In testing conditions
August 1st 2005

Changes in standards and legislation are increasing concerns about safety in electrical testing. Mark Hadley of Megger discusses the changes, their implications, and what can be done to minimise the risks involved in testing work.

If mistakes were never made in electrical testing, there would be far fewer safety concerns. In practice, mistakes are inevitable, especially as most testing is carried out in poor working conditions, on badly labelled circuits or under pressure as testing is inevitably the last stage of a project, and everyone wants the job finished.

The potential for testing mistakes has recently been further increased by the introduction of a new standard for cable colours. This means that electrical contractors are working with an unfamiliar colour system and, in many cases, with a potentially confusing mix of old and new cable colours.

Another important factor is the introduction of Part P of the Building Regulations. This has led many plumbers, heating installers and the like to become involved with electrical testing, so that they can certify their own work. Here the temptation may be to choose the cheapest instruments available without considering aspects such as safety and reliability, something that is not obvious from a cursory look at the instrument. If mistakes in testing are going to be made, what can be done to minimise the safety implications? Fortunately, quite a lot.

A common mistake is to connect a test instrument to a live supply when it is set for a test that should be carried out on a dead circuit. When this happens, particularly if three-phase supplies are involved, the instrument is severely stressed. Older types will almost invariably be damaged, and the user may be exposed to electric shock risks. Good modern instruments, however, incorporate a range of safety features to guard against these problems. As a minimum, they are ‘three-phase compliant’, which means that they will remain safe and undamaged if accidentally connected to a live supply.

Further, the best of modern test instruments do much more to increase testing safety. For example, live circuit warning and automatic test inhibit are highly desirable features. With these, if the instrument is inappropriately connected to a live circuit, audible and visual warnings immediately alert the user. In addition, the selected test is instantly terminated.

Not all tests can be performed without making circuits live. In loop impedance testing, for example, the circuit is tested live. This presents few problems if the earth bonding of the installation is satisfactory but, if it is not, metalwork such as radiators and plumbing may become live, creating a shock hazard. This problem is best addressed by using an instrument with a touchvoltage provision. Before starting the main test, this performs a safe low-voltage check on the circuit to confirm the adequacy of the earth bonding. If there i a problem, an alarm alerts the operator and the main test is inhibited. The touch-voltage provision is so called, as some older instruments require users to touch a metal pad while the test is being carried out. With newer instruments, this is unnecessary, which means that the test can be fully automatic, providing the highest possible level of safety. One further, less obvious, safety measure which leading instrument manufacturers are addressing is simplicity. As we have seen, testing is usually carried out under pressure, and sometimes by people with limited experience and expertise.

Making the test instruments as easy as possible to use by eliminating unnecessary features, and by providing clearly labelled functions – possibly even colour coded – makes a big contribution, therefore, to safety. As a bonus, this simplification also speeds up testing which helps users to address those ever-present time pressures.

A general guide to safe instruments is to look for the IEC61010 and EN61557 declarations that should be on every instrument or in its user guide. Conformance to these standards is a good indication that the product should be safe to use and meet the necessary performance requirements.

With many factors currently conspiring against safety in electrical testing, it’s important to take measures to minimise risk. These include discarding old instruments which don’t meet modern standards, and choosing new instruments wisely. Not only should they provide the safety features discussed, they should be sourced from a manufacturer like Megger that has a long and demonstrable commitment to safety in testing.

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