrument could measure 0.025 V, but this range would only go to 9.999 V. After this the range would change to 10.00 V and now measure up to 99.99 V before changing to 100.0 V.

The right hand figure in each of these ranges denotes the resolution of the range.  

Likewise the instrument cannot display 1.087338 V, as the limitations of the display only allow for 4 digits. Consequently the instrument would likely display 1.087 V, rounding up or down the last digit.  

In reality the electronics are usually designed to manage measurements in the range that match the display. For example, an instrument with electronics that can measure to a resolution of 1 uV at 10 V is pointless if the display is limited to 4 digits. It is also a gross waste of money.  

For the example above, the instrument could measure 0.025 V, but this range would only go to 9.999 V. After this the range would change to 10.00 V and now measure up to 99.99 V before changing to 100.0 V. 

No instrument can provide perfect measurements in all conditions. Therefore an instrument declares an accuracy within which the instrument should perform for any measured value.

Accuracy figures are usually given as a percentage, followed by a number of digits. It typically looks like this example:

Accuracy = ± 5% ± 2d

The percentage is simply the amount the displayed value can deviate from the real value, as a percentage.

The ± 2d (or 2 digits) means how much the least significant number in the display an also vary. This depends on the resolution of the instrument and range selected. 

So putting this together, measuring a voltage of 100 V on an instrument with a display that shows 100.0 V with an accuracy of ± 5% ± 2d could give a range of values as below: 

From:   100.0 V less 5% = 95V              Less 2 digits = 0.2 V      = 94.8V

            To:       100.0 V plus 5% = 105%            Plus 2 digits = 0.2 V       = 105.2V 

            Total range of variation is 10.4%

So in this case the percentage has a much bigger effect than the digits

However a small value, say 0.5 V, is affected by the digits far more than the percentage, as below:

            From:   0.5 V less 5% = 0.025 V   Less 2 digits = -0.2 V  = 0.275V rounded to 0.27 V

            To:       0.5 V plus 5% = 0.525V     Plus 2 digits = +0.2 V             = 0.725 V rounded to 0.73 V

Total range of variation is 92%

So it is very important to note that on very low values the number of digits in the accuracy statement can have significantly more effect than the percentage accuracy.

On an analogue display the situation is slightly different. Accuracy is usually stated as a percentage of full range. So, where an instrument has a range of up to 100V, a 1% accuracy would be equal 1V.

In this case the accuracy is stated as 1% of Full Range.