Continuity in testing

29 March 2017

Published in 1882, the earliest forerunner of the current 17th Edition of the IET Wiring Regulations warns, “the difficulties that beset the electrical engineer are largely invisible, and they can only be effectively guarded against by testing,” so it’s fair to say that testing has been with us since the earliest days.

The equipment available in 1882 for testing electrical installations was, however, basic in the extreme. Insulation tests, for example, were carried out using batteries and a galvanometer. That changed in 1897, when Sidney Evershed of Evershed & Vignoles developed the first practical insulation tester and, just a few years later in 1903, registered the trademark Megger for his ground breaking instrument. 

Since then, Megger – the name was eventually adopted for the company as well as for the product – has been continuously involved in the development and manufacture of innovative test equipment. Today the company continues to produce some of the UK’s most popular and successful ranges of instruments for installation testing.

Unsurprisingly, since 1903, test equipment has changed almost beyond recognition. Insulation testers are still widely used, but today’s models measure with electronic circuitry rather than with moving coil meters and they no longer require the user to crank a handle to generate the test voltage. 

The best current models have dual analogue/digital displays. These are useful because the stability of insulation resistance, which tells an experienced user a lot about potential problems, is most easily seen on an analogue display whereas the exact value of the insulation resistance is easier to read on a digital display.

Insulation testing is, of course, not the only form of testing that’s needed for electrical installations. It’s essential, for example, to be able to measures earth loop impedance and testers for this purpose first appeared around 1970. 

Early models used a high test current, typically around 10 A, which gives fast accurate results. High-current testing is preferred in most  applications, but it does have a big drawback – if the circuit is protected by an RCD or RCBO, it will cause them to trip. To address this problem, loop testers have been developed that use much smaller test currents – just a few milliamps – which will not trip an RCD or RCBO. 

Until recently, these low-current testers needed a three-wire connection (line, neutral and protective conductor) to the circuit under test. Usually this isn’t a problem but there are some situations – at a light switch, for example – where a neutral connection isn’t available. A recent development from Megger, however, is low-current non-tripping loop testers that need only a two-wire connection (line and protective conductor). This is very convenient unless high levels of electrical noise are present in the circuit under test, when it is still preferable to use three-wire testing.

RCDs and RCBOs are now very common, particularly since the introduction of the 17th Edition of the IET Wiring Regulations, which requires them to be used on the majority of circuits in domestic and commercial properties. Instruments for testing RCDs first started to appear in the 1980s, but the latest versions from Megger incorporate important additional features such as automatic testing, which is a big time saver.

Without it, users have to walk between the tester and the RCD to reset it for every one of the five tests that have to be performed. With automatic testing, the user simply initiates the whole test sequence once, then walks to the RCD and resets it each time it trips. He or she doesn’t need to return to the instrument until the whole test sequence has been completed.

In addition to the developments discussed so far, the last 20 years has seen another significant change in installation test equipment: the growth in popularity of multifunction testers which has largely been driven by the success of Megger’s MFT1500-series and more recently MFT1700-series instruments. These provide all of the functionality needed for testing electrical installations – and often much more besides – in a single instrument. 

Many users prefer these because they are easier to carry around than separate instruments for each type of test, and because the cost of a multifunction tester is usually somewhat lower than a set of individual instruments that would provide similar functionality.

Finally, let’s look at an issue that makes modern instruments very different from their early ancestors – safety. Early instruments had few safety features and relied on expert, careful usage to guard against damage and injury. The best of modern instruments, in contrast, have safety built in. Megger’s latest testers, for example, have CAT IV 300 V safety ratings in line with IEC 61010 and protection features that make them resistant to damage even if they are connected to a three-phase supply after a dead-circuit test has been selected and locked on.

For some engineers and technicians, vintage test equipment has the same sort of appeal as a vintage car. The same provisos also apply, however. You probably wouldn’t want to rely on a vintage car to get you to and from work every day, and you would certainly hope that it’s safety features – or lack of them – were never put to the test. In short, whether it’s a car or an installation test set, the past is interesting but staying up to date brings a lot of benefits!


1882    First UK wiring regulations published

1897    Sidney Evershed invents the world’s first practical insulation tester

1903    Evershed & Vignoles, forerunner of Megger, registers Megger trademark

1908    Evershed & Vignoles introduces Ducter low-resistance ohmmeters

1923    ACWEECo, now part of Megger, launched the Avometer, the world’s first multimeter

1960    First earth resistance testers

1970    Introduction of loop impedance testers

1980    First modern RCD testers

1996    Megger introduces its first multifunction installation tester

2003    Megger’s market-changing MFT1500 series multifunction testers launched

2011    Megger releases MFT1700 series multifunction testers with unique two-wire loop test feature

2016    Megger launches MFT1701 series multifunctional tester with added features