When testing electrical systems of any kind, it’s essential to make sure that the test equipment being used is suitable for the task in hand. If it’s not, there is a significant risk not only of damage to the test equipment and the installation, but also of injury to the user. That probably seems so obvious that it’s hardly worth mentioning. After all, how many technicians or engineers would use unsuitable equipment for testing? The answer is that few would do so knowingly, but many may be doing so every day without even realising that there’s a problem. And that problem relates to transients. All electrical installations experience transients, which are voltage spikes that are super-imposed on the normal supply. Although these spikes are usually of very short duration – typically they last just a few microseconds – their amplitude can be thousands of volts. These transients come from a variety of sources, but one source that is surprisingly common even in temperate climes is lightning strikes. Note that a direct hit on the installation doesn’t have to be involved, nor even a hit on the power lines supplying it; a nearby strike is often enough to produce a large transient.
But what have transients got to do with testing and safety? To answer this question, let’s examine what happens if you’re carrying out a test – which could be something as simple and routine as checking the voltage of an LV supply – when it experiences a transient. If the instrument in use has not been specifically chosen to be suitable for the type of work being carried out, there’s a very real risk that the transient will cause a flash over inside the instrument and set up an arc.
Because its duration is very short, the transient itself is unlikely to have enough energy to do a lot of damage. Unfortunately though, once it is established, the arc provides a low
impedance path for current from the mains supply. That current flow releases a lot of
energy inside the instrument. Of course, the circuit’s protective device, whether it’s a fuse
or circuit breaker, will quickly disconnect the supply and interrupt the fault current.
Before this has time to happen, however, the energy released within the instrument is enough to cause real problems. In the worst cases, the instrument may explode, injuring or even killing the person who is using it. Even in less severe cases there is a definite risk of fire and damage to the equipment under test as well as to the instrument itself.
It’s clearly important, therefore, to choose an instrument that has been designed to withstand the level of transients it’s likely to encounter in use. But how can you tell? The
answer is to look at the instrument’s category rating, which is more commonly called its CAT rating.
CAT ratings are defined in the IEC 61010-1 standard, and are specifically intended to address the issue of transients in the testing of low-voltage installations. To understand how the ratings work, it’s necessary to look at what happens to transients as they pass through a typical electrical installation.
Outside the building and at the point where the mains supply enters the building, the transients have their highest amplitude. For testing in these locations, only instruments with a CAT IV rating are suitable. Transients are, however, quickly attenuated by the wiring and equipment in an electrical installation. Once the supply has passed through the main switchboard, therefore, the amplitude of the transients is much lower, and instruments with a CAT III rating (or higher) can be safely used. At the final circuit outlets, the transient levels are lower still, and CAT II or higher instruments can be used without problems.
What about CAT I instruments? These are for use within appliances such as VDUs and photocopiers. In practice, major suppliers of instruments designed for professional use are
unlikely to offer CAT I or CAT II instruments, as their area of safe usage is so limited. That’s not quite the whole story, as CAT ratings must always include a voltage – for example, CAT IV 300 V. This voltage is the maximum RMS phase-to-earth voltage of the system on which the instrument is suitable for use. This means, for example, that instruments with a 300 V rating can be used on singlephase systems up to 300 V and three-phase systems up to 520 V, making them suitable for the vast majority of low-voltage applications.
There’s one final point to mention. It would be easy to think that insulation testers and other instruments designed for use on dead circuits didn’t need a CAT rating. Remember, however, that these instruments could be accidentally connected to a live supply, and also that many of them incorporate facilities for some live circuit tests, such as measuring the supply voltage. The CAT rating is, therefore, still relevant for these types of instruments.
Once the significance of the CAT rating system is understood, it’s not difficult to choose an instrument that’s appropriate for the type of work being undertaken. As a general rule of
thumb, a CAT III 300V rating is likely to be the minimum acceptable for general use.
It is, however, well worth considering investing in CAT IV instruments, as these can be used without restrictions anywhere within a normal installation. Many utility companies
and other major purchasers of instruments are, in fact, now specifying CAT IV instruments as standard, since they deliver an extra level of safety in return for a very modest additional investment.
By Simon Wood, Megger UK wholesales and distribution sales manager. Excerpt from the July 2013 edition of Megger’s Electrical Tester.
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