Feature Circuit Protection
David Pitt of Eaton discusses the changes to the BS EN 60947-2 standard that covers low voltage circuit breakers and their implications for industry
The impact of change T
he revision means that, for the sake of consistency, the term ‘dis- crimination’ when referring to protection systems is no longer used. Instead, the new version of the annex always uses the term ‘selectiv- ity’. The terms are virtually inter- changeable, but by sticking to one rather than using both, the new annex eliminates a possible source of confu- sion and matches the terminology used in the IET Wiring Regulations.
Test procedures
The changes to the test procedures pre- scribed in the annex, particularly in relation to tests designed to evaluate selectivity, are rather more far reaching. In the previous versions of the annex, the focus of testing had been to ensure that, in systems employing more than one breaker in series, adequate back- up protection was provided. In other words, the tests were designed to show that, under all fault conditions, the upstream breaker would always limit the let-through current and energy to values that could be withstood by the downstream breaker. This is clearly an essential requirement for safety and, in practice, adequate back-up protection often has the additional bene- fit that a smaller and less costly breaker can be used in the downstream position. However, a potential drawback is that in clearing a fault, it’s possible that the upstream breaker will trip as well as the downstream breaker - meaning that many more loads will be disconnected than strictly necessary to deal with the problem.
Selectivity is key
The answer to this problem is selectiv- ity. The characteristics of the upstream and downstream circuit breakers are co-ordinated so that, as far as possible, only the downstream breaker trips under fault conditions, even though the upstream breaker may contribute to limiting the fault current and energy. Achieving selectivity can be treated as a desktop exercise achieved by compar- ing the published characteristics of the upstream and downstream breakers, but this is only an approximate method at best. Much more meaningful results are obtained by testing but, unfortunately, until the latest revision, BS EN 60947-2 and its annexes provided no definitive guidance on selectivity testing.
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For this reason some manufacturers, not unreasonably, carried out selectiv- ity testing on their products using the same arrangement as that prescribed for back-up testing. This requires, for example, that the breakers under test should be connected by cables that are just 0.75m long. This approach is per- fectly valid, although in most real world applications, the cables would be significantly longer.
One consequence of the short cables is that the results are rather pessimistic. A 250A upstream breaker and a 60A downstream breaker that have been tested for back-up at 20kA might, for the sake of argument, give selectivity up to 12kA when tested. Of course, the breaker manufacturer would like to say that the combination provides selectiv- ity up to the full 20kA - and with the old version of Annex A, it probably could. All that is likely to be needed is to make those cables somewhat longer and test for selectivity once again. The result will undoubtedly be better and, with a bit of luck, will hit the highly desirable
Above: David Pitt, product marketing manager - Power Distribution Components for Eaton
Below: all of Eaton’s LV ACB and MCCB products have been tested in line with the provisions of the new annex of the BS EN 60947-2 standard
Amendment 2 - has closed these loop- holes by specifying the cable lengths that must be used for selectivity testing, and also by making it mandatory to use worst case trip settings - that is, the upstream breaker must be set to mini- mum, the downstream to maximum. The new annex also goes further. Previously, tests on two breakers in series were carried out by applying a fault to the downstream end of the cir- cuit while both breakers were closed. This test is still required, but subse- quently the test must be repeated with the fault present, the upstream breaker closed, and the downstream breaker open. The downstream breaker is then closed onto the fault.
This second part of the test is much more demanding, not least because there is no way of controlling the point on the waveform at which the contacts in the downstream breaker make. Finally, the new annex requires mea- surements to be made of contact separa- tion in the upstream breaker during selectivity tests. In most cases, even if the upstream breaker doesn’t trip when the fault is applied to the circuit, its con- tacts will open momentarily due to magnetic repulsion. Indeed, this is one of the ways in which the upstream breaker limits let-through energy. The opening of the contacts does, however, mean that circuits fed from the breaker, even if they are not directly affected by the fault, experience a tran- sient break in supply. Clearly, if the contacts are open too long this could be problematic for sensitive loads. The new annex therefore requires that the duration of the contact opening should be no longer than 30ms.
Summary
20kA mark. This was permissible with the old version of Annex A1, because it prescribed cable lengths only for back- up testing, not for selectivity testing. The old version also permitted another ‘fiddle factor’ in that it said nothing about the breaker trip settings during selectivity testing, so the breaker manufacturer was free, if it so wished, to choose settings that gave the most optimistic selectivity results. Overall, the real problem was that the lack of detailed test procedures in the old version of the annex meant that there was no way for users to know the conditions under which a manufacturer had assessed the selectivity of its break- ers and, therefore, it was impossible to compare the figures given by different manufacturers in their data sheets. The new version of the annex - in
The introduction of the new version of Annex A to BS EN 60947-2 has two very important consequences for breaker users. The first is that selectivity data for all breakers tested in line with the new annex is now directly comparable. The second is that the detailed selectivity test specification included in the new annex will encourage testing, rather than desktop selectivity assessments. This is likely to lead to substantial cost savings, as the test results are almost invariably better than those based on paper comparisons, which must always include a substantial margin for error. In short, the new annex is an impor- tant step forward that will provide benefits for breaker manufacturers and users alike. One word of advice for users, however, is to ensure that the breakers they specify and buy have actually been tested in line with the provisions of the new annex.
Eaton
www.eaton.com T: 0161 655 8900
Enter 211 OCTOBER 2013 Electrical Engineering
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