Who tests the testers?

It’s all very well testing electrical installations in line with the IEE Wiring Regulations, but how can you be sure that your results are accurate? Unless you check your instruments regularly, the answer is you can’t.

Unless there’s an obvious problem, few contractors give a thought to the accuracy of their instruments. Yet inaccurate test results are worse than none at all, as 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’s easy to assume annual calibration satisfies the requirement, but that’s not always the case. A lot can happen to an instrument within a year and although modern instruments are remarkably resilient, there’s no doubt that accidents which adversely affect calibration can and do occur.

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, a number of suppliers have now started to offer test boxes.

The idea is simple enough. The boxes contain known standard components which can be used to evaluate the performance of insulation, continuity, earth loop impedance and rcd testers. They’re designed to be relatively easy to use and usually come with comprehensive instructions. Unfortunately, products currently on the market don’t deliver what they promise.

Firstly, let’s look at checking continuity testers and the continuity functions of multifunction testers. Most test boxes just provide a couple of resistors of known values, typically a few ohms. To check an instrument, the user plugs it in to the test box and, providing readings are very close to the values written on the test box, everything is ok…or is it?

Certainly, this simple test confirms the instrument can measure a couple of specific values of resistance correctly. But what about other values? Most continuity testers have a measurement range of around 200 ohms and confirming correct readings with regards to a few values at one end of the range doesn’t give an indication of overall performance.

If a tester has a range of 3000 megohm, what good are a couple of tests at around 0.5 megohm?

And this is not the only problem. To comply with the IEE Wiring Regulations, a continuity tester must be able to deliver a short circuit current of 200 mA and an output voltage between 4 V and 24 V. The simple check with a known value resistor does nothing to confirm the instrument meets these requirements. And if it doesn’t, then it doesn’t comply.

When we look at insulation testing, the situation is similar. The usual test box provides a resistor or two, often around 0.5 megohm. If the instrument under test displays this value correctly, all is considered to be well. But, if a tester has a range of 3000 megohm, what good are a couple of tests at around 0.5 megohm?

Also, insulation tests must be carried out at a defined voltage – often 500 V. The simple test box does nothing to confirm the instrument is delivering the correct voltage, nor that it is capable of delivering a current of 1 mA, as required by the regulations. A test carried out at reduced voltage places less stress on the insulation, which means faults can be missed.

The situation is no better with loop testing. Most of the test boxes are arranged to allow the instrument being tested to measure the loop impedance of a circuit chosen by the user. Then a 1 ohm resistor is added and the measured result should show a 1 ohm increase.

This is fine, except that many loop testers can read up to around 3 kilohms, and testing the accuracy right at the bottom of this range provides little information about accuracy at the higher readings. If anyone thinks this doesn’t matter, they should remember that, in TT systems, acceptable loop impedance values in excess of 100 ohms are sometimes found.

Finally, let’s look at rcd testing. The problem here is not so much the validity of the results provided, but with the inconvenience of getting them. When used to verify the performance of an rcd tester, all test boxes will trip any rcd protecting the circuit from which they are powered. The only solution is to power them from a circuit that is not rcd protected, but this can be difficult or even impossible.

There’s no doubting the need for instrument test boxes as they have an important role to play in guaranteeing the reliable test results needed to ensure the safety of electrical installations. Those currently available, however, leave much to be desired – clearly there’s a need for better ways of testing the testers.