Preparing Your Students for an Online Exam
Provide clear instructions.
Give clear written and verbal instructions for the students on how they access the exam, the format, expectations, open and close times, and how to ask you questions during the exam.
Provide your contact information. 
Ensure you have clearly posted your phone number, email address, or other forms of communication students can immediately reach you at should they not have access to the exam, get locked out, or experience a technical difficulty.
Consider an online discussion forum for fielding questions.
Consider running a BigBlueButton session where students can send you chat questions regarding the exam, which all students can see your responses to.
Suggest students use a word document to write their answers in.
As a precaution against technical difficulties, suggestion students answer any written/essay-style questions in a word document and copy/paste their answers into the Moodle quiz. This ensures that if they lose internet connectivity they will still have a copy of their work and could, as a last resort, email you a copy of their word document as an attachment.
Provide a back-up plan. 
Before the exam, give clear instruction to the students on what will happen if there are major technical issues such as Moodle crashing, internet connectivity issues, or power outages. Having a word document version of the exam ready to email out for students to complete and email back might be a simple solution for some courses.