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Published by Megger
Nov 2010 |
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| The industry's recognised information tool |
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ELECTRICAL
TESTER |
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In this issue |
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Keep the noise down! |
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Warning! Safety first |
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Q&A - Earth Testing |
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Keep the noise down! |
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| Paul Swinerd |
| Product Manager |
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| Electrical noise is the enemy of accurate measurement, and it is often a particularly acute problem in high-voltage insulation testing. But what exactly is electrical noise, what are its effects and what can be done about it? Paul Swinerd supplies the answers. |
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| What is electrical noise? |
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| The term electrical noise is used to describe a whole range of phenomena, but the most general definition is spurious electrical or electromagnetic energy that produces an unwanted effect. In a hi-fi system, the unwanted effect might, for example, be background hiss, but in measurement systems it most usually manifests itself as inaccurate or unstable readings. |
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| In measurement applications, noise takes the form of voltages and currents induced from adjacent equipment. This is very common in substations, and particularly in high voltage substations where induced noise at power frequencies predominates. |
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| What is the effect of noise? |
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| In insulation testing, electrical noise superimposes an AC signal on the DC test current. This can cause readings to vary erratically, and can even prevent a reading of any kind being obtained if the level of noise exceeds the capabilities of the instrument. Many operators see this as something they have to live with – a form of occupational hazard – but, as we shall see, this doesn't have to be the case. |
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| One solution of course, is to implement a complete shutdown of adjacent plant to eliminate the noise source, but this is in many cases both costly and inconvenient. More often, noise leads to tests being omitted, which is very undesirable since the objective of diagnostic insulation testing is to prevent expensive and dangerous failures. |
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| How can insulation testers deal with noise? |
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| Noise immunity can be designed into insulation testers and indeed, instruments that are sold in the EU must meet the EMC requirements of the latest edition of IEC 61326-1, which came into force in February 2009. Megger has its own EMC laboratory at its Dover site, which it has used to ensure that all of its latest MIT and S1 5 kV and 10 kV not only meet this standard, but also conform to the requirements for heavy industrial use. |
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| However, even that is not always enough. Experience has shown that in extreme environments such as HV substations, instruments can be subjected to levels of noise far in excess of those laid down in IEC 61326. That's why it's essential not only to look for standards compliance, but also to take into account the noise immunity specification of an instrument. |
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| What do noise immunity specifications mean? |
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| It's very easy for an instrument manufacturer to say that its HV insulation testers have high noise immunity, but unless the level of immunity is specified, such claims are worthless. But exactly how is noise immunity specified? |
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| A typical specification might say that the instrument has an immunity of 2 mA at 50/60 Hz. This means that if the noise current induced in the test circuit at power frequency is 2 mA or less, the instrument will give accurate and reliable results. |
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| Specifying the maximum permissible noise current at power frequency is actually considering the worst case, as instruments usually incorporate capacitive filtering that increases in effectiveness as frequency rises. As a result, the noise immunity of the instrument also increases with frequency. This can be very useful, for example, with corona discharge on bushings, which typically generates electrical noise with frequencies in the kilohertz range. |
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| While noise immunity of 2 mA is adequate for the majority of applications, there are extreme environments such as substations operating at 300 kV and above, where even this isn't enough. For applications of this type, Megger has produced HV insulation testers that not only incorporate specially developed input filtration to minimise the effects of high frequency noise, but also employ firmware filtering to remove low frequency effects. |
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| These instruments have a noise immunity of 4 mA at power frequency, and have been successfully used in some of the world's noisiest switchyards. Nevertheless, the question remains – can noise levels exceed 4 mA and if they do, what can be done about it? |
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| The answer is that noise levels of more than 4 mA are very rare, but they are not unknown. They may be encountered for example, where making connections to the bushings on the top of a transformer involves the use of very long test |
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| How can the effects of noise be reduced? |
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| One of the most effective ways of minimising noise pick up is to take care with the test lead layout. In particular, keep the leads as short as possible and route them near to grounded objects, such as the casing of a transformer. |
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| leads, since these act as effective aerials for picking up noise. In these circumstances, the best course of action is to take steps to minimise noise pick up in the first place. |
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| Another effective option is to use screened test leads. The short test lead between the insulation tester and ground will not pick up enough noise to cause problems, but it is often beneficial to use a screened lead for the longer connection to the equipment under test. Megger offers suitable leads in lengths of 3 m, 10 m and 15 m. |
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| When screened test leads are used, the screen is connected to the guard terminal on the insulation tester. This ensures that noise currents are diverted away from the measuring circuits and are, therefore, ignored. The guard terminal is also used in the normal way to eliminate the effects of leakage currents. |
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| It is important to note that shielding is only effective in reducing noise pick up on the test leads. If noise is picked up on the test piece itself, as might well be the case with long overhead power cables, for example, there is no substitute for using an instrument with high noise immunity. |
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| Electrical noise is undoubtedly a troublesome issue in HV insulation testing, especially in substation environments. By choosing instruments with high noise immunity however, and using shielded test leads where appropriate, it should be possible to make accurate and dependable measurements in even the most challenging of circumstances. |
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Warning! Safety first |
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| Jeff Jowett |
| Applications Engineer |
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| Safety should be the first concern when preparing to carry out electrical testing, but all too often it's taken for granted. Lack of attention to safety requirements and lack of sufficient expertise can, however, have life threatening consequences. This two-part article, the second part of which will appear in the next issue of Electrical Tester, explains how safety requirements should be systematically and thoroughly assessed before testing commences. |
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| The first essential for safe testing is to keep in mind that safety involves equipment, procedure, and the test item. Each should be considered in turn. Beware of focusing entirely on one aspect and letting the others take care of themselves. Equipment, for instance, may have adequate or even superior safeguards while the item being tested presents an overlooked danger. Of paramount importance is the degree of protection against arc flash/arc blast. This combines elements of all three: the test instrument, the test item, and procedure. |
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| Arc flash protection is covered by the standard EN61010-1:2001, issued under the aegis of the International Electrotechnical Commission (IEC). The rating defines the level of spike or surge transient the instrument has been designed to withstand. Remember, the rating pertains to transient voltage, not line voltage. Spikes can be many multiples of line voltage and can cause test instruments that happen to be connected at the time to arc internally. The arc can produce tremendous heat, violent expansion of air in a small space, exploding the instrument and exposing the operator to burns, shock waves and flying particles. The key to safety is to design the instrument so as to minimize the risk of internal arcing, but that isn't enough. The operator must understand the rating system and use the instrument accordingly. The standard defines clearance and creepage distances between critical parts within the instrument. The degree of protection is interpreted as a Category ("CAT") rating, plus a voltage limitation. |
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| CAT ratings are assigned from I to IV, although CAT I is nowadays of little practical use. Instruments with higher category ratings have better capacity to withstand transients. The ratings indicate the position of the circuit under test “downstream” from the transformer serving the premises. Energy dissipates with attenuation and therefore so does risk. CAT IV is assigned to the utility feed from the transformer to the service entrance, CAT III is from the fuse panel to an outlet, and CAT II is downstream of the outlet. |
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| The rating doesn't stop here, though, but also must include a voltage limit for the rated voltage of any system being tested, because CAT rating is based on multiples of system voltage. Some instruments list a CAT rating but do not specify the voltage. These should be avoided, as it is an indication of shortcutting for economy in design. To ensure safety, the operator must use an instrument with a CAT rating matching or exceeding that of the system being worked on. |
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| A recent study by a utility company has indicated that using an instrument rated one category lower than the application requires increases the chances of an accident by a factor of 30. Looked at another way, this means that if 100 operators used instruments of the wrong rating connected to live systems for one hour per day over a 200-day year, a dangerous situation is likely to occur every 18 months. These are not good odds for anyone intending to spend a career in the industry! |
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| After CAT rating, it's time to consider the many other safety features that are designed into quality equipment. A worthy illustration relates to the common practice of insulation testing. Years ago, poorly designed test equipment would leave the operator unprotected, and extra diligence was |
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| required to avoid being shocked, or worse. Remember, while many in the industry take non-lethal “shocks” for granted, they can produce unexpected consequences like falling from a ladder or catwalk or jostling a nearby person who is working close to dangerous machinery. Better to eliminate the issue altogether than to try to live with it. |
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| Testers now come with additional safeguards that didn't exist years ago. A special safety hazard exists with insulation testing because the tester will charge up the capacitance and absorption inherent in the item being tested. This is a prime example of how the test item can be an unrecognized source of danger. Insulation tests are always performed on de-energized equipment. Therefore, given the safety features of the tester itself, it's easy for the operator to become complacent and think that he or she is working in a completely safe environment. Not necessarily so! |
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| The test item can store a lethal static charge in its capacitance and polarization of molecules in the insulating material. At termination of the test, with the field gradient provided by the tester now removed, the charged item will generate a relaxation or reabsorption current. The operator does not want to become part of this discharge circuit! |
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| Older testers at best had a discharge switch, if any protection at all. Operator involvement was paramount, as it was easy to forget to engage this switch. Modern testers virtually eliminate the chance of operator error by providing automatic safe discharge. When such a tester senses extraneous voltage (not provided by the tester itself), visual and audible warnings are engaged, and the actual voltage may be automatically displayed, with flashing symbols to ensure the operator's attention. |
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| A discharge circuit will then safely dissipate the stored charge and monitor it on the display, so that the operator will not get across the terminals until the voltage falls to less than 50 V. This warning circuit will of course, operate at any time during the course of a test, so that if the operator accidentally connects to a live circuit, or someone closes a breaker or flips a switch while the test is in progress, the operator will be immediately warned. But beware of fuse protection; make sure it's properly integrated with other functions. In some units, a blown fuse disables the protection circuits; in others, it doesn't. If a blown fuse has disabled the alarms, the operator may not realize what has happened and not be warned if the tested equipment becomes live. |
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| One might also want to consider job protection along with personal protection. Before instruments were provided with modern safeguards, it was common for operators to cook not themselves but the tester. Manufacturers of test equipment were flooded with “warranty” returns that had burn tracks across the terminal boards. And their answer was invariably, “Sorry, the warranty does not cover connection to a live high voltage circuit.” Operators were disregarding voltage indications, if they existed at all, and proceeding to engage the test button. As soon as they did, the tester was ruined. |
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| Well-designed modern testers remove this source of anxiety and embarrassment by disabling the test circuit in the presence of extraneous voltage. The operator presses the test button and nothing happens. Phew! De-energize the circuit and proceed with the test! One factor to be aware of, however, is the possibility of low voltage background noise, such as crosstalk on communications circuits. Testing may have to proceed in the presence of this, so the tester must have an appropriate threshold that will not disable it in such situations. |
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| This article is continued in the next issue of Electrical Tester. The second part will cover further issues relating to the safe use of test equipment and will include information about test leads, multifunction testers and ground testers. |
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Q&A - Earth Testing |
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| In this issue, experts from the Megger Technical Support Group supply answers to the questions they most frequently receive about earth testing. |
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| Q: Most of the time I find that clamp-type testers provide a quick and convenient way of measuring earth resistance. Where space is tight, however, I sometimes have trouble getting the clamp round the cable or earth strap. Is there a solution? |
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| A: The root of this problem is two fold - most earth resistance testers have clamps with a round aperture and a long body length. While round clamps make life easier for the instrument manufacturer, they're not really ideal for getting into inaccessible places, or for clamping round earth straps, which usually have a rectangular cross section. However, Megger's new DET14C and DET24C models, have elliptical clamps and a short body length making them much more versatile. In addition, the Megger instruments have a pre-hold key, which makes them easier to use in restricted locations as the meter can be simply clamped around the cable or electrode and then withdrawn before reading the display. |
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| Q: After I've been using my earth clamp meter for a while, especially in harsh environments, I find that it starts to give inconsistent and inaccurate results. Why is this and what can I do about it? |
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| A: This kind of problem is usually an indication that the jaws of the clamp are not closing properly. In many earth clamp meters, the mating faces of the jaws take the form of laminations that interlock when the clamp is closed. This arrangement is particularly susceptible to contamination – it only takes a small particle of grit or dirt to prevent the laminations from interlocking and, when this happens, the meter will not give accurate or reliable results. The short |
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| term solution is to carefully clean the mating faces of the jaws. There is, however, another problem associated with instruments of this type – the laminations in the jaw faces are thin and so are easy to damage. When this happens, the only certain remedy is to return the instrument to the manufacturer for repair and recalibration. Attempts to straighten the bent laminations without the proper equipment are likely to make things worse and, after an ad-hoc "repair" of this type, there's no way of knowing whether the instrument is giving accurate readings. A better and longer lasting solution is to choose an earth clamp tester that has flat jaw faces, as these are much easier to keep clean and far less susceptible to damage. |
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| Q: What is the point of the ground leakage current measurement feature that is offered by some earth resistance clamp meters? |
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| A: Measuring earth currents, preferably with an instrument that gives true RMS readings, is a good way of pre-empting potential problems with earth resistance tests, as large standing earth currents can affect the accuracy of the results obtained. The current measurements also provide a very useful indication of the overall dynamics of the earth system. In addition, should it become necessary to disconnect an electrode, it's a very good idea to measure the leakage current flowing. A high leakage current will become a dangerous live voltage when the cable is disconnected. Again, Megger's DET14C and DET24C have been designed with this in mind. The instrument has a very high resistance to 'noise' current, and includes an automatic firmware filter to smooth out varying readings. Another additional user benefit is the automatic current warning which operates even when in resistance range – just in case the user forgets to measure it. |
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