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The structure and chemistry of two electrical trees (designated Tree A and Tree B) grown in low density polyethylene have been studied by a combination of confocal Raman microprobe spectroscopy, optical microscopy and scanning electron microscopy. Despite being grown under similar conditions (A, 30 °C and 13.5 kV; B, 20 °C and 13.5 kV), these two trees exhibit very different structures. Tree A exhibits a branched structure while Tree B is more bush-like. In Tree A, the very tips of the structure are made up of hollow tubules, which exhibit just the Raman signature of polyethylene. On moving towards the high voltage needle electrode, fluorescent decomposition products are first detected which, subsequently, are replaced by disordered graphitic carbon. From the relative intensity of the graphitic sp2 G and D Raman bands, the constituent graphitic domains are estimated to be ∼4 nm in size, which leads to a local tree channel resistance per unit length of 1–10 Ω µm−1. These structures are therefore sufficiently conducting to prevent local electrical discharge activity. In Tree B, the observed fluorescence increases continuously from the growth tips to the needle. Here, the tree channels are not sufficiently conducting to prevent electrical discharge activity within the body of the tree. These results are discussed in terms of mechanisms of tree growth and, in particular, the chemical processes involved.

A simple accumulated damage analysis method and an empirical field-driven tree growth model are proposed to characterize and describe the spatial and temporal development of electrical trees. Examples are presented for trees grown in CT200 and CY1311 epoxy resin pin-plane samples subjected to a wide range of 50 Hz alternating current electrical stress. It is shown that a material's resistance to treeing may be described quantitatively, allowing the relative performance of different synthetic resins to be easily compared. For CY1311 epoxy resin, tree structural characteristics change progressively from branch to bush structures as the stressing voltage is increased. It is shown that the time to failure is primarily influenced by the local electric field and the resultant tree geometry and fractal dimension of tree growth and is not simply dependent on the applied voltage.

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Combined partial discharge detection and video monitoring of the tree growth have shown a strong correlation between the partial discharge activity and the spatial and temporal development of electrical tree growth in CY1311 epoxy resin. CCD imaging of the spatial distribution of light emitted, due to partial discharges in the tree structure, has shown that the different modes of partial discharge behaviour reflect their different spatial distribution within the existing tree structure, with new growth occurring at those parts of the tree in which the partial discharges are active. The dynamics of the partial discharge behaviour, namely the frequency and duration of two modes of activity, is controlled by the experimental conditions (voltage and pin - plane spacing) and determines the type (fractal dimension) of the resultant tree. During one mode of activity, rapid low-fractal-dimension radial growth of the tree occurs. During the other mode, new growth occurs at a slower rate from the tree structure near the pin electrode, leading to an increase in the overall fractal dimension of the tree structure.

Electrical treeing is of interest to the electrical generation, transmission and distribution industries as it is one of the causes of insulation failure in electrical machines, switchgear and transformer bushings. Previous experimental investigations of electrical treeing in epoxy resins have found evidence that the tree structures formed were either electrically conducting or non-conducting, depending on whether the epoxy resin was in a flexible state (above its glass transition temperature) or in the glassy state (below its glass transition temperature). In this paper we extend an existing model, of partial discharges within an arbitrarily defined non-conducting electrical tree structure, to the case of electrical conducting trees. With the inclusion of tree channel conductivity, the partial discharge model could simulate successfully the experimentally observed partial discharge activity occurring in trees grown in both the flexible and glassy epoxy resins. This modelling highlights a fundamental difference in the mechanism of electrical tree growth in flexible and glassy epoxy resins. The much lower resistivities of the tree channels grown in the glassy epoxy resins may be due to conducting decomposition (carbonized) products condensing on the side walls of the existing channels, whereas, in the case of non-conducting tree channels, subsequent discharges within the main branches lead to side-wall erosion and a consequent widening of the tubules. The differing electrical characteristics of the tree tubules also have consequences for the development of diagnostic tools for the early detection of pre-breakdown phenomena.

BackgroundClinical care alone at the end of life is unlikely to meet all needs. Volunteers are a key resource, acceptable to patients, but there is no evidence on care outcomes. This study aimed to determine whether support from a social action volunteer service is better than usual care at improving quality of life for adults in the last year of life.MethodsA pragmatic, multi-centre wait-list controlled trial, with participants randomly allocated to receive the volunteer support intervention either immediately or after a 4 week wait. Trained volunteers provided tailored face-to-face support including befriending, practical support and signposting to services, primarily provided within the home, typically for 2–3 hours per week. The primary outcome was rate of change of quality of life at 4 weeks (WHO QOL BREF, a general, culturally sensitive measure). Secondary outcomes included rate of change of quality of life at 8 weeks and Loneliness (De Jong Gierveld Loneliness Scale), social support (mMOS-SS), and reported use of health and social care services at 4 and 8 weeks.ResultsIn total, 196 adults (61% (n = 109) female; mean age 72 years) were included in the study. No significant difference was found in main or secondary outcomes at 4 weeks. Rate of change of quality of life showed trends in favour of the intervention (physical quality of life domain: b = 3.98, CI, –0.38 to 8.34; psychological domain: b = 2.59, CI, –2.24 to 7.43; environmental domain: b = 3, CI, –4.13 to 4.91). Adjusted analyses to control for hours of volunteer input found significantly less decrease in physical quality of life in the intervention group (slope (b) 4.43, CI, 0.10 to 8.76). While the intervention also favoured the rate of change of emotional (b = –0.08; CI, –0.52 to 0.35) and social loneliness (b = –0.20; CI, –0.58 to 0.18), social support (b = 0.13; CI, –0.13 to 0.39), and reported use of health and social care professionals (b = 0.16; CI, –0.22 to 0.55), these were not statistically significant. No adverse events were reported.ConclusionsClinicians can confidently refer to volunteer services at the end of life. Future research should focus on ‘dose’ to maximise likely impact.Trial registrationThe trial was prospectively registered. ISRCTN Registry: ISRCTN12929812, registered 20 May 2015.

A dynamic bipolar charge recombination model is proposed to explain the charge injection and electroluminescent behaviour found experimentally in pin-plane polymeric resin samples under high alternating voltage stress. The model is described for two electrode geometries producing parallel and divergent electric fields. The long term rise in electroluminescence intensity at constant alternating stressing voltage is accounted for by charge trapping in the polymer close to the electrode and recombination via mobile charge recombining with trapped charge of opposite polarity. The phase profile of the electroluminescence is a consequence of charge injection across a potential barrier and field limiting behaviour occurring at the injection electrode. The saturation electric field under field limiting conditions and the injected current density as a function of time can be calculated from the experimental conditions and knowledge of the charge injection law. Field limiting behaviour provides a feedback mechanism that maintains net charge neutrality within the resin over time.