Alcohol interlocks stop the vehicle from starting when alcohol is detected in the driver’s breath sample. They are used as a safety measure in commercial transport and as a condition after sanctions for drink-driving, supporting the legal duty to drive sober in Norway.
How alcohol interlocks work
An interlock combines a breath handset and a control unit wired to the starter circuit. If the measured value exceeds the set threshold, the engine remains locked and the event is stored for follow-up.
| Component | Function | Notes |
|---|---|---|
| Handset | Guides the driver through the breath test | Displays status and fault codes |
| Measuring unit | Analyses alcohol level | Needs regular calibration |
| Control unit | Allows or blocks start | Records every attempt |
| Relay harness | Interrupts the starter circuit | Must be installed professionally |
Thresholds and monitoring
| Parameter | Typical value | Action when exceeded |
|---|---|---|
| Start limit | 0.10 mg/l (≈0.2‰) | Engine blocked |
| Retest | 5–30 minutes | Requires a rolling retest |
| Data retention | 6–24 months | Authorities may request the log |
For the legal basis, see Vehicle regulations.
When interlocks may be mandatory
| Situation | Example | Legal reference |
|---|---|---|
| Commercial transport | School transport contracts | Requirement in tender specifications |
| Sanctions | Alternative to a longer disqualification | Decided by authorities |
| Corporate policy | Internal HSE programme | Agreed within the company |
Many municipalities demand interlocks in Traffic control and flow to protect vulnerable passengers. Drivers with a drink-driving history can receive interlock conditions to regain their licence, see Disqualification, probation and penalty points.
Daily use
- Perform the breath test before start; the display confirms ready to drive
- Complete rolling retests safely when prompted while driving
- An interrupted or manipulated test blocks the next start and creates a log entry
- Keep the handset clean and warm in winter to secure accurate readings
For required routines, see Duty to remain sober.
Installation and approval
| Action | Responsible | Key requirement |
|---|---|---|
| Installation | Approved workshop | Follow the manufacturer’s instructions |
| Calibration | Certified technician | Normally every 6–12 months |
| Log review | Employer or Norwegian Public Roads Administration | Detects misuse or manipulation |
| Documentation | Driver/owner | Must be available during inspections |
Tampering with an interlock can trigger further penalties, as outlined in Sanctions: fines, confiscation, pledges and points.
Consequences of misuse
| Breach | Possible reaction | Outcome |
|---|---|---|
| Attempt to start with alcohol present | Log warning, start denied | Follow-up by employer or authorities |
| Tampering | Contract sanctions, licence issues | May extend disqualification |
| Missed calibration | Warning, usage ban | Service required before the next trip |
Regular follow-up lowers the risk of serious incidents; see Alcohol, BAC limit and drugs.
Safety benefits
- Prevents impaired driving within high-risk groups
- Strengthens corporate HSE work and documentation
- Builds trust among guardians and clients in passenger transport
- Provides clear sanction options for repeated breaches