Megger's new TrueLoop technology, only available in the Megger MFT1845 multifunction tester
When you use an ordinary multifunction installation tester (MFT) to perform non-trip loop tests, it typically makes the measurement over a fixed time period of 15 seconds. That’s not long, but if the result isn't quite what you expected – possibly because of noise in the circuit under test – you may well decide to do a second test “just to be sure”, and maybe even a third! So now testing this circuit has taken a minimum of 45 seconds, but probably longer. And now there’s another issue: which of the three test results do you put on the certificate. It’s very unlikely they will be identical!
There has to be a faster and better way – and now there is! A new MFT – the Megger MFT1845 – has recently been launched that incorporates a unique “TrueLoop” technology.
The principle is simple: when you carry out a non-trip loop test, your tester actually makes a large number of measurements before using these to calculate the result it shows on the display. The confidence function continuously monitors these measurements, and tells you when they become stable.
On a quiet circuit, which has low levels of electrical noise, this can take as little as eight seconds, which is a big time saving compared with 15 seconds or more with a conventional MFT. On a noisy circuit, the test may well take a little longer than 15 seconds, but there’s still a very important benefit – the result will be right first time.
You never need to perform multiple tests to make sure of getting a reliable result, and you never need to decide which of several different results should go on the test certificate. So, whether the circuits you’re working on are noisy or quiet, using an MFT with ”TrueLoop” will save you valuable time, as well as making sure that you can complete the test certificate with a high level of confidence.
LOOP IMPEDANCE TESTING AND NOISE
Why do non-trip loop impedance tests on noisy circuits sometimes give unexpected results? And what can be done about it? To answer these questions, we first need a generalised look at how non-trip loop testing works. Essentially, the instrument monitors the current and voltage in the circuit under test, and looks at the supply voltage difference made by adding a small extra load. With most loop testers, this extra load is designed to draw a current of around 15 mA; it can’t be more, or there’s a significant risk that the RCD or RCBO protecting the circuit would trip.
Because of this, the instrument actually makes multiple measurements, taking into account the point on the supply waveform where each measurement is made. After a preset time period, typically 15 seconds, the instrument takes the average of all the measurements and displays this as the test result.
This arrangement works well provided that the supply waveform is a good, clean sine wave. Noise distorts the waveform, from harmonics or by adding positive- and negative-going spikes. If one of the instrument’s measurements coincides with a distortion, the measurement can be wrong and, if there are enough incorrect measurements during the 15-second measuring period, they can significantly affect the average that the instrument displays as the result.
This is why non-trip loop impedance measurements on noisy circuits often produce unexpected results and why, if the test is repeated on such circuits, the result may well be different every time.
What can be done about this? Megger has carried out extensive research and development to answer this question and the result is the new MFT1845 multifunction installation tester which incorporates an innovative “TrueLoop” function for non-trip loop testing. The tester works in a similar way to its predecessors, making multiple measurements during the test period, but it continually monitors these measurements and uses a unique new algorithm – which minimises the impact of noise – to evaluate them.
In fact, the MFT1845 even incorporates a “TrueLoop” display – an arc that diminishes in length as the instrument becomes more and more confident of the reliability of the result. When the arc reduces to a single segment, this indicates a result within the accuracy specification has been achieved. This can take as little as 8 seconds on circuits with little noise present. It takes longer on noisy circuits – sometimes more than the customary 15 seconds – but it has the very important benefit of providing right-first-time results.
Non-trip loop testing has always been a concern for those involved with testing electrical installations because of its tendency to deliver inconsistent results. This problem has now been solved by the new Megger MFT1845 multifunction installation testers with their groundbreaking test algorithm and their innovative confidence function.