A non-defining relative clause adds extra information to a sentence. The extra information is not essential and is separated from the essential information using commas.
Sometimes called a non-restrictive relative clause.
- The boy, who had just eaten a sandwich, was still hungry.
(non-essential information/who had just eaten a sandwich)
To test if you have a non-defining relative clause in your sentence simply remove it and if the sentence still sounds good it is a non-defining relative clause.
- The boy was still hungry.
(still sounds good)
- Conrad, who is still on the train, will arrive this evening.
- My grandfather, who’s 87, goes running every day.
- The award was given to Mr. Dickens, whose short story impressed everyone.