Interposing relay energises but the load contact won't switch
A PLC/control output energises an interposing relay (you can hear/see it pick up), but the load it's meant to switch — a contactor coil, valve, or lamp — doesn't operate.
Safety first
The relay's contact side often switches a higher voltage than the coil side. Don't assume the contact side is dead just because the coil is low voltage — confirm isolation before probing the output side.
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
Worn, pitted, or welded relay contact
Most likelyThe coil picks up fine, but the switching contact is burnt or not making, so the output side stays open even though the relay 'operated'.
- 2
Wrong contact used (NO vs NC)
#2Wiring the load through a normally-closed contact, or the wrong pole, means the output is open when the relay is energised — common after a relay swap.
- 3
No supply to the relay's contact (output) side
#3The contact is fine, but the switched supply feeding it is missing — a blown control fuse or open upstream of the contact.
- 4
Relay not fully seating in its base
Least likelyA relay not pushed home, or a tired socket, can energise the coil yet not connect the contact through to the base terminals.
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 ModeConfirm the coil side: correct voltage across the coil and the relay visibly/audibly operating.
Rated coil voltage present and the relay operating.
Coil side is healthy — the fault is on the contact/output side.
If the coil isn't getting its voltage, fix the command/coil feed first.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Relay energises, load contact dead
→ step 2 - 2decision
Is the coil side healthy (relay operating)?
Yes→ step 3No→ step 4 - 3decision
Supply on the common and contact making when energised?
Yes→ step 5No→ step 6 - 4result
Fix the coil command/feed first — see relay coil not energising.
- 5result
Output is switching — fault is downstream in the load or its wiring.
- 6result
Contact not switching / wrong pole / no feed — rewire to correct NO pole or replace the relay.
Common mistakes apprentices make
- Hearing the relay click and assuming the output must be working.
- Wiring the load through an NC contact after a relay change.
- Forgetting the contact side needs its own supply, separate from the coil command.
- Not checking the relay is fully home in its socket.
When to stop & escalate
If interposing relays in the panel are repeatedly failing on their contacts, escalate to review whether the relay rating suits the load (inductive/inrush) rather than just swapping like-for-like.
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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