The Supreme Court of Canada has upheld a BC case (see R. v. Goleski, 2014 BCCA 80, March 16, 2014 blog entry) which concluded that once the Crown has proven a proper demand, a failure or refusal to provide the required breath sample and an intention to fail or refuse to provide the required sample, the persuasive burden then shifts to the accused to show a "reasonable excuse" for not providing a breath sample:
In our view, the British Columbia Court of Appeal correctly concluded that s. 794(2) properly interpreted, imposes a persuasive burden on the accused to prove an “exception, exemption, proviso, excuse or qualification prescribed by law”. R. v. Goleski, 2015 SCC 6