A contactor’s contacts can fuse — weld — under fault current or after long switching wear, leaving the charger’s main switch stuck closed and unable to cut power. Welded contact detection is how the charger proves its switch really opened: it checks for voltage where none should exist, or reads a mechanically linked auxiliary contact, and locks the unit out if the switch is closed when commanded open. Every other protection in the charger acts through that one switch, so proving it still moves is not a refinement — it is the foundation.
Key Takeaways
- Contacts weld through arc erosion, fault current and wear — invisibly, and more readily at commercial duty.
- Detection proves the switch opened, by downstream voltage sensing, a mirror contact, or both — backed by lockout logic.
- Every protection in the charger acts through the main switch; weld detection verifies that single point of failure.
- Buyers specify the function with four written questions: method, timing, response, and test documentation.
How Contacts Weld
Welding is a contact-material failure, not an electronics fault. Three mechanisms drive it. Switching under load draws an arc as contacts part or bounce on closing, melting microscopic spots that can fuse. A downstream short-circuit sends very high current through already-closed contacts, heating the faces until they bond. Wear matters too: every operation erodes the surface, and pitted contacts run hotter and weld more easily. Risk rises with current and operations — a public station switching all day accumulates in months what a home wallbox sees in years. None of it is visible from outside the cabinet, which is why the charger must test for it electrically.
How Detection Works
| Method | How it proves the switch opened | Notes |
|---|---|---|
| Downstream voltage sensing | With the contactor commanded open, the controller measures the output; voltage present means the contacts did not part | Uses the charger’s existing measurement chain; must read correctly with a car connected or not |
| Auxiliary (mirror) contact | A small contact mechanically linked to the main contacts reports their true position to the controller | A direct mechanical witness, specified so it cannot report “open” while the main contacts are welded |
| Detection plus lockout logic | A welded result blocks the next session and raises a fault; no retries into the fault | The response matters as much as the sensing — detection without lockout only records the failure |
The check belongs at both ends of a session: at switch-off, to catch a weld as it happens, and at start-up, so a unit welded while idle never energises. A charger that detects a weld stays out of service until the switching device is replaced — there is no user reset, by design.
Why the Safety Case Depends on It
Every protection in a charger ends at the same place: the main switch. Leakage protection detects a fault — and opens the contactor. Overtemperature decides to stop — and opens the contactor. The session ends: the contactor opens. A welded contactor silently defeats every path while leaving the charger looking normal. That is why certification reviews treat the ability to disconnect as a core safety function — and why the protection layers in our guide to GFCI protection for EV chargers assume a switch that works. Detection turns that assumption into a verified fact, every session.
Specifying It in Procurement
No EVDANCE product page states a weld-detection method for a model — the method lives in the design and certification file. A brand, importer or fleet buyer can put four questions in writing: the method; when the check runs (switch-off, start-up, or both); what the charger does on a positive result; and where the function appears in the model’s test documentation. EVDANCE — a Shenzhen manufacturer founded in 2011, exporting to 30+ countries, private label from 100 units, 3-year standard warranty — supplies the EV-AC03201T AC station and EV-RHDC30K-L DC charger through its commercial charging channel, where detection questions belong in the technical file request via the contact page.
Frequently Asked Questions
If everything else works, is a welded contactor really dangerous?
Yes, because “everything else” acts through it. Perfect detection electronics change nothing if the switch they command cannot open — and the charger is not safely off for the next user, or for service, until the switch is proven open.
How would a driver notice a welded contact?
Usually not — that is the point of automatic detection. What a driver sees is the detection working: the charger refuses sessions or shows a fault. A unit that fails this way needs its switching device replaced, not repeated restarts.
Is weld detection the same as the start-up self-test?
The weld check is one element of the start-up routine, but not only a start-up function: a complete design also checks at switch-off, when a weld occurs, so the fault is caught before the unit sits idle in that state.
Related Reading
- Does an EV Charger Need GFCI Protection?
- How Do Home EV Chargers Work?
- EV Charger Certifications Explained: CE, UKCA, UL, ETL, CB and What Buyers Should Ask For
