Contactor noisy only when the load is connected
The contactor pulls in and holds quietly with no load, but buzzes or hums once the load is connected and drawing current.
Safety first
Buzzing under load means arcing/heating at the contacts. Isolate before inspecting contact faces. Treat the load side as live.
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.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Worn/pitted main contacts (poor make under load)
Most likelyContacts that make poorly under load current arc and buzz, even though they seat fine unloaded.
- 2
Coil voltage sagging when the load draws current
#2If the control supply shares with the load and sags under load, the contactor can't hold cleanly.
- 3
Loose load-side connection causing arcing
#3A loose load terminal arcs and vibrates only when current flows.
- 4
Under-rated contactor for the load
Least likelyA contactor too small for the load struggles under current.
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.
Measure coil voltage with the load connected and drawing current.
Steady rated coil voltage even under load.
Coil voltage steady — inspect the main and load-side contacts.
Coil sags under load — separate/strengthen the control supply.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Buzzes only under load
→ step 2 - 2decision
Does coil voltage stay steady under load?
Yes→ step 3No→ step 4 - 3decision
Are contacts clean and load terminals tight?
Yes→ step 5No→ step 6 - 4result
Coil sags under load — separate/strengthen the control supply.
- 5result
Check the contactor rating against the load.
- 6result
Worn contacts / loose terminal — repair or replace.
Common mistakes apprentices make
- Testing only with no load and missing the fault.
- Sharing a marginal control supply with the load.
- Overlooking a loose load-side terminal that arcs under current.
- Fitting an under-rated contactor.
When to stop & escalate
If the control supply sags under load across devices, review the control supply arrangement. An under-rated contactor for the duty should be replaced with the correct device.
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.
Related faults
Contactor chattering or buzzing instead of holding in
The contactor rapidly clicks/buzzes, pulls in and drops out repeatedly, or hums loudly without seating cleanly. Often comes with arcing noise and heat.
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.
Loose connection overheating (discolouration / smell)
A terminal or connection is overheating — discoloured insulation, a burning smell, or heat you can feel — a common cause of nuisance faults and a real fire risk.