A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation”
This definition has not changed in the 17th Edition. In fact, it is basically the same as it was when it was first introduced in the 15th Edition. It is worth just taking a look at it to remind ourselves of its meaning.
It is a conductive part that is not part of the electrical installation - so it is capable of conducting an electric current but that is not its intended function. Typical examples are: metallic service pipes; structural steel work; etc. It is not part of the electrical installation so we do not expect that it would become ‘live’ under fault conditions.
In our context - what we mean by ‘become live’ is attain a potential above zero volts that is sufficient to cause a dangerous current to flow through a person or animal depending on the situation.
Key Fact Sheet Extraneous Conductive Parts Explanation 2016-08-23
Extraneous Conductive Parts by Electrical Contractor's Association