Extension Lead Getting Hot or Tripping? Here's Why

When Your Extension Lead Becomes a Problem

Extension leads are essential on site, but they can also be dangerous. A lead that gets hot, trips the RCD, or has intermittent power is trying to tell you something. Here's what's happening and what to do.

Why Extension Leads Get Hot

Overloading:

The most common cause. Extension leads have amp ratings for a reason.

• Check the rating on the cable (usually 10A or 13A)
• Add up the wattage of connected tools
• 1000W = roughly 4 amps at 230V
• Running more than the rating = overheating

Coiled cable:

• A coiled lead can only handle fraction of its rated load
• Coiled cable creates inductive heating
• Always fully unwind extension reels
• Most reels show reduced rating for coiled use

Damaged cable:

• Cuts or nicks in insulation
• Internal conductor damage (often from running over with vehicles)
• Corrosion at connections
• Damaged insulation creates hot spots

Poor connections:

• Loose plugs or sockets
• Damaged pins
• Corroded contacts
• Bad connections create resistance and heat

Why Extension Leads Trip RCDs

Earth leakage:

• Damaged insulation allowing current to escape
• Moisture in cable or connections
• Connected tool has a fault

Overload tripping (MCB, not RCD):

• Too much current for the circuit
• Multiple high-draw tools
• Starting current of motors (temporary high draw)

Testing which:

• Disconnect all tools, reset and reconnect one at a time
• If it trips with nothing connected, lead is faulty
• If it trips when specific tool connects, tool is suspect

Intermittent Power Problems

Loose connections:

• Power cuts out when cable moves
• Often at plug or socket end
• Check for loose wires inside plug

Internal break:

• Conductor broken but insulation intact
• Usually from repeated bending or crushing
• Power works in some cable positions but not others

Arcing at damage point:

• Sparking sound when cable moves
• Extremely dangerous - replace immediately

When to Repair vs Replace

Can be repaired:

• Damaged plug - replace the plug
• Single clean cut - proper splice or shorten cable
• Loose connections - rewire properly

Should be replaced:

• Multiple damage points
• Melted or burned insulation
• Internal conductor damage
• Badly corroded
• Unknown fault (not worth debugging)

Extension leads are cheap. Your safety isn't. When in doubt, replace.

Choosing Quality Extension Leads

For site use:

• Heavy-duty rubber or PVC sheath
• Adequate amp rating (13A minimum for power tools)
• Cable diameter appropriate for length (longer = thicker needed)
• Quality plugs and sockets
• RCD-protected or use with RCD

110V site leads:

• Yellow leads for 110V site supplies
• Correct plugs for 110V system
• Safer voltage but still needs respect

Safe Extension Lead Practice

• Always fully unwind reels
• Don't daisy-chain multiple leads (increases resistance)
• Keep connections off the ground, especially in wet
• Inspect before each use
• Don't run under carpets or through doors (crushing damage)
• Route to avoid trip hazards and vehicle traffic
• Disconnect when not in use

The Legal/Site Position

On many sites:

• All extension leads require PAT testing
• Damaged leads must be removed from service
• RCD protection is mandatory
• Only 110V systems on construction sites

The Bottom Line

A hot or tripping extension lead is a warning. Find the cause - usually overloading, coiled use, or damage. Fix what's fixable, replace what isn't. Electrical fires kill people; don't take risks with dodgy leads. Use appropriate leads for the job, inspect them regularly, and retire them when they're worn out.

Your power tools deserve reliable power delivery.