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Don’t take a chance on your CAT!

Simon Wood UK wholesale and distribution sales manager

HV Supply

W

EN 61010-1 categories

ment is suitable for use. This means, for example, that instruments with a 300 V rating can be used on single Outside the building and at the point where the phase systems up to 300 V and three-phase systems up mains supply enters the building, the transients have to 520 V, making them suitable for the vast majority of their highest amplitude. For testing in these loca- low-voltage applications. tions, only instruments with a CAT IV rating are There’s one final point to mention. It would be easy suitable. to think that insulation testers and other instruments Transients are, however, quickly attenuated by designed for use on dead circuits didn’t need a CAT ratthe wiring and equipment in an electrical installa- ing. Remember, however, that these instruments could tion. Once the supply has passed through the main be accidentally connected to a live supply, and also that switchboard, therefore, the amplitude of the tran- many of them incorporate facilities for some live circuit sients is much lower, and instruments with a CAT tests, such as measuring the supply voltage. The CAT III rating (or higher) can be safely used. At the final rating is, therefore, still relevant for these types of incircuit outlets, the transient levels are lower still, and struments. CAT II or higher instruments can be used without Once the significance of the CAT rating system is unproblems. derstood, it’s not difficult to choose an instrument that’s appropriate for the type of work being undertaken. As What about CAT I instruments? These are for use within appliances such as VDUs a general rule of thumb, a CAT III 300V rating is likely and photocopiers. In practice, major suppliers of in- to be the minimum acceptable for general use. struments designed for professional use are unlikely to offer CAT I or CAT II instruments, as their area It is, however, well worth considering investing in CAT IV instruments, as these can be used without reof safe usage is so limited. But what have transients got to do with strictions anywhere within a normal installation. Many testing and safety? companies and other major purchasers of instru That’s not quite the whole story, as CAT ratings To answer this question, let’s examine what happens must always include a voltage – for example, CAT utility are, in fact, now specifying CAT IV instruments if you’re carrying out a test – which could be some- IV 300 V. This voltage is the maximum RMS phase- ments as standard, since they deliver an extra level of safety in thing as simple and routine as checking the voltage to-earth voltage of the system on which the instrureturn for a very modest additional investment. 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, Measurement Categories there’s a very real risk that the transient will cause a flash over inside the instrument and set up an arc. hen 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.

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 with-stand 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

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a typical electrical installation.

Measurement Categories are used to rate test instruments on their ability to resist a voltage spike, which is applied through a specific resistance. The higher the category, the more risk there is that a high voltage can overload a circuit and cause electrical and physical damage. Usually, the higher the CAT (category) rating, the safer the rating. Let’s go over an example. Measurement Categories are used in the maximum switching voltage rating in the specifications for National Instruments switches. The PXI-2584 specifications are: Maximum switching voltage Channel-to-ground....................................................... 300 V Category II, 600VDC, V ACpk Category I. Using the chart below we deduce the following: This specification informs the user this module is rated for 300V CAT II and 600V CAT I. In other words, this module can withstand up to 2,500V impulse voltage. This specification additionally informs the user this module must not be connected to MAINs CAT II circuits when operated above 300V. Lastly, the user should not use this module with Category III or IV circuits.

150 V 300 V 600 V 1,000 V Resistance

CAT IV 4,000 V 6,000 V 8,000 V 12,000 V 2 ohms

CAT III 2,500 V 4,000 V 6,000 V 8,000 V 2 ohms

CAT II 1,500 V 2,500 V 4,000 V 6,000 V 12 ohms

CAT III 2,500 V 4,000 V 6,000 V 8,000 V 2 ohms

CAT II 1,500 V 2,500 V 4,000 V 6,000 V 12 ohms

CAT I 800 V 1,500 V 2,500 V 4,000 V 30 ohms

News From Megger August 2013


News from Megger No. 6