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.
Safety first
Overheating connections are a fire risk and can fail suddenly. Isolate before touching. Discoloured/charred terminations may be brittle — handle with care.
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
Loose terminal (high-resistance joint)
Most likelyA loose connection has high resistance, so it heats under load — the most common cause.
- 2
Under-torqued or over-torqued termination
#2Incorrect tightening (too loose, or damaged by over-tightening) creates a poor joint.
- 3
Corroded / contaminated contact
#3Oxidation or contamination raises contact resistance and heat.
- 4
Overloaded connection / undersized for current
#4A termination carrying more current than it's rated for runs hot.
- 5
Mixed metals / poor lug crimp
Least likelyDissimilar metals or a bad crimp degrade over time and overheat.
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.
Safely identify the hot spot (thermal imaging or careful inspection) under load.
A clear localised hot connection.
Hot spot found — isolate and inspect/repair that termination.
If widespread heat, consider overload across the connection point.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Connection overheating
→ step 2 - 2decision
Is there a clear localised hot connection?
Yes→ step 3No→ step 4 - 3decision
After isolating, is the termination sound (just needs re-terminating)?
Yes→ step 5No→ step 6 - 4result
Widespread heat — consider overload at the connection point.
- 5decision
Is the connection within its current rating, and cool after repair?
Yes→ step 7No→ step 8 - 6result
Damaged/charred — cut back and remake properly.
- 7result
Repaired and cool — consider why it loosened.
- 8result
Overloaded/undersized — address loading/sizing.
Common mistakes apprentices make
- Just re-tightening a charred terminal instead of remaking it.
- Over-tightening and damaging the termination.
- Ignoring mixed-metal or poor-crimp joints.
- Not checking the connection is rated for the current it carries.
When to stop & escalate
Badly damaged terminations or evidence of an overloaded connection point should be repaired and the cause addressed (re-termination, correct torque, or design review). Treat overheating as urgent due to fire risk.
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
Voltage drop on a long cable run
Equipment at the end of a long run misbehaves — dim lights, a contactor that won't hold, a motor struggling — because volt-drop along the cable leaves too little voltage at the load.
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Learn the theory
How the gear and circuits behind this fault actually work.
Ohm's law & power
The relationship between voltage, current and resistance — and how it gives you power.
Voltage drop
Volts lost along a cable's resistance under load — why the far end of a long run can misbehave.
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.
Earth-fault loop impedance
The path a fault current takes back to the source, and why its impedance decides whether protection trips in time.
Cable current capacity & derating
Why the same cable can safely carry less current in some installations than others — it all comes down to heat.