Control relay contacts sticking or welded
A control/interface relay's contacts stick closed (or won't make cleanly) — the controlled circuit stays on when it should be off, or switches unreliably.
Safety first
A stuck relay can keep a circuit energised with no control. Don't assume the controlled circuit is off because the relay is de-energised — isolate and prove dead.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Contacts welded by switching too heavy a load
Most likelySwitching an inductive or high-inrush load beyond the contact rating arcs and welds the contacts.
- 2
Worn / pitted contacts near end of life
#2Aged contacts develop pitting and stick or make unreliably.
- 3
No arc suppression on an inductive load
#3An inductive load without suppression arcs hard at switch-off, eroding/welding contacts.
- 4
Mechanical sticking
Least likelyContamination or a sticky mechanism keeps the contacts from releasing.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeDe-energise the relay coil and check whether the controlled circuit actually opens.
Controlled circuit opens when the coil is de-energised.
Contacts release fine — the fault is elsewhere.
Circuit stays made with the coil off — the contacts are stuck/welded.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Relay contacts sticking/welded
→ step 2 - 2decision
Does the controlled circuit open when the coil is de-energised?
Yes→ step 3No→ step 4 - 3result
Contacts release fine — look elsewhere.
- 4decision
Is the load within the contact rating with suppression where needed?
Yes→ step 5No→ step 6 - 5result
Replace the worn/welded relay.
- 6result
Over-rating / no suppression — fit a suitable relay or add suppression.
Common mistakes apprentices make
- Switching an oversized load directly on a small relay contact.
- No arc suppression on an inductive load.
- Assuming the controlled circuit is off when the relay has welded.
- Replacing the relay without fixing why it welded.
When to stop & escalate
Recurrent welding means the contact is under-rated for the load or lacks suppression — correct that, or interpose a suitably rated contactor, rather than just swapping relays.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
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Related faults
Control relay coil not energising
A plug-in or interface relay isn't picking up — its indicator stays off and its contacts don't change, so whatever it controls never operates.
Contactor contacts welded closed — load won't switch off
The contactor won't drop out when the coil is de-energised. The load stays powered even with the control circuit off, because the main contacts have welded together.
PLC output stuck on (device won't switch off)
A field device stays energised even though the program has turned the output off — the output won't release, so the device runs continuously.
Learn the theory
How the gear and circuits behind this fault actually work.