If you can get hold of it from your uni library, there's a useful book for those approaching the PhD exam called "The Doctoral Examination Process: A handbook for students, examiners and supervisors" by Penny Tinkler and Carolyn Jackson - it doesn't answer your question exactly (well, not on a quick perusal) but it does offer a lot of advice on the implications of selecting known/unknown examiners, etc. and it covers a lot of ground for those new to the exam process. It's a quick read, small paperback, 200 or so pages and easy to dip in and out of.