Bung: He doesn't have a hole yet. If you already have drilled a pattern of holes into gib, join up the dots and remove a small piece of gib. You'll be able to glue it back in. It is possible you do have a steel plate protecting cables if it is close to the meter box.
^ This. I've found it easiest to cut a square hole, then once done, replace and fill the joins. If you don't have something solid to glue it to, you can run a spar of something across the inside of the hole. Superglue works well for that.
I would however suggest some investigations with a strong magnet first. A steel plate in your wall could be whether strapping, joining or cable protection but you'll quickly (and non destructively) figure out it's boundaries sweeping a magnet across it.