I'm trying to hang a ceiling lamp from what appears to be a reinforced concrete ceiling. My anchors require a 40 mm deep hole, but I don't succeed in drilling more than about 35 mm deep with my hammer drill, and judging from unused 35 mm deep existing holes in another room, nor did a previous resident. I thought that meant I am hitting a stud/beam (which is why I think it's probably reinforced concrete), so I would need a stud finder, but those appear to be for use in buildings using wood or drywall (see also this question); my walls and ceiling all appear to be solid concrete (with a maybe 15 mm plaster layer). The local Baumarkt rents metal detectors, which are advertised to avoid cables, wires, and pipes. Is this also the right tool to find places in my ceiling where I can drill in-between the studs of reinforced concrete, or do I need a different tool for this?
(I could also buy shorter anchors and screws, but I would still need to avoid cables)