Feature Test & Measurement
Making the right choices M
ulti-function testers designed for general purpose installa- tion testing and handheld insulation test sets are also useful for testing industrial installa- tions. This is because most of the elec- trical installations in a typical factory closely resemble the installations found in commercial and domestic premises. And, of course, all UK installations have to conform to the same requirements laid down in the IET Wiring Regulations. However, there are some factors that are essential to take into account when choosing instruments for industrial use. For example safety - industrial installations are likely to be subject to a higher level of transients than com- mercial or domestic installations and testing is more likely to be carried out
like voltage measurement, which are used for testing on live circuits. Even if the instrument doesn’t have these func- tions, it’s easy to accidentally connect a dead-circuit tester to a live circuit. An appropriate CAT rating is therefore essential for all types of instrument.
Differences
Above: Simon Wood, Megger
Left and right: multi-function installation testers are eminently suitable for a wide range of industrial testing applications
While many parts of industrial electri- cal installations are similar to those in commercial and domestic properties, there are parts that are significantly dif- ferent. In particular, the use of 3-phase supplies is common. Lack of 3-phase facilities limits the usefulness of older multi-function installation testers in industry, but not the latest models. The best of these can check phase rotation, fully test three-phase RCDs without neutral connections, and carry out phase-to-phase loop and PSCC tests. The better multi-function testers also provide extra facilities that are of frequent use in industrial applications. These include the ability to measure the resistance of earth electrodes, to test the less common types of RCD, including programmable types, and to accurately measure earth leakage cur- rents. Some of these options may require the use of accessories, but these accessories always cost much less than a specialist instrument.
near to the source of supply. This makes the instrument’s IEC 61010 CAT rating particularly important. For industrial use, instruments should be rated CAT IV 300V or better for connection to low voltage systems without additional protective devices. This means they are safe for connec- tion anywhere on the system right up to the source transformer. It’s tempting to think that CAT rat- ings are relevant only for instruments that are routinely used for live circuit tests and not instruments like insula- tion testers that should be used only on dead circuits. However, it’s important to remember that most modern insula- tion testers have ancillary functions
Electrical Engineering DECEMBER/JANUARY 2013
Insulation testers Turning to insulation testers, a hand- held instrument with an appropriate CAT rating, capable of testing at 250, 500 and 1,000V will be suitable for much of the testing required in indus- try. The instrument must, however, give meaningful readings up to very high insulation resistance values - ide- ally 100GΩ or more.
This is because trending insulation resistance values over time gives an excellent indication of the true state of insulation. If the values are falling con- sistently, investigation is necessary, even if the actual insulation resistance value is above the ‘minimum accept- able’ value. Only numerical results can be trended and ‘infinity’ isn’t a number, so a tester that reads infinity for all values above 1GΩ wont provide any indication that insulation has fallen in resistance from, say, 20GΩ to 5GΩ, even though such a large fall suggests that there is a problem.
While specialist equipment is essential for electrical testing in some parts of industry, many everyday requirements can be satisfied with the same equipment that’s routinely used to test building installations. However, this does hinge on whether the tester has been chosen wisely. Megger’s Simon Wood explains
In industrial applications, an insula- tion tester with facilities for dielectric absorption ratio (DAR) and polarisation index (PI) testing is even more useful, as these tests provide results that can be compared with known typical values for a specific item of equipment, such as a motor. DAR and PI values are also largely independent of temperature.
Note, however, that these values are derived from insulation resistance mea- surements, so it is still important that the instrument can meaningfully mea- sure high values of resistance. For instruments to be used in industry,
there’s another important aspect to con- sider - ruggedness. No one ever intends to drop an instrument or to use it where it will get splashed with water, but these things happen. Therefore, if the instru- ment is to have a long life, it must feature rugged construction and a good ingress protection rating - IP54 is ideal.
Summary
When their versatility is taken into account, the best multi-function instal- lation testers and handheld insulation testers offer outstanding value for money and, as we’ve seen, they are eminently suitable for a wide range of industrial testing applications. So, if you buy or specify test equipment for industrial use, don’t neglect contractor type products. Chosen wisely, they provide a lot of functionality for sur- prisingly little money.
Megger
www.megger.com T: 01304 502 101
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