Megger’s Simon Wood claims that when it comes to checking the accuracy of test instruments used for certifying electrical installations in line with the IEE Wiring Regulations, using a handful of resistors of known value simply isn’t good enough
The IEE Wiring Regulations make it clear in no uncertain terms that the accuracy of instruments used for testing and certifying electrical installations must be checked regularly. The question is, what is the best way to carry out these checks? Unfortunately, it seems that a lot of bad advice is being given.
For example, it has been suggested that all contractors need to do is to spend a few pounds on a few resistors and then make sure that their instruments show the correct value when these resistors are connected across the test leads. Although this may at first sound like a perfectly satisfactory solution (and it’s certainly very cheap) it simply isn’t good enough.
The reason is that tests carried out on electrical installations always involve more than one parameter. For example, think about measuring insulation resistance. The result you’re looking for is the resistance, but to meet the requirements of the IEE Wiring Regulations, the tester has to deliver a specified voltage when measuring that resistance.
Connecting a resistor across the test leads will indeed check whether the instrument reads resistance correctly, but what information does it provide about the test voltage? The answer is absolutely none. The instrument could be testing at 100V rather than 500V, for example, and the ‘resistor test’ wouldn’t provide any hint of a problem.
The situation is similar for continuity tests. A ‘resistor test’ will show that the instrument reads correctly, but the Wiring Regulations state that continuity testers must be capable of delivering a short circuit current of 200mA and that the tests must be carried out at a voltage between 4V and 24V. Once again, the ‘resistor test’ gives no indication of whether these conditions are being met.
A question that’s sometimes asked is why instrument manufacturers don’t build diagnostics into their products that would reveal problems like low output voltage on insulation tests. The answer is simple. It would be almost impossible to do so. If an insulation tester is inadvertently connected to a short circuit, for example, its output voltage will fall to near enough zero. This isn’t an indication that there is anything wrong with the instrument, but a diagnostic system would almost certainly see it as a fault.
Clearly a handful of resistors aren’t up to the job of verifying instrument performance. So, what should be used instead? The solution is a purpose designed test box. However, when choosing one of these, it’s still important to proceed with care. Some test boxes are nothing more than that handful of resistors in a fancy box. These units inevitably share the same shortcomings as the resistors – they don’t check the secondary test parameters like voltages and currents.
However, Megger’s MTB7671 checks all of the parameters associated with each test it does and also checks instruments at several points throughout their measuring ranges. To see why this is important, we need to look at loop testing.
Test boxes generally check the operation of loop testers by allowing them to measure the loop impedance of a local circuit. Then a 1O resistor is added to the circuit, and the measured result should, of course, show a 1O increase. This is fine, except that many loop testers can read up to 3,000O! Testing the accuracy at 5O and 6O for example (two points right at the bottom of the measuring range), says nothing about accuracy at the higher end of the range.
It’s tempting to dismiss this shortcoming as unimportant. After all, it is a widely held belief that loop impedances are always down at the low end of the range. However, this simply is not the case. With TT systems, loop impedance values in excess of 100O are encountered on a regular basis. The MTB7671 solves this problem by checking the local loop value (Zs), then not only at Zs + 1O, but also at Zs + 180O. Therefore, the accuracy of the instrument over a much wider range of readings is verified.
The need for calibration
Clearly good test boxes have a lot to offer, but there is one more important and frequently asked question that needs to be considered. If a contractor uses a good test box, is there any need for them to have their instruments professionally calibrated? The answer is an emphatic yes.
Calibration is carried out in a laboratory using specialised equipment, the accuracy of which can be traced back to international standards. It makes sure that the instrument is giving accurate results over its entire range, which even the best test box can’t do. In addition, while they’re working on an instrument, the calibration technicians will make any minor adjustments needed to ensure that it keeps on delivering the best possible results.
Regular use of a check box could mean that calibration is needed less frequently, but it is important to remember that the actual calibration interval should always be decided on the basis of a risk assessment.
In conclusion, if you are a conscientious contractor and you want to be sure that your testing and certification work always meets the requirements of the IEE Wiring Regulations, throw away that pocketful of resistors and buy a good test box.