You work as a designer for a manufacturer of multifunction electrical test meters. For the last few years you've made meters which provide a standalone solution to most of the testing requirements of the 17th Edition of BS7671. Your meters are robust, reliable and easy to use. But you need to sell more meters. So what do you do to differentiate your new products from the rest of the fold?
For some manufacturers, you look at all the glossy, colour screen smart devices everybody has, and you incorporate this technology into your meter. Instead of switches and dials, your device uses screen taps, and instead of a fixed LCD display, you use a full colour LED display.
But is this what your customers want? A quick straw poll of domestic and commercial electricians gave the following unscientific wish list for an MFT meter:
- Fast to use - get the results you want quickly without messing around.
- Robust - can be used in working environments, with dusty or damp hands, or while balanced on top of a ladder in the rain.
- Reliable - every day in a service workshop is a day's lost work.
- Ergonomic - for example, leads don’t get tangled constantly, test can be triggered directly from test probe.
It's evident that while colour touch screens may look nice, they don’t really address any of these points. My concern is that incorporating a touch-screen interface may actually be a step backwards, in terms of usability and reliability. To borrow an example from a different industry, professional video cameras have eschewed multi-function touch screens with layers of recursive menus, retaining a whole lot of dedicated buttons over the casing. Why? Because, even in darkness, or while moving, without having to look, the operator can find the exact function they need instantly. With a test meter, having to navigate through a series of menus seems less user-friendly than being able to turn a dial to access a function immediately.
Megger have beaten a different path with their new range of testers. The form and function remain broadly the same. Instead, Megger have chosen to address a functional issue: how to obtain accurate and reliable test results from low-current earth fault loop impedance testing.
To understand the problem, take your existing test meter and apply it to a non-RCD-protected circuit, or to a consumer unit incomer directly. Take an earth fault loop impedance (Zs or Ze) measurement using the normal measurement setting, and now take exactly the same measurement using your meter’s RCD-compatible mode, which uses a lower test current. Are they the same? Most likely they're not. One is inaccurate; it's almost certainly the RCD compatible measurement. Other MFT manufacturers may claim this is because of electrical noise on the circuit, and they think they've found a way to make this measurement both reliable and accurate. Later on we’ll find out if they're right.
Out of the Box
The MFT1471 is Megger’s new flagship meter, so unlike some of their lower models, it actually comes with rechargeable batteries and a charger - although how Megger think it’s reasonable to sell a meter for around £1,000 and still charge extra for one with rechargeable batteries is beyond me.
In the UK, Megger are synonymous with electrical test meters (we’ve all seen fittings which have “Do Not Megger" printed on their casing, meaning “Do Not Apply a 500V DC Insulation Resistance Test Voltage,” not that you can just use a different instrument and avoid frying the device.) Therefore you’d expect something of decent quality, and most people won’t be disappointed.
The meter itself is rugged and chunky, with a durable plastic casing. I didn’t test it by throwing it down some stairs or dropping it off a ladder, as it was a loan unit, but I’ve heard comments from long-time Megger users that the meters are pretty much bomb-proof.
Test buttons, in Megger’s traditional style, are positioned both on the far left and far right of the meter, so that it can be used left- or right-handed, and crucially, also on one of the supplied test probes. My everyday meter is a not a Megger, and daily I curse the lack of a test button on the probe ends as I try to juggle multiple probes in each hand whilst pressing the test button with my nose or a foot. In fact this is arguably a safety issue, as it leaves hands free for holding on to other things. However, Megger have gone a step further and implemented automatic testing for several functions, which means you don’t even need to use the probe button.
The meter takes standard 4mm “Banana” probe leads, which is a great benefit over proprietary connectors, as it means if you damage a lead you can use one off any other meter or from any supplier. Other manufacturers sometimes use proprietary connectors at their meters, which means you’re stuck with their (sometimes substandard) leads. Bonus points to Megger.
The leads are of good quality, with a choice of tips including probes, crocodile clips and an elongated fine probe for specialist use. My particular bugbear with leads is ones that twist up in use, meaning you have to constantly untangle them as you work; I found these leads didn’t suffer from this problem.
In the sturdy kit bag provided, you’ll also receive a dedicated 3-pin plug lead, manuals on CD, a calibration certificate and a neck strap. Annoyingly, if you want to access the manual online, you’ll have to sign up for an account on Megger’s website. Megger’s 3-pin plug adopts a flexible approach to which lead is used where and allows you to carry out any variation of testing without having to buy a break-out plug. This includes R1+R2 testing.
One thing I’m still waiting for in a test meter is a clip on the neck strap to store the probe ends when not in use. Crocodile clips can be clipped to the neck strap, but probe ends end up in a pocket, or put down somewhere and lost. I wouldn’t have thought this would be rocket science.
The MFT1741 supports Megger’s ICLAMP and VCLAMP leads, for stakeless earth electrode testing, but these come at a substantial additional cost.
In Use
Some simple yet effective design features make the MFT1741 a pleasure to use. In particular, automatic testing is provided for many key measurements, including fault loop impedance and resistance (such as R1+R2 resistance testing). This means you don’t have to press a button – just connect the test leads, and when the meter senses the correct combination of expected resistances and voltages, it’ll run the test automatically. If you’re used to using a meter which doesn’t have this feature, you’ll grow to love it very quickly.
A good example of this is when you are testing earth fault loop impedance at different points on a circuit for an EIC or EICR. The MFT1741 will record the maximum Zs over several sequential tests. It’s simply a case of applying the probes, waiting for the meter to automatically run the test, then moving on to the next test point. It almost makes EICR testing pleasant.
Insulation Resistance testing requires the test button to be held; presumably as it’s the only test where you can actually get a high-voltage shock from the meter by touching the probe at the wrong time. However, the integrated test button on the line probe tip makes this quick and easy without having to possess lots of limbs.
RCD testing is, as with all Megger meters, fast and automated. Once the function’s triggered, simply reset the RCD 4 times in quick succession and return to the meter for the results. However, the MFT1741 doesn’t have a matrix display, which means the display lacks flexibility, so reading the results back requires 4 button presses to switch between screens (for I, 5I, 0° and 180° tests). In contrast, other meters with a matrix display can display a complete set of results at once. In turn this means you can instantly compare all 4 results, rather than have to make a mental note of each one whilst flicking through to find the maximum time for each value of I. I’m not entirely sure why Megger haven’t gone down the matrix display route as this would make this and other functions clearer and easier to use.
Another limitation of the display arises with impedance testing: the MFT1741 puts a very small and inconspicuous “k” on the screen to differentiate between 1Ω and 1kΩ. I found this far from obvious, at one point believing I’d measured 2.3Ω in an R1+R2 test on a lighting circuit, which would have been a pass, when upon closer inspection the impedance was 2.3kΩ, which was a very definite fail.
I found the twin dials fast and intuitive for selecting the function I needed. An additional button allows further options to be selected. Others have said that this can be confusing - set one dial, set another dial, then press a button, apparently arbitrarily - but I found in practice it was all straightforward.
Automation goes further with some clever features which aren’t obvious at first. For example, if you have 2-lead low-current fault loop testing selected, if you connect a third lead, the MFT1741 recognises this and automatically switches to 3-lead mode. On the one hand, this ensures you don’t overlook the setting and end up in a sub-optimal mode, but some may prefer to have full control over the meter and not have it make decisions for them.
Low-Current Earth Fault Loop Impedance Testing
As mentioned, the MFT1741’s flagship new feature is improved accuracy and reliability of low-current, RCD-friendly earth fault loop impedance testing. Megger explain that this is achieved by the meter sampling electrical noise over the testing period, and factoring this into its ultimate calculation.
Not only should the result be more accurate, but the meter will also indicate the level of confidence through the use of a graphical bar. The narrower the bar, the more confident you can be in the result.
In order to test this, we put the MFT1741 up against an older Megger meter (MFT1731 model) and a meter from another manufacturer. All three meters support a low-current earth fault loop impedance test which is intended to be used on RCD-protected circuits with low probability of tripping the RCD.
Methodology
The test supply was a normal single-phase domestic supply. During testing the supply voltage was not observed to vary, and the load was broadly constant. Each meter was tested using its own supplied test leads. 3 tests were conducted for each setting on each meter; between tests, all test leads were disconnected and reconnected.
The Megger meters were tested using 2-lead high-current, 2-lead low-current and 3-lead low-current modes. The other test meter only supports 3-lead testing so was tested using 3-lead high-current and 3-lead low-current modes.
Mean averages and standard deviation were calculated for each of the 3 sets of results.
The mean of all high-current results was taken and assumed to be the “accurate” result: 0.283Ω. Each of the means was then compared to this “accurate” result and the difference (delta) calculated. Note that we have no way of verifying that this result is indeed correct. Measurement for the circuit; but in the absence of any further information, it is a reasonable proxy.
Primarily of interest is the range of results, which gives a measure of how reliable each test was (i.e. how much variation there is between successive tests), and the difference from the “accurate” result, which gives a measure of how accurate the low-current tests were for each meter.
Results
The graph shows the mean test results for each meter and testing type, with the red line denoting the “accurate” result. A smaller black bar range indicates a greater reliability. The further the mean from the red line, the less accurate the result.
High-Current Testing
In high-current testing, although numerically the MFT1741 was actually the least accurate and least reliable of all three meters, this should be viewed in the context of the precision of the measurements for all meters only being to within 1/100th of an Ohm, and the ranges of all results being very narrow.
All three meters gave mean results within 0.01Ω of the “accurate” result. This gives confidence that when testing circuits where an RCD is not present, it’s straightforward to obtain an accurate earth fault loop impedance measurement.
3-Lead Low-Current Testing
In the three-lead, low-current test, which is where the MFT1741’s new functionality comes into play, the results were markedly different. The other meter was by far the least accurate and least reliable of the three meters in low-current testing, with a consistent substantial over-measurement. (One could argue that in this situation, a high reading is far better than a low reading - as any error will be on the side of caution.)
The older MFT1731 was actually on paper the most reliable of the three meters in low-current testing, although both Megger meters gave substantially more reliable results than the non-Megger instrument. However, crucially, the MFT1741’s low-current results were just as accurate as the MFT1731’s high-current results. This somewhat vindicate’s Megger’s claims for the accuracy of their new testing architecture, and suggests their new approach is indeed a significant step forward.
2-Lead Low-Current Testing
Finally, in the 2-lead low-current modes, both Megger meters over-read by a substantial margin, comparable to the other test meter in 3-lead low-current mode, although both were slightly more reliable than the results of the other meter's 3-lead tests. From this we can conclude that if you desire accurate and reliable results, don’t use the 2-lead testing modes.
Conclusion
With the MFT1741, Megger set out to improve the accuracy of low-current fault loop impedance testing. The testing I did gives me confidence they’ve succeeded. For a low-current test to give the same level of accuracy and reliability as the high-current test is a worthwhile and well-planned step forward.
The main impression I was left with is that this meter really delivers on all the key points: it’s fast, easy to use and robust. Working with the MFT1741 for a few days left me pretty much hating the older test meter I have to go back to; every feature of the more expensive Megger gives a better, more efficient experience, and I really wanted to keep it.
If you already have a decent meter you’re happy with, the features of the MFT1741 may not persuade you to throw it away and buy a new one. However, next time you’re in the market for a meter, you’ll find this one hard to beat.