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Who tests the testers?
Story
It’s all very well testing electrical installations in line with the IEE Wiring Regulations, but how can you be sure that the test results are accurate? Unless you check your instruments regularly, the answer is that you can’t.
Accuracy should be checked regularly
Unless there’s an obvious problem, few contractors give a thought to the accuracy of the instruments they’re using. But they should – inaccurate test results are worse than no test results at all because they can create the impression that an installation is safe when it isn’t. This is recognised in the guidance notes that accompany the IEE Wiring Regulations, which make it absolutely clear that the accuracy of test equipment must be checked regularly.
It would be easy to think that annual calibration would satisfy this requirement, but that’s not necessarily true. A lot can happen to an instrument over the course of a year and, although modern instruments are remarkably resilient, there’s no doubt that accidents can and do occur which adversely affect calibration.
Impractical to calibrate more than once a year
What’s needed is a way of checking the performance of instruments much more regularly. Returning them to the supplier several times a year for calibration would, however, be both inconvenient and costly.
To solve this problem, Megger has introduced the MTB7671. Unlike some of the other test boxes currently available, the MTB7671 genuinely looks at all key aspects of instrument performance, rather than just focusing on a single aspect of each test.
Loop testers test up to 3000 ohms not 1 ohm
For example, lets look at loop testing. Many test boxes are arranged to allow the instrument that’s being checked to measure the loop impedance of a circuit chosen by the user. Then a 1 ohm resistor is added to the circuit, and the measured result should, of course, show a 1 ohm increase.
This is fine, except that many loop testers can read up to around 3 kilohms! Testing the accuracy right at the bottom of this range provides little information about accuracy at the higher readings. With the MTB7671, this issue doesn’t arise, as the test box checks the accuracy of the instrument at multiple points.
If anyone thinks that accuracy only matters at the low end of a loop tester’s measuring range, they should remember that, in TT systems, acceptable loop impedance values in excess of 100 ohms are sometimes found.
This is just one example of how Megger’s MTB7671 sets a new industry benchmark in testing the testers.
Won’t trip RCD protected supplies
Other examples include RCD testing, where the problem here is not so much with the validity of the results provided by the test box, but with the inconvenience of getting those results.
When used to verify the performance of an RCD tester, all currently available test boxes will trip any RCD protecting the circuit from which they are powered. One solution is to power the test box from a circuit that’s not RCD protected, but often that’s not convenient. Once again, the MTB7671 has addressed this problem, and checks loop testers without any risk of RCD tripping.
Instrument test boxes have an important role to play in guaranteeing the reliable test results needed to ensure the safety of electrical installations. However, not all test boxes are the same. In particular, Megger’s MTB7671 looks at all key aspects performance of insulation test, continuity test, loop test and RCD test instruments. Only an instrument that does this provides users with complete assurance that their test equipment is compliant with all regulations.
