Question:
Our company use a fleet of Megger MFT1720 multifunction testers to complete inspection, testing and certification of both new and existing electrical installations. However, when we carry out a loop test at the same socket outlet, each tester displays a varied reading which may differ by 0.20 Ohms. Why is this when the make and model of each tester in use are identical?
Answer:
Regardless of the model of the tester that you are using, it is common place for a number of identical testers to display different readings during loop impedance tests.
The tester that you are using is a Megger MFT1720 Multifunction Tester. If we take a closer look at the manufacturer’s technical specification for this unit, we see that Megger allow +/- 5% and a +/- 0.05 Ohm tolerance.
In simple terms, if we were to perform a loop test with each of the testers across a fixed impedance of 1 Ohm within a controlled environment, we would expect to see test results as low as 0.90 Ohm and as high as 1.10 Ohms. When using the low current ‘anti-trip’ loop test, we must also allow for electrical noise on the circuit. On the contrary, a higher current loop test cancels out electrical noise, providing an uninterrupted signal for the multifunction tester allowing for a quicker and more accurate measurement.
Outside of controlled environments in the ‘real world’, we also have to think about variables which may also affect the test result. For example, weather and environmental changes, test lead condition and battery charge to name a few.
Having taken the above into consideration and you are still experiencing very random test results, please feel free to call us on 01132 48 99 66 to speak to one of our electricians or calibration engineers. We will be happy to try and overcome any issues that you may have.