Revision summary

Electrical input, power and energy

Kettles, ovens and heaters use the heating effect of current in a resistive element. Electrical work increases the material's internal energy; energy then transfers to water, food, air and surroundings. Input energy need not all become the named useful output.

Power: P = VI

P is energy transferred per second, in W. Use V in volts across the chosen component and I in amperes through it. One watt is one joule per second.

Energy: E = VIt = Pt

With steady operating values, use t in seconds for E in joules. E = Pt also works with the average power over the interval. At fixed V, an average I can be used; if both change, account for the changing power.

Conversions

1 kW = 1000 W; 1 mA = 0.001 A; 1 mV = 0.001 V; 1 min = 60 s; 1 h = 3600 s.

A low-voltage determination needs an ammeter in series, a voltmeter across the same component and a measured time. Choose suitable ranges and check whether the readings remain steady. Electrical readings alone do not determine useful output energy.

Kilowatt-hours and cost

Energy in kW h = power in kW x time in h. One kW h is 3 600 000 J. Cost = energy in kW h x the stated tariff per kW h. A rating gives power; operating time or average power is also needed for energy use.

For the supplied heater: 1.15 kW x 0.20 h = 0.230 kW h. At the example $0.30 per kW h, cost is $0.069 = 6.9 cents. Keep intermediate values before rounding.

Connections and live-wire interruption

• Live: brown, substantial alternating p.d. relative to earth; connects through the plug fuse.
• Neutral: blue, normal load return, near earth potential in the supply model.
• Earth: green/yellow, protective connection to exposed conductive parts where required; negligible normal current.
• Plug: identify L/N/E by terminal labels and viewpoint. The cable grip secures the outer sheath.
• Switches, fuses and breakers: must interrupt live. Opening only neutral can stop current while leaving the appliance connected to live.

Match protection to the fault

• Damaged insulation: exposed live parts or a live-to-case contact can create an unwanted current path.
• Overheated cable: excessive current can damage insulation or cause fire.
• Damp conditions: lower resistance or new conducting paths can increase current through a person.
• Fuse: excess current heats and melts its link. Its rating is in amperes; actual operating time depends on the current.
• Overcurrent breaker: opens contacts automatically under its operating conditions and can be reset after the fault is addressed.
• Earthing: a complete low-resistance protective return allows appropriate protection to disconnect live after a casing fault.
• Double insulation: two independent barriers or equivalent reinforced insulation protect accessible parts without relying on protective earth.

Normal load current returns through neutral. Protective earth has a different role. An RCCB detects an imbalance between supplied and returned current; a fuse or overload breaker responds to excessive current.