The simple and easy to machine transverse-magnetic-field (TMF) contact design with a tapered cross-sectional profile toward the outer edge and a swastika-shaped slot design has been successfully used in millions of commercial vacuum interrupters throughout the world, for the last 40+ years. But is it the optimum design? For one example, is 4 the optimum number of petals? And is the answer dependent on the rated voltage or the interruption current? To answer those questions, a series of spiral TMF contacts with different numbers of petals, ranging from 2 to 7, but otherwise identical, were built into commercial vacuum interrupter (VI) envelopes and tested with synthetic testing at 38kV 40kA and direct testing at 17.5kV 63kA. Preliminary results showed that while the 5-petal pair of contacts had a slight edge over the 4-petal pair, interruption performances were very similar among 4 to 7 petals. Even the 2-petal pair managed to interrupt a couple of shots, although arc voltage traces showed evidence of lack of significant arc column motion, due to the large petal size relative to the diameter of the columnar arc ignited in between the contacts. A VI with a higher number of the petals showed more shots with the arc-voltage showing rapid arc motion. Post-mortem examinations, however, revealed a deeper melt depth and rougher surface deformation in contacts with the higher number of petals, due to the smaller cross-sectional size of the petal. This may indicate a higher contact surface temperature and more fluid molten metal flow at current zeros for contacts with a higher number of petals. Taking also into consideration electrical erosion rate and mechanical strength, it is preliminarily concluded that while 4-petal provides a very cost effective solution at the 12∼17.5kV level, for higher voltage ratings such as 38kV, 5 appears to be the optimum number of petals.

This report presents five case studies of long-term partnerships (over three years) between arts organisations and schools. The Start programme enables arts venues and schools to work together to offer disadvantaged young people opportunities to engage in creative activities that inspire them and enhance their experience of the arts. It is organised and funded by the Prince’s Foundation for Children & the Arts. The study finds that Start has acted as a catalyst for growing and sustaining creative activities and created a lasting legacy of arts engagement between case-study schools and arts venues. Children and young people involved in Start activities experienced a wide range of positive changes. They were enthusiastic about arts and cultural activities, developed a wide range of creative and interpersonal skills and became aware of the careers available to them in the creative industries. Key findings: The effectiveness of Start partnerships between schools and arts organisations depended on a strong mutual commitment to ensuring children and young people were engaged in arts and cultural activities and involving them in authentic creative activities Schools’ sustained involvement with arts organisations led to an expansion in their arts and cultural offer. Start’s impact and legacy was enhanced by arts partners providing opportunities for continuous professional development (CPD). This equipped and motivated teachers to use their creative skills in the classroom in the longer-term.

The use of vacuum interrupters (VIs) as the current interruption component for switches, circuit breakers, reclosers and contactors operating at distribution voltages has escalated since their introduction in the mid-1950’s. This electrical product has developed a dominating position for switching and protecting distribution circuits. VIs are even being introduced into switching products operating at transmission voltages. Among the reasons for the VI’s popularity are its compactness, its range of application, its low cost, its superb electrical and mechanical life and its ease of application. Its major advantage is its well-established reliability. In this paper we show how this reliability has been achieved by design, by mechanical life testing and by electrical performance testing. We introduce the “sealed for life” concept for the VI’s integrity. We discuss this in terms of what is meant by a practical leak rate for VIs with a life of over 30 years. We show that a simple high voltage withstand test is an easy and effective method for monitoring the long-term vacuum integrity. Finally we evaluate the need for routine inspection of this electrical product when it is used in adverse ambient environments.

Experiments were performed with vacuum interrupters containing Cu-Cr (25 wt%) and W-Cu (10 wt%) contacts. The vacuum interrupters were placed in a spring mechanism, which was placed in a tuned, capacitor bank electrical test circuit. The capacitor bank was charged to 25 kV, which allowed a symmetrical fault current of 50 kA (peak) at 30 Hz. As the vacuum interrupter's contacts closed a prestrike arc occurred when the contact spacing was small enough. This contact gap was recorded. The prestrike arc initiated the ac current, which was interrupted by the test circuit after one half cycle. The contacts were then opened with no current. This process was repeated 5 times. As the experiment progressed the prestrike arcing time increased; i.e. the contact gap broke down at larger and larger gaps during the closing operation resulting in longer and longer prestrike arcing times. We explained this phenomenon by considering the effect of the prestrike arc and the subsequent contact welding on the surface structure of the contacts. The change in the contact's surface structure resulted in an increase of the field enhancement factor, which, in turn, led to the vacuum breakdown of the contacts at increasing contact gaps. For the Cu-Cr contacts the prestrike arcing time was eventually long enough that the contacts formed a weld that the mechanism could not break. Although the prestrike arcing time with the W-Cu contacts did increase, the mechanism always broke any welds that formed.

Experiments were performed to determine the electrical life of a vacuum interrupter as a function of current. The test currents ranged from the normal load currents of 630 A (rms) to 3150 A (rms) to higher currents of 20% to 100% of the rated short-circuit current. The two vacuum interrupter contact styles were evaluated: the transverse magnetic field (TMF) contact and the axial magnetic field structure (AMF). For the short-circuit current switching the AMF contact shows a longer electrical life, but the erosion may not be the only limitation on the life of the vacuum interrupter. The results will be discussed in terms of the expected performance of the two contact designs and on the operating life of a vacuum interrupter.

During the development of a vacuum interrupter for a 63k.A, 15kV generator breaker it was observed that the transverse magnetic field (TMF) contact structure operated well under the difficult short circuit conditions expected from such a circuit breaker. The TMF contact successfully endured the long arcing times associated with no current zero periods of very high asymmetric currents and also interrupted them successfully. This development resulted in the question: how would the interruption performance of axial magnetic field (AMF) contacts compare with the TMF designs for short circuit currents and for long arcing times? A controlled experiment using both contact structures was thus developed. Contacts using TMF and AMF structures with Cu-Cr contacts and the same dimensions were manufactured into experimental vacuum interrupters that had the same diameter. A 12kV, 16Hz ac circuit was arranged using our tuned capacitor circuit in our High Power Test Lab. The two contact structures were then evaluated for symmetric short circuit currents in the range 16kA (rms.) to 2SkA (rms.). It was observed that the TMF contact successfully interrupted the circuit for currents all the way up to the 25kA level 100% of the time. The AMF contact interrupted the circuit 100% of the time for currents up to 22.5kA, but had a distinct drop in its ability at 25kA where it was successful only 80% of the time. These data will be discussed in terms of the quite different vacuum arc characteristics expected from the TMF and AMF contact structures.

Generator circuit protection applications present some of the most severe short circuit interrupting conditions encountered for medium voltage circuit breakers. The proximity of the circuit breaker to both a generator on one side and a large step-up transformer on the other side produces short circuits with high short circuit interrupting stresses. Traditionally, generator circuit breakers with the largest ratings are physically very large units. Generator circuit breakers of this type usually utilize air blast or SF6 blast interruption techniques. Vacuum circuit breakers have in recent years been developed that can perform well in these applications at the lower end of the generator rating spectrum.

The effects of Cr content in CuCr contact materials, especially on high current interruption, have so far mostly been examined in experimental chambers rather than in actual vacuum interrupters (VIs). Possible interactions of the contacts with their surroundings, both electromechanical and gaseous, may require commercial VIs for a more realistic test determination of the effects of Cr content on the overall performance. In this work, the Cr content in CuCr contacts is varied from 5 to 75 wt% and tested in their 38 kV axial magnetic field type VIs. The material properties of electrical conductivity, hardness and total VI resistance are measured as a function of the Cr content. AC and impulse voltage withstand capabilities are tested before and after short-circuit current interruption tests. Contact erosion behavior is examined with VIs subjected to only a half cycle arcing duty.

Vacuum interrupters offer the lowest environmental impact of all medium voltage switching technologies over the entire product life cycle. Vacuum Interrupters are comprised of materials that are all environmentally benign and safe to handle during periodic out-of-service maintenance and at end-of-life disposal. Vacuum Interrupters can perform well in all medium voltage switching applications required in modern power systems with exceptionally long life and low maintenance. In contrast, SF6, the other major technology for circuit interruption, is an extremely potent greenhouse gas. In addition, SF6 arc by-products possess significant health concerns for handling and disposal. Environmental concerns have spurred increasing regulation of the manufacture and use of SF6, leading to higher costs for purchase, usage and disposal. Accordingly, environmental protection agencies have recommended that alternatives to SF6 be used wherever such alternatives are viable. By choosing switching devices employing vacuum interrupters, the user can obtain the benefits of both excellent switching performance and low environmental impact.

In this article, we step through the development process, from fundamental investigations of the arc-contact interaction to performance and precertification testing of manufactured vacuum interrupters (VI). We also describe how VI models are tested above their arc-contact interaction to performance and rated current to monitor the quality of their materials and processing. The present work began with movies of arcs between spiral-type VI contacts, which generate a radial magnetic field to drive the high-current arc column along the spiral arms. The end-of-life condition of identical contacts and shields from manufactured VIs tested repeatedly under the corresponding conditions, was also systematically studied, along with the arc voltage traces. From the integration of these experimental results, guidelines have been developed for using the arc voltage waveforms of spiral-contact VIs to determine when in a high power arcing cycle the arc is stationary, moving, breaking up, diffuse or causing melting of the arc-contacts. In particular, anchoring of a high-current arc, and the resultant intense localized heating of the contacts, can increase the probability of long VI arcing times. Inspection of arc voltage traces is thus a method to monitor for contact damage and reduced internal performance during testing. The results are applicable to analysis of arc voltages and contact travel records from breaker certification tests.

It is well known that the ability of a gap in vacuum to withstand high voltages is a property enhanced by conditioning with high voltage[1]. Moreover, the conditioning effect is actually produced by small spark discharges that occur while this high voltage is applied. These spark discharges, or disruptive discharges in the language of test standards, are therefore required to produce the desired conditioning effect in the vacuum interrupter. In addition, the few disruptive discharges that sometimes occur during the process of performing impulse voltage testing in a completed circuit breaker are normal. Although observing such discharges is not new to those active in vacuum technology, they have only recently become an issue in performing impulse voltage certification testing as design voltages have increased to 36 and 38 kV and as the demand for ever smaller interrupters has led to higher stressed designs. The procedure for performing impulse voltage tests must therefore allow for a few disruptive discharges in a way that does not erroneously consider them as evidence of failure in the test.

The Oligocene Little Cottonwood stock in the central Wasatch Mountains, Utah is an elliptical, 80 km2, compositionally and texturally zoned pluton, with a more felsic interior. The stock ranges in composition from monzogranite to granodiorite with three distinct groundmass textures: coarse- (>5 mm), intermediate-(2-5 mm), and fine-grained (<2 mm). Modally the coarse-grained facies (CGF) rock composition is a porphyritic monzogranite to porphyritic granodiorite, while the intermediate- (IGF) to fine-grained facies (FGF) rock composition is a porphyritic granodiorite, with the three facies concentrically arranged. Differentiation-related major oxide variation within the stock is extensive and spatially systematic, but trace element abundances were not strongly influenced by differentiation. Mineral assemblages and calcu-lated Zr saturation temperatures indicate crystallization between 750" and 850°C at oxygen fugacities (fo2) ranging from 1 0-l2 to 1 0-l5 bars. The pressure-depth of emplacement of the stock ranges from 300 MPa (1 1 km) on its western margin (corresponding to the CGF rocks) to 150 MPa (6 km) on its eastern margin (cor-responding to the FGF rocks). The rather steep and fairly linear inverse variation of Fe, Mg, Ca, Ti, and Zr with Si02 suggests that biotite, hornblende, titanite, magnetite, and zircon, accumulated or fractionated to produce the chemical variation of these elements between the FGF, LGF, and CGF rocks of the stock. The concentric compositional pattern could have been produced by early separation of mafic minerals along magma chamber walls with inward transport of the remaining more felsic liquid. and trace-element Cottonwood stock show the high-K granitoid series. Low Ti02 concentrations, major- and trace-element-tectonic-discrimination dia-grams, and high LlUHFS ratios are consistent with magma genesis related to subduction and magmatic-arc processes inland from the western edge of the early Cenozoic North American plate. Arc related generation of the Little Cottonwood stock during Oligocene time is consistent with models pertaining to early Cenozoic southward progression and westward retreat of magmatism.

Using high-speed movies of vacuum arcs between spiral contacts in conjunction with floating arc shields, the arc voltage, current and contact travel have been correlated with arc behaviors. These arc behaviors can be described by arc appearance diagrams in gap-current space. These allow the waveforms from high-power tests on spiral-contact vacuum interrupters to be evaluated for information on the internal arcing behavior and arc-induced damage. We have found that arc voltage traces can be examined during interruption testing to help in judging the probability that a vacuum interrupter will continue to pass additional interruption trials. The arc voltage also reveals signs of damage to the contacts, such as melting caused by a prolonged interval of a stationary high-current arc column. This information enhances the statistics we use in single-phase pass/fail testing of interrupters to assess manufacturing quality and the probability that new designs will pass three-phase certification. Correlation of arc voltage traces with the arc appearance and arc motion is also discussed for butt-type contacts, axial magnetic field contacts, and radial magnetic field slotted-cup contacts

A vacuum circuit breaker (VCB) has demonstrated its ability to interrupt short circuits with faster than normal rates of rise of transient recovery voltage (TRV) at levels greater than those produced by most transformer secondary faults. Two recent exploratory test programs evaluated the interrupting ability of a 15 kV VCB containing interrupters of the rotating arc type with contacts made from a chromium-copper powder metal mixture. The interrupting conditions covered a wide range of currents from 10% to 130% of the 28 kA rated short circuit current of the tested circuit breaker and a wide range of TRV rates of rise. These tests showed that the interrupting performance of the tested VCB was unaffected by the TRV rate of rise to the fastest rates available in the test laboratory. Such a VCB can therefore be used without TRV modifying capacitors to slow down the rate of rise provided by the power system. This ability is particularly important if analysis shows that the expected TRV from a transformer secondary fault has a fast rate of rise beyond the recognized ability of an older circuit breaker to acceptably interrupt.<>

The design and application requirements for solid state distribution current limiters and circuit breakers are described and compared to those for the electromechanical switchgear used in present day distribution systems. Control strategies are provided for current limiting application in the power transformer main circuit and for current limiting and interruption for the bus tie location.<>