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.
The challenge for the maker is that a 15mA load creates a very small voltage drop. This is buried amongst the mass of noise and disturbances of the same AC voltage created by the downstream loads and the upstream supply. 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, it 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 by adding harmonics and/or positive- and negative-going spikes. If one of the instrument’s measurements coincides with this 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 MFT1741 multifunction installation tester which incorporates an innovative “confidence 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 MFT1741 even incorporates a confidence 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 dot, this indicates a high-confidence result 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 impedance 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 MFT1741 multifunction installation testers with their groundbreaking test algorithm and their innovative confidence function.
Imaging the time you could save if your multifunction installation tester provided reliable non-trip loop testing at the first time of asking.
Fast and repeatable non-trip loop testing is now easily achieved with a high level of confidence using the new MFT1741 multifunction installation tester from Megger.
Are you sick of fluctuating measurements when you are loop testing? The latest innovation from Megger may just have the answer!
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