If a person is in charge of a motor vehicle on a road or other public place when that person is over the prescribed limit then that is a contravention of Section 5(1)(b) of the Road Traffic Act. 

The penalty for this offence is endorsement of the licence with 10 penalty points or imposition of a period of disqualification. 

It is a defence if a person can prove at the time of the offence that there was no likelihood of driving whist the accused was over the prescribed limit. It is usual that an expert toxicology report would be required to establish this defence.