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Published by Megger
May 2010 |
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ELECTRICAL
TESTER |
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In this issue |
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How safe are magnetic fields? |
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Megger shows off its wares worldwide |
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Oil Testing without slicks! |
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Guided innovation |
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How safe are magnetic fields? |
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| Dr Stan Zurek |
| Magnetics Technical Specialist |
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| We are all surrounded by magnetic fields emanating from the Earth and from various electromagnetic devices: motors, transformers, relays and even such common items as fridge magnets. But do we know how strong these fields are? And are they safe? As engineers and electricians, should we be worried when working in a “magnetised” area? Read on for some interesting answers. |
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| High frequency |
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| High frequency magnetic fields (GHz upwards) are capable of producing ionisation. The health implications of some types of field, such as those associated with mobile phones, are still being debated, whereas for the other types of field, like X-rays, the harmful effects are much better understood. |
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| From a health and safety point of view, ionising fields are treated almost as radioactive sources and are subjected to special rules and working restrictions. Their use is strictly controlled so there is generally a much smaller chance of dangerous exposure. For example, the radiated power of mobile phones is limited even though, after several years of research, there is no clear and conclusive evidence that the high frequency electromagnetic field that phones generate in normal use is dangerous. Nevertheless, the higher the frequency, the lower are the limits of safe exposure. |
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| Low frequency |
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| Lower down the spectrum, the mains frequency of 50/60 Hz produces electromagnetic fields of very long wavelength (over 6000 km) and therefore their influence is confined to the so-called “near field” region, where Faraday’s law of magnetic induction applies. The International Commission on Non-Ionizing Radiation Protection (www.ICNIRP.de) has published exposure guidelines, based on the current densities induced by time-varying fields. The safe DC field values are much higher, because a stationary field is not capable of inducing a current. |
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| Levels of magnetic field |
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| Magnetic fields are generated around electric currents. In air, gases and vacuum the field is directly proportional to the value of the current. The immediate implication is that for the same power, lower voltage systems will generate higher fields because the currents will be higher. For instance, a 20 V, 500 W battery operated drill will require 25 A DC. Similar drill supplied by mains of 230 V will consume only around 2 A. Thus the magnetic field in the first case will be over 10 times greater, but it will also have different influence on health because it is a DC current. (For the sake of simplicity, we are neglecting issues like electric field, arcing, etc.) |
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| Let us calculate the fields for the four simple configurations shown in Fig. 1: a single wire, two wires, a transformer and a very large coil that could surround the whole human body. |
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| Fig. 1. The magnetic field was calculated for four simplified cases (at 5 m distance) in relation to the body centre: single wire, a transformer (1 m high), two wires (separated by 0.5 m) and also for the whole body placed inside a 2 m coil |
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| The intensity of the field (flux density) reduces with the distance from the source. For a single wire the flux density values are as shown in Fig. 2. We can see that if the distance increases by a factor of ten, the field decreases by the same factor. |
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| Because the space between the source and the human is, in practice, filled mostly by air, the resulting flux density values are relatively small when compared, for example, with values inside magnetic cores. Low DC current (below 10 A) does not generate fields that are greater than magnetic field of the Earth. Such values are, therefore, perfectly safe. |
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| Fig. 2. Magnetic field levels around a single wire at various currents |
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| The ICNIRP guidelines for safe levels of static and time-varying electromagnetic fields were based on many |
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| research reports, including data from World Health Organisation. It was recommended that for the general public the safe level of DC magnetic field is up to 400 mT. As can be seen from Fig. 2, very large electric currents at short distances would be required to generate this level of field in air. Such fields can be generated with the help of permanent magnets and magnetic cores, but their range will be limited so that only fingers and, at worst, limbs will be affected. Of course for special cases, like for people with heart pacemakers, the safe limits are much lower – a value of 0.5 mT is mentioned. |
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| For AC fields the values are much lower and they depend inversely on frequency – the higher the frequency the lower the limit. For continuous exposure at 50/60Hz the recommended safe limits are 100 nT for the general public and 500 nT for professionals (occupational). Much lower AC currents are sufficient to generate these levels of magnetic field. |
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| A hand-held mains-supplied drill with a 2A current in the worst case of very close proximity to operator’s hands can generate field, which is slightly above the limit. However, such a tool would not be used all the time and the real exposure should be calculated as the average value over time. Much higher peak values can be safely tolerated if the exposure time is short. |
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| For instance, if the field is 10 times the safe limit, then as a rough approximation it can still be tolerated safely if the exposure is limited to a tenth of the time, e.g. six minutes in an hour. So it will be still perfectly safe to walk slowly past a 100 A busbar, provided that the exposure time is suitably limited, whereas touching such an energised high- or medium-voltage conductor would cause almost certain death, but for electrical rather than magnetic reasons. |
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| Magnetic field at a 5 m distance |
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| Let us look again at the fields from various sources as shown in Fig. 1. The 5 m spacing was chosen here arbitrarily, but engineers and electricians often work at this sort of distance from overhead lines, cables, transformers, etc. while still remaining safe from an electrical point of view. Even if the spacing were reduced to 2.5 m, the fields would simply double. |
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| Almost the worst-case scenario for normal circumstances is the magnetic field generated by a single wire (or a single busbar) as shown in Fig. 3. The magnetic field around a transformer will be significantly lower. This is because the field will be contained mostly in the magnetic core and the oil tank will provide further shielding. In fact, the highest magnetic field can be expected at the lower voltage bushings of the transformer – where the currents are highest. |
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| With a single phase supply the current flows in one wire and returns in another. The magnetic fields from the wires cancel to a large extent and so the resulting field is much lower. Similar effect occurs for three-phase supply, because the currents also balance out. The spacing affects the cancelation, but the multi-wire field will be always lower. The most effective cancellation happens in cables, where the wires are placed as close as possible to each other. |
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| Fig. 3. Magnetic field levels for the configuration from Fig. 1 |
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| In order to expose the body to a static field of over 2 T, the person would have to be INSIDE a large coil with 50 turns and over 50 kA current. Under such unreal conditions, the mechanical dangers (we are not even considering the electrical hazards) are actually much greater than the magnetic ones. The surrounding magnetic field will magnetise any tool made of iron or steel and this will cause a mechanical force proportional to the square of flux density and the area of the tool. For example a 10mm cube made of steel can be magnetised up to 2 T, which can result with a force of 160 N. Imagine what effect would have on a spanner in your pocket … |
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| This is exactly the reason that a person undergoing magnetic resonance imaging (MRI) has to strip almost naked and cannot have any metal parts on or in their body (e.g. implants). The MRI scanner can generate fields up to 7 T, which could potentially rip those metal bits from human flesh. |
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| Because of the lower exposure limits for AC fields, the situation is slightly different for alternating currents. Nevertheless, at a 5 m distance a three-phase overhead line carrying 30A is roughly within the safe limits, whereas the 1000 A cable is well within limits. And as mentioned above, much higher currents can be safely tolerated if the exposure is limited in time. |
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| So the conclusion has to be that low frequency magnetic fields are quite safe from magnetic point of view. They can also be extremely dangerous, of course, but for mechanical or electrical rather than magnetic reasons! |
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Megger shows off its wares worldwide |
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| A host of product innovations, especially in the power testing sector, combined with the cautious return of global business confidence are contributing to the enthusiastic response Megger is seeing to its presence at exhibitions around the world. |
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| In the first quarter of 2010, major exhibitions attended by the company included Elecrama in Mumbai, India; Middle East Electricity Exhibition in Dubai, UAE; IEEE PES Transmission & Distribution Conference and Exposition in New Orleans, USA; and Hannover Fair in Hannover, Germany. |
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| Elecrama, Mumbai |
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| “We’ve made a big investment in exhibitions this year to give as many people as possible the opportunity of seeing first hand our many new products,” said Nick Hilditch, Megger’s |
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| Group Marketing Services Manager. “These include, for example, innovative test equipment for evaluating the condition of power transformers, versatile and readily portable HV insulation testers, and ground-breaking solutions for testing IEC 61850 based sub-station installations.” And this investment has been well rewarded. Our stands have been busy at every single exhibition, with visitors showing a high level of genuine interest in our products and the opportunities that they open up for faster, more reliable and more convenient testing. On the basis of this evidence, we confidently predict that 2010 will be an excellent year for sales growth around the world.” |
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| Hannover, Germany |
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Oil Testing without slicks! |
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| Paul Swinerd |
| Product Manager |
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| Testing the quality of insulating oil is an invaluable guide to the condition of many kinds of large electrical plant, including power transformers. However, oil testing is frequently seen as a messy and inconvenient procedure that all to often produces results of dubious accuracy. These problems, however, have their roots not in the testing techniques themselves, but in the shortcomings exhibited by many of the oil test sets currently in use. |
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| Let’s take a look at the most important of these shortcomings, and see what might be done to address them. One of the most common complaints from users of oil test sets is that they are messy. In most models, for example, any oil spilled in the instrument is retained – which is good, because oil that finds its way onto the floor is a definite safety hazard. |
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| The problem comes when its time to clean the instrument and remove the oil from its spill container. Almost invariably, the only way to do this is to use paper towels or similar absorbent material to soak up the oil, a procedure that can hardly be described as convenient. Might not a better solution be a built in drain tube, which can be used to transfer the spilled oil to a suitable container for disposal or recovery? |
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| Then there are the test vessels themselves. Traditionally, these have been made of glass, to ensure freedom from interaction with the oils under test. Glass vessels are, however, expensive to produce and easy to break. Today, however, glass is not the only material that’s suitable for use in oil test vessels. New moulded materials are now available that will not react in any with insulating oils, and these are a far better option. |
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| These moulded vessels are relatively inexpensive, which means, for example, that testing laboratories can easily afford to keep separate vessels for each different type of oil that they test, as many prefer to do. Moulded vessels are also much more robust than their glass counterparts – they’re not guaranteed to bounce if they’re dropped, but there’s a very good chance that they will. |
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| The use of these new moulded materials also allows much more freedom in the design of the vessel. This means that an effective pouring lip can readily be incorporated, allowing the vessel to be emptied conveniently without dripping hazardous oil everywhere. The vessels can also be produced in a shape that makes them quick and easy to clean. On the subject of cleaning, another common complaint about current oil test sets is the difficulty of accessing the electrodes to allow thorough cleaning. This is purely a matter of design, of course, and there are no intrinsic reasons why machines can’t be produced that allow unhindered electrode access. |
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| Many existing oil test sets also have operational shortcomings. Precise setting of the electrode spacing is, |
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| for example, essential if reliable test results are to be obtained. But precise setting alone is not enough – provision must be made to ensure that the setting is not accidentally altered during testing. Test sets often fail in this respect by providing the facilities needed to set the electrode spacing accurately, but no option to lock the electrodes in place after the spacing has been set. |
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| Then there’s the question of test options. Portable oil test sets capable of testing at up to 80 kV are few and far between, as are instruments with the option of carrying out tests that detect breakdown by monitoring current, as required by IEC standards, or by monitoring voltage, as required by ASTM standards. |
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| Of course, there are many other features that would be useful to some, but not necessarily all users. These include, for example, an integral printer or, for portable models, a choice of battery types. Rather than increase costs by building these into every instrument, why not build the instruments to order, to the exact specification required by the customer? |
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| In the design of its new range of oil test sets, Megger has addressed all of the points raised in this article. The new testers are easy to use, easy to clean and employ the same robust test vessel for every model. In addition they all have a locking mechanism on the electrode adjusters to prevent the electrode gap from moving. |
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| The OTS range includes five models. The OTS60AF, OTS80AF and OTS100AF are primarily intended for use in fixed locations, such as laboratories, and offer maximum test voltages of 60 kV, 80 kV and 100 kV respectively. The OTS60PB and OTS80PB are compact lightweight instruments for portable use and offer maximum test voltages of 60 kV and 80 kV respectively. |
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| All models feature a colour display with straightforward menu navigation, which makes them fast and easy to use. In addition, the laboratory models have a large keypad to facilitate rapid data entry. A configure-to-build service is offered across the complete range. |
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Guided innovation |
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| Mark Hadley |
| Product Manager |
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| “Innovation is the key to success” is a good maxim for any company, and it is particularly appropriate for companies in the technology sector. However, it is also very apparent that not all innovation leads to success. |
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| In fact, it’s disconcertingly easy to come up with instances of innovations that have led to resounding failure – the Sinclair C5, for example, that was going to revolutionise urban transport or, going a little further back in history, the Hughes H-4 Hercules aircraft that was intended to be the world’s first “jumbo” scale passenger carrier, but ended up flying just once – and then only just. |
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| So what is it that separates successful innovation from potentially disastrous innovation? The answer has two components – in order to be successful, the innovation must lead to a product or service that the marketplace wants, needs and can afford, and it must also respect the limitations of the available technology. |
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| The logical way to satisfy the first requirement is to ask potential customers what they want. It’s acceptable to offer them suggestions, of course, if only to keep the process on track, but the real essential is to listen to what those customers have to say. This may sound completely obvious, but companies have time and time again placed on the market products that have flopped, simply because they thought they knew what their customers wanted better than the customers themselves. |
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| To satisfy the second requirement – respecting the limitations of technology – companies need to have access to genuine and well-proven expertise. The kind of experts needed are those who are not afraid to push the limits of what current technology can dependably deliver, but know their field well enough so that they never allow their enthusiasm to take them beyond those limits. |
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| It is taking heed of these requirements that has helped Megger and its predecessors to remain in the vanguard of successful electrical test instrument innovation for well over a century. |
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| The company’s approach to innovation is well illustrated in the recent development of its new range of portable appliance (PAT testers). The first step was to talk to users in general terms. What did they most want from a PAT tester? |
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| It was interesting to note that speed and ease of use were the top items on the list, but perhaps this shouldn’t have been surprising given that PAT testing is a very competitive business and that making a good profit depends on being able to carry out tests quickly and efficiently. It’s also worthy of mention that, for most users, having the instrument offer a wide range of functions, most of which would be rarely used, didn’t figure on their wish list at all. |
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| Pages of features may be exciting for the members of the marketing team who love all of those bullet points and for the product engineers who can’t wait to add in a few more extras, but what the vast majority of PAT testing customers really want is a simple instrument that’s easy to use! |
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| After the users had discussed general requirements for PAT testers, they were asked about more specific issues and it soon became apparent that two common problems were causing a lot of annoyance and aggravation. |
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| The first was the time that it takes the testers to start up. This doesn’t really matter for the first time it’s used on a particular site, but it matters a lot when the instrument has only been switched off briefly to allow the user to move, say, from one desk to another or between rooms. Since someone carrying out PAT tests on a large site may have work in tens of rooms, having to wait even a minute or two for the PAT tester to reboot after each move soon becomes very annoying! |
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| Megger addressed the boot up time issue by adding a back-up battery to its new instrument, which keeps it ticking over for up to five minutes after the power has been switched off. The result, instant restart when moving from room to room. No it wasn’t rocket science, but it was genuine innovation and it makes users very happy! |
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| To address the shutdown issue it was necessary for the development engineers to look to the latest technology in power supply components, and to choose devices that would enable them to build a compact, lightweight power section into the instrument, which was, nevertheless continuously rated. Users could now carry out any reasonable number of high-current bond tests sequentially, and their test set would just keep on going. |
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| The more general issues that had been identified for PAT testers were addressed with equal care. The new testers were equipped with full colour displays to show the test data and results clearly and unambiguously, and with clearly identified pushbuttons to initiate test sequences. |
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| The testers were provided with a pass/fail with user-defined limits that makes testing for standard assets very quick and easy, and arrangements were made for them to be supplied pre-programmed for automatic operation so they are ready to go straight from the box. |
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| None of these innovations stands out on its own as earth shattering but, in combination, they represent a big step forward for PAT testing instruments. They are a response to genuine customer requirements, and they make good use of the latest technology. Megger believes that it is innovations like this that are the true key to success, and the company’s enviable track record over the last 100+ years suggests that it may just be right. |
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| The second problem that users were having was PAT testers that regularly shut themselves down. It wasn’t hard to trace the root of the problem. For high current bond tests, the instruments are required to deliver a current of up to 25 A. All can do this once in a while but when called upon to carry out several of these tests sequentially, a lot of testers overheat, and are forced to shut themselves down to cool off and prevent damage. |
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