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The purpose of an alcohol interlock is to prevent drivers from starting their vehicle if they have consumed alcohol and have a blood or breath alcohol concentration (BAC; BrAC) over a preset limit (usually between .025% and .05%). An alcohol ignition interlock is a breath testing device that connects to the starter, ignition or other on-board computer system of a vehicle. The device requires the driver to blow into a breath alcohol testing device in order to start the vehicle. The device prevents the vehicle from starting if breath test results reveal a breath alcohol concentration (BrAC) is found to exceed a certain pre-set limit. This device also requires the driver to continue to pass repeated breath tests while the vehicle is in use to ensure that the driver remains sober while driving.

Alcohol interlock devices must meet a range of technical criteria before being approved for use in a jurisdiction. These devices are also installed at a service centre by professional installers that are approved by the jurisdiction. Alcohol interlocks have a wide range of programmable options that can be tailored to meet the requirements set by the jurisdiction. In addition, several anti-circumvention features have been incorporated into the alcohol interlock to ensure efficient functioning and minimize tampering and circumvention attempts.

The alcohol interlock device is connected to the vehicle's starter or ignition switch. The interlock interrupts the power supply by creating an open relay that prevents power from flowing to the vehicle starter. This prevents the car from starting until it has received a breath sample with a low breath alcohol concentration (BrAC of less than .02%). Figure 1 demonstrates this process.

If the breath test reveals a BrAC in excess of the threshold value, the vehicle will not start and the driver must wait a period of time before trying again. If the driver's BrAC is less than or equal to the pre-set threshold value, the vehicle is operational. Once the vehicle has been started, the interlock system requires periodic running retests to ensure the driver's BrAC does not rise above the threshold value while the vehicle is in use. A data recorder captures the results of all vehicle events and breath tests for later review.

The Alcohol Interlock System