I shouldn't think that you are liable. You are working for someone else and they should be responsible. OK, it was careless of you to leave it on but on the other hand had the equipment been electrically tested. Each item should have a small green sticker on giving the date of testing. Was this so? If not they are breaking the law. And all those wires trailing about the place could be deemed hazardous - I should mention Health and Safety to them. Finally, it was dealt with in a very poor way. A receptionist (presumably in a public place) should not be talking to you in this way - very unprofessional I would say.