Boundary layer models for calving marine outlet glaciers

We consider the flow of marine-terminating outlet glaciers that are laterally confined in a channel of prescribed width. In that case, the drag exerted by the channel side walls on a floating ice shelf can reduce extensional stress at the grounding line. If ice flux through the grounding line increases with both ice thickness and extensional stress, then a longer shelf can reduce ice flux by decreasing extensional stress. Consequently, calving has an effect on flux through the grounding line by regulating the length of the shelf. In the absence of a shelf, it plays a similar role by controlling the above-flotation height of the calving cliff. Using two calving laws, one due to Nick et al. (2010) based on a model for crevasse propagation due to hydrofracture and the other simply asserting that calving occurs where the glacier ice becomes afloat, we pose and analyse a flowline model for a marine-terminating glacier by two methods: direct numerical solution and matched asymptotic expansions. The latter leads to a boundary layer formulation that predicts flux through the grounding line as a function of depth to bedrock, channel width, basal drag coefficient, and a calving parameter. By contrast with unbuttressed marine ice sheets, we find that flux can decrease with increasing depth to bedrock at the grounding line, reversing the usual stability criterion for steady grounding line location. Stable steady states can then have grounding lines located on retrograde slopes. We show how this anomalous behaviour relates to the strength of lateral versus basal drag on the grounded portion of the glacier and to the specifics of the calving law used

Grounding System Adequacy of HV/MV Substations in Areas With Reduced Accessibility

The adequacy of grounding systems has to be verified periodically in the operational time. With urban development and buildings growth adjacent to power systems as HV/MV substations, it is very rare to have area around with sufficient accessibility for installing the potential and current electrodes. This paper discusses a safety criterion to verify the effectiveness of a grounding system. This criterion suggests conservative tests for both ground potential rise and touch voltages and step voltages that allow to verify the grounding systems effectiveness in areas with reduced accessibility and to monitor its evolution in the time

A Guide to Ground in Kant's Lectures on Metaphysics

While scholars have extensively discussed Kant’s treatment of the Principle of Sufficient Ground in the Antinomies chapter of the Critique of Pure Reason, and, more recently, his relation to German rationalist debates about it, relatively little has been said about the exact notion of ground that figures in the PSG. My aim in this chapter is to explain Kant’s discussion of ground in the lectures and to relate it, where appropriate, to his published discussions of ground

The ATLAS SCT grounding and shielding concept and implementation

This paper presents a complete description of Virgo, the French-Italian gravitational wave detector. The detector, built at Cascina, near Pisa (Italy), is a very large Michelson interferometer, with 3 km-long arms. In this paper, following a presentation of the physics requirements, leading to the specifications for the construction of the detector, a detailed description of all its different elements is given. These include civil engineering infrastructures, a huge ultra-high vacuum (UHV) chamber (about 6000 cubic metres), all of the optical components, including high quality mirrors and their seismic isolating suspensions, all of the electronics required to control the interferometer and for signal detection. The expected performances of these different elements are given, leading to an overall sensitivity curve as a function of the incoming gravitational wave frequency. This description represents the detector as built and used in the first data-taking runs. Improvements in different parts have been and continue to be performed, leading to better sensitivities. These will be detailed in a forthcoming paper

Electrical Properties of Carbon Fiber Support Systems

Carbon fiber support structures have become common elements of detector designs for high energy physics experiments. Carbon fiber has many mechanical advantages but it is also characterized by high conductivity, particularly at high frequency, with associated design issues. This paper discusses the elements required for sound electrical performance of silicon detectors employing carbon fiber support elements. Tests on carbon fiber structures are presented indicating that carbon fiber must be regarded as a conductor for the frequency region of 10 to 100 MHz. The general principles of grounding configurations involving carbon fiber structures will be discussed. To illustrate the design requirements, measurements performed with a silicon detector on a carbon fiber support structure at small radius are presented. A grounding scheme employing copper-kapton mesh circuits is described and shown to provide adequate and robust detector performance.Comment: 20 pages, 11 figures, submitted to NI

Safety issues in PV systems: design choices for a secure fault detection and for preventing fire risk

Photovoltaic systems have played a key role over the last decade in the evolution of the electricity sector. In terms of safety design, it's important to consider that a PV plant constitutes a special system of generation, where the Direct Current (DC) presence results in changes to the technical rules. Moreover, if certain electrical faults occur, the plant is a possible source of fire. Choices regarding the grounding of the generator and its protection devices are fundamental for a design that evaluates fire risk. The subject of the article is the analysis of the relation between electrical phenomena in PV systems and the fire risk related to ensuring appropriate fault detection by the electrical protection system. A description of a grid-connected PV system is followed firstly by a comparison of the design solutions provided by international Standards, and secondly by an analysis of electrical phenomena which may trigger a fire. A study of two existing PV systems, where electrical faults have resulted in fires, is then presented. The study highlights the importance of checking all possible failure modes in a PV system design phase, to assess fire risk in advance. Some guidelines for the mitigation of electrical faults that may result in a fire are finally provided

The effect of buttressing on grounding line dynamics

Determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice-sheet models. Here, we investigate the role of lateral shear and ice-shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets. We derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non-local ice-shelf properties. Application of these results to a laterally confined version of the MISMIP 1a experiment shows that the boundary condition at the ice-shelf front (i.e. the calving law) is a major control on the location and stability of the grounding line in the presence of buttressing, allowing for both stable and unstable grounding line positions on downwards sloping beds. These results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice-shelf properties, in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed. Consequently, the marine ice-sheet instability hypothesis may not apply to buttressed marine ice sheets

Simplified conservative testing method of touch and step voltages by multiple auxiliary electrodes at reduced distance

Grounding systems (GSs) must be tested periodically in order to maintain the touch voltage (TV) and step voltage (SV) below a safe value in all of the zones of the installation. Measurements of the ground resistance and of the TV and SV are typically done by the fall-of-potential (FoP) method, locating the auxiliary current electrode at remote distance to test the effective behavior of the GS. In urban areas, it could be very complicated or impossible to install the auxiliary current electrode as required, not having area around with sufficient accessibility. At this aim, this paper describes a methodology of using multiple current electrodes at short distances, modifying the classic FoP practice, so that the measurements of TV and SV are always conservative. The adequacy of a GS is verified if the values of the TV and SV, tested inside and in the vicinity of the GS, are below the permissible limits, regardless if they are true or conservatively increased. Thus, the measured TV and SV by the suggested method, always conservative, allow verifying the adequacy of GSs, in the cases where it is impossible to locate the remote auxiliary electrode

Nuclear Magnetohydrodynamic EMP, Solar Storms, and Substorms

In addition to a fast electromagnetic pulse (EMP), a high altitude nuclear burst produces a relatively slow magnetohydrodynarnic EMP (MHD EMP), whose effects are like those from solar storm geomagnetically induced currents (SS GIC). The MHD EMP electric field E < 10^-1 V/m and lasts < 10^2 sec, whereas for solar storms E > 10^-2 V/m and lasts >10^3 sec. Although the solar storm electric field is lower than MHD EMP, the solar storm effects are generally greater due to their much longer duration. Substorms produce much smaller effects than SS GIC, but occur much more frequently. This paper describes the physics of such geomagnetic disturbances and analyzes their effects.Comment: 29 pages, 14 figures, 5 table