The Tuck-Shop Extension Lead That Gets Warm
The owner of a busy tuck-shop at a bus rank has one 240 V wall socket, protected by a 15 A circuit breaker at the distribution board and by a 13 A fuse in the plug. She buys a four-way extension lead marked '10 A maximum' and plugs in an electric kettle rated 2000 W, a hot plate rated 1500 W, a refrigerator rated 150 W and a phone charger. On a cold morning the kettle and the hot plate are switched on together while the fridge is running. After several minutes the extension lead feels warm along its length, and the plug fuse has still not blown. She asks an electrician whether she can simply fit a 20 A fuse 'so it stops nuisance-blowing', because she cannot afford to close the shop while a second socket is installed. The lead currently runs under a rubber mat to keep it out of the way.
1. Calculate the current drawn by each appliance and the total, and use the figures to explain why the lead is warm.
Read the worked answer
Currents, using I = P/V:
- Kettle: 2000/240 = 8.3 A
- Hot plate: 1500/240 = 6.3 A
- Fridge: 150/240 = 0.6 A
- Charger: negligible (well under 0.5 A)
Total when all run together: 8.3 + 6.3 + 0.6 = about 15.2 A.
Compare with the ratings: the lead is rated 10 A and the plug fuse 13 A. Both are exceeded - the lead by about 50%, the fuse by about 17%.
Why the lead is warm: the copper conductors in the lead have resistance, so each metre dissipates heat at a rate I^2R. At 15.2 A instead of 10 A the heating is (15.2/10)^2 = about 2.3 times what the lead was designed to shed. Wound on a reel or covered by a rubber mat, the heat cannot escape, so the temperature keeps rising. The mat makes a dangerous situation worse.
Why the fuse has not blown yet: a fuse is a thermal device. A modest overload of 15 A through a 13 A fuse may take many minutes to melt the element, and the plug is in open air so it cools. The absence of a blown fuse is not evidence of safety - the warm cable is the real warning, and it appears long before the fuse acts.
Note: if the supply is nearer 230 V, the currents are slightly higher still.
2. Explain what is wrong with fitting a 20 A fuse, including what each protective device is actually there to protect.
Read the worked answer
The rule: a fuse must be rated at or below the current-carrying capacity of the cable it protects. Its job is to protect the flexible lead, not the appliance and not the wall wiring.
Fitting a 20 A fuse would:
- Remove protection from a lead rated 10 A. A 15.2 A load would run indefinitely - the fuse would never act, because 20 A is above the load.
- Allow the lead's insulation to soften and crack through sustained I^2R heating. Once the live and neutral conductors touch, the result is a short circuit, sparks and possibly a fire under a rubber mat in a wooden stall.
- Exceed the rating of the socket outlet itself, which is a 13 A accessory. The plug, socket and lead would all be operating outside their design limits.
What the 15 A breaker protects: the fixed wiring inside the wall, from the distribution board to the socket. It is not a substitute for the lead's protection, because 15 A is still above the lead's 10 A rating - the breaker would happily pass a current that destroys the lead.
Judgement: the proposal is exactly backwards - it removes the weak link just as the weak link is being overloaded. The warm lead, not the intact fuse, is the accurate message from the system. The electrician should refuse and record why.
3. Recommend a practical plan for the owner, separating what she can do today from what needs an electrician.
Read the worked answer
Today, no tools needed:
- Do not run the kettle and hot plate together. Kettle + fridge = 8.3 + 0.6 = 8.9 A, which is inside the 10 A lead rating; hot plate + fridge = 6.9 A, also fine. Only the combination of the two 1 kW-plus appliances is unsafe.
- Uncoil the lead fully and take it out from under the mat so heat can escape; keep it away from water and from customers' feet.
- Replace the 13 A plug fuse with the correct 13 A fuse - never a higher one, and never foil or wire.
- Feel the plug and lead periodically; warmth means stop.
Needs an electrician:
- Install a second socket on its own circuit, with cable sized for the hot plate (at least 2.5 mm2 for a 15/16 A radial circuit) and its own breaker. This is the only real solution for a shop that needs both appliances.
- Check the existing circuit's cable size and earthing, since the shop has been running near its limit.
Judgement: the immediate fix is behaviour, not hardware. The permanent fix is a circuit designed for the load. Swapping the fuse for a bigger one solves the symptom (nuisance blowing) by removing the safety device, which is the one thing that must never be done.