under 17th edition is bathroom radiator classed as an exraneous conductive part

I think that for people to fully answer your question and give the best response they can, they need to know a little more, e.g. whay your asking or whats prompted the question? Firstly the definition of an extraneous-conductive-part is "aconductive part not forming part of the electrical installation and liable to introduce an electric potential" so a steel radiator in a bathroom could be viewed to be an extraneous conductive part,and has been so under previous editions of the wiring regulations. This is complicated by the premise "liable to become live". In most domestic installations it is normal to have protective earth bonding to the main water pipework (if its metal) and the consumer side gas pipework. With main bonding in place, it can be argued that the radaitor is not liable to become live under fault conditions as there will be a parrallel earth path through the central heating and hot and cold pipework, however in a lot of installations the radiator has an earth connection to it which is often referred to as supplementary bonding (i.e. it supplemnts the main protective earth bonding). Under the 17th edition regulations, supplementary bonding in the bathroom is not required provided that; all electrical equipment in the bathroom(light, extractor fan, electric shower, etc) are RCD protected and that the main protective earth bonding is in place. I hope this helps, and I'd be happy to provide more information if you could elaborate on your original question.

I was going to reply, but Electrical Safety Services has covered it very thoroughly, nice work Chris