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1. A well installation comprising downhole power delivery apparatus for delivering electrical power from a surface power source to a downhole tool, the well installation comprising downhole metallic structure running down into a borehole and the apparatus comprising an electrical cable for carrying electrical power from the surface power source, a power transfer arrangement and a power pick up arrangement, the cable running down into the borehole together with the metallic structure to the power transfer arrangement which is provided at a first downhole location for transferring electrical power carried by the electrical cable onto the downhole metallic structure for onward conduction, and the power pick up arrangement being provided at a second downhole location, spaced from the first, for picking up electrical power from the downhole metallic structure for supply to the downhole tool, wherein the power transfer arrangement is arranged to transfer electrical power onto the downhole metallic structure at a location above a packer provided in the well installation, wherein the downhole tool is arranged to signal back towards the surface by modulating a load which the power pick up arrangement puts on the power delivery system, wherein such changes in the load in use cause a variation in the potential across the surface power source and data is encoded by the tool modulating the load and extracted at the surface by monitoring these changes in potential difference across the surface power source or changes in potential in the cable relative to a reference.
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