Although I'm only half way though my PhD at the moment the fact that a couple of people in my research group have just had their external examiners appointed has got me thinking.

As the research field I'm in is small there are a limited number of people that would be suitable externals, the thing is as I have done a Masters in this field at one University and am doing a PhD at another, and have collaborated with a number of other people, that covers a significant proportion of the researchers in the field.

So I'm quite curious to know what the general restrictions on external examiners are. For example can your external be soeone you've published with?, or someone who has written you a reference? or someone that has supervised you at masters/undergrad level?

Anyone any ideas?

The external cannot have been involved in your research in anyway. Usually, the Uni will have their own additional specifications, such things as the examiner not being registered for a research degree themselves, needing to have examined at the required level previously, having not examined a PhD at your institution in recent years et cetera. Read the relevant section in your research student handbook or ask the postgrad office for the procedures for selecting an examining team.

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.