Electrical, microstructural, physical and chemical characterization of HV XLPE cable peelings for an electrical aging diagnostic data base

The aim of the European project "ARTEMIS" is to develop a diagnostic system for assessing aging in power cable insulation. Its first task was to make a thorough characterisation of the cable insulation before aging. This is intended to provide a background against which any changes introduced by thermo-electric aging can be identified. The aging markers derived from this initial characterisation will be considered both as diagnostic indicators in their own right, and also to develop an aging model for predictive purposes, if and when possible. This stage of the ARTEMIS programme is now complete and we will present an analysis of the results, and show how they may be correlated with the concepts proposed in aging theories.     

Anomalous dielectric response of very small quantities of virgin, aged and failed silicone oil

A technique is described for making dielectric spectroscopy measurements of very small quantities (<1 /spl mu/l) of oil. The technique utilises surface tension to hold the oil between the plates of a capacitor, the inter-electrode distance being controlled by a micrometer. Breakdown strength can also be estimated using this technique. Three samples of silicone oil, used in cable sealing ends, were tested: virgin, used and failed. A major component in the frequency dependent impedance had the form Z(/spl omega/) = B(1-b(i/spl omega/)/sup 1-p/). This component was interpreted in terms of a fractal percolation model, and the anomalous thickness dependence predicted by the model verified by varying the inter-electrode distance. The difference observed for the three different samples indicate that conducting contaminants are responsible for the percolation system.     

Comparative evaluation of National Committee for Clinical Laboratory Standards broth macrodilution and agar dilution screening methods for testing fluconazole susceptibility of Cryptococcus neoformans

A simple screening method for fluconazole susceptibility of Cryptococcus neoformans using 2% dextrose Sabouraud dextrose agar (SabDex) with fluconazole was compared to the National Committee for Clinical Laboratory Standards (NCCLS) broth macrodilution method. By this method, fluconazole-susceptible C. neoformans isolates are significantly smaller on medium with fluconazole than on fluconazole-free medium. Isolates with decreased susceptibility have normal-size colonies on medium containing fluconazole. The 48-h NCCLS broth macrodilution MICs (NCCLS MICs) for isolates with normal-size colonies on 8- or 16-microg/ml fluconazole plates were predicted to be > or =8 or > or =16 microg/ml, respectively. On medium with 16 microg of fluconazole per ml, all strains (84 of 84) for which the NCCLS MICs were <16 microg/ml were correctly predicted, as were all isolates (7 of 7) for which the MICs were > or =16 microg/ml. Agar dilution appears to be an effective screening method for fluconazole resistance in C. neoformans.     

Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps     

Detection of Candida dubliniensis in oropharyngeal samples from human immunodeficiency virus-infected patients in North America by primary CHROMagar candida screening and susceptibility testing of isolates

Candida dubliniensis has been associated with oropharyngeal candidiasis in patients infected with human immunodeficiency virus (HIV). C. dubliniensis isolates may have been improperly characterized as atypical Candida albicans due to the phenotypic similarity between the two species. Prospective screening of oral rinses from 63 HIV-infected patients detected atypical dark green isolates on CHROMagar Candida compared to typical C. albicans isolates, which are light green. Forty-eight atypical isolates and three control strains were characterized by germ tube formation, differential growth at 37, 42, and 45 degreesC, identification by API 20C, fluorescence, chlamydoconidium production, and fingerprinting by Ca3 probe DNA hybridization patterns. All isolates were germ tube positive. Very poor or no growth occurred at 42 degreesC with 22 of 51 isolates. All 22 poorly growing isolates at 42 degreesC and one isolate with growth at 42 degreesC showed weak hybridization of the Ca3 probe with genomic DNA, consistent with C. dubliniensis identification. No C. dubliniensis isolate but only 18 of 28 C. albicans isolates grew at 45 degreesC. Other phenotypic or morphologic tests were less reliable in differentiating C. dubliniensis from C. albicans. Antifungal susceptibility testing showed fluconazole MICs ranging from 相似文献   

Comparative evaluation of macrodilution and alamar colorimetric microdilution broth methods for antifungal susceptibility testing of yeast isolates

A comparative evaluation of the macrodilution method and the Alamar colorimetric method for the susceptibility testing of amphotericin B, fluconazole, and flucytosine was conducted with 134 pathogenic yeasts. The clinical isolates included 28 Candida albicans, 17 Candida tropicalis, 15 Candida parapsilosis, 12 Candida krusei, 10 Candida lusitaniae, 9 Candida guilliermondii, 18 Torulopsis glabrata, and 25 Cryptococcus neoformans isolates. The macrodilution method was performed and interpreted according to the recommendations of the National Committee for Clinical Laboratory Standards (document M27-P), and the Alamar colorimetric method was performed according to the manufacturer's instructions. For the Alamar colorimetric method, MICs were determined at 24 and 48 h of incubation for Candida species and T. glabrata and at 48 and 72 h of incubation for C. neoformans. The overall agreement within +/- 1 dilution for Candida species and T. glabrata against the three antifungal agents was generally good, with the values for amphotericin B, fluconazole, and flucytosine being 85.3, 77.9, and 86.2%, respectively, at the 24-h readings and 69.3, 65.2, and 97.2%, respectively, at the 48-h readings. Most disagreement was noted with fluconazole against C. tropicalis and T. glabrata. Our studies indicate that determination of MICs at 24 h by the Alamar colorimetric method is a valid alternate method for testing amphotericin B, fluconazole, and flucytosine against Candida species but not for testing fluconazole against C. tropicalis and T. glabrata. For flucytosine, much better agreement can be demonstrated against Candida species and T. glabrata at the 48-h readings by the Alamar method. Excellent agreement within +/- dilution can also be observed for amphotericin B, fluconazole, and flucytosine (80, 96, and 96%, respectively) against c. neoformans when the MICs were determined at 72 h by the Alamar method.     

Optimum estimators for the Weibull distribution from censored testdata. Progressively-censored tests [breakdown statistics]

The paper considers the estimation of the parameters of the 2-parameter Weibull distribution from tests in which data has been progressively censored in different ways (50% censoring and two 30% censoring schemes). Four techniques are compared using Monte-Carlo simulations: maximum likelihood, least squares using the Bernard and Weibull rank estimators, and the White technique. The latter three have been specially adapted for use with censored data. It is found using several criteria that the White technique never performs badly and usually performs best. The maximum likelihood technique is reasonable under most conditions for estimating the scale but not the shape parameter. The least squares techniques generally introduce severe errors. It is recommended that the White technique is adopted widely     

Book reviews

Managing Cities. The New Urban Context. P. Healey, S. Cameron, S. Davoudi, S. Graham & A. Madani‐Pour (Eds) Chichester, John Wiley and Sons, 1995, £14.95 pb, ISBN 0–471–95533–7

Industrial Property: Policy and Economic Development. Rick Ball & Andy Pratt (Eds). London, Routledge, 1994, £45.00 hb, ISBN 0–415–09152–7

Grassroots Environmental Action: People's Participation in Sustainable Development. Dharam P. Ghai & Jessica M. Vivian (Eds). London and New York, Routledge, 1995 (pb), 347 pp., ISBN 0–415–12703–3

Services and Space: Key Aspects of Urban and Regional Development. N. Marshall & P. Wood. Essex, Longman Scientific and Technical, 1995, 294 pp., £17.99 pb, ISBN 0–582–25162–1

Servicing The Middle Classes: Class, Gender and Waged Domestic Labour in Contemporary Britain. Nicky Gregson & Michelle Lowe. Routledge, 1994, vii + 327 pp., £13.99 pb, ISBN 0–419–08531–4

Urban Development Planning: Lessons for the Economic Reconstruction of South Africa's Cities. Richard Tomlinson. Johannesburg, Witwatersrand University Press, 1994, 277 pp., £14.95 pb, ISBN 1–868–14258–2     

Electric field criteria for charge packet formation and movement inXLPE

The formation of space charge packets in crosslinked polyethylene (XLPE) tapes from un-aged cable insulation has been studied utilizing the pulsed electroacoustic (PEA) technique. The 150 μm thick sheets were studied under constant applied DC field of 120 kV/mm at a temperature of 20°C for a period of 48 h. After an inception period of ~3.5 h, during which heterocharge accumulates at the anode and increases the local field there, a sequence of positive charge packets was observed to transit the sample starting from near the anode. Calculation of the internal field showed that the packets required a field of ⩾140 kV/mm for their initiation. Reduction of the applied field step-wise from 120 to 80 kV/mm indicated that the charge packet would keep moving as long as the local field at its front was >100 kv/mm, but with a reducing magnitude. A return to an applied field of 120 kV/mm confirmed that the local field required to initiate a new packet was >135 kV/mm. The results are discussed in terms of current theories of charge packet formation. The first packet appears to be a moving front of field ionization. The generation of subsequent packets is governed by the field at the anode and the balance of charge injection and extraction process, which occur there. The nature of the negative charges produced at the ionization front is not clear, but they are unlikely to be electrons