Fuses

The purpose of a fuse is to be a deliberate weak link in the wiring ‘circuit’. If the current in the circuit exceeds the level intended, the wire in the fuse overheats and melts, breaking the circuit so electricity can no longer flow. Fuses are available in 0.5A, 1A, 2A, 3A, 5A and 13A ratings or values (3A and 13A are most common).

You would expect an electric kettle to have a higher fuse value than a table lamp as it uses more electricity, i.e., higher wattage and amps. Apart from the fuse in the plug (or other connection device) there will be a consumer unit/distribution board in dwellings which is also fused. the mains electricity passes through a meter of some sort and then enters the consumer unit. The output of this unit is split into various circuits, most commonly: upstairs lighting, downstairs lighting, upstairs power sockets, downstairs power sockets, plus cooker and immersion heater supplies. Each of these have different fuse values.

RCCB (Residual Current Circuit Breaker)

In many modern domestic electrical supplies the fuses have been superseded with an RCCB (residual current circuit breaker). These breakers are sensitive to both the amount of current flowing in a circuit and the rate at which it changes. The extra current when, for example, a washing machine changes to fast spin, will not trip the breaker, but if the electric motor overheats or seizes up, then the circuit breaker will trip, as it would if you touched something live, and the electricity found a path to earth through you. It is unlikely that a consumer unit or appliance fuse would blow or trip under these latter circumstances, but an RCCB would. This is why they afford greater safety and are being use more and more.

Watts

The power rating of a device or appliance is measure in 'watts', named after James Watt. A light bulb will be marked, e.g., 60W (sixty watts), an electric iron will usually be between 1,000 and 1,500W, an electrical kettle around 2,000W and an electric heater, a maximum of 3,000W.

Anything over 1,000W may be marked using the letter k to stand for one thousand, e.g., 1.75kW equals one thousand, seven hundred and fifty watts, (1,750W), note that the k is lower case and the W upper case.

Ohm’s Law

Georg Simon Ohm was a German Physicist who published in 1826 the results of his research into what happens in an electric circuit. From his work he proposed Ohms Law, that the voltage (potential difference) across a “conductor” is proportional to the current flowing through it. In other words R = V/I where V is the voltage, I is the current in amps and R is the “resistance” which the conductor offers to the current flow. This is often shown in triangular form. The triangle indicates the three way relationship and can be used to calculate other values, e.g., V = I x R or I = V/R.

For our purposes, this can be translated into the following more practical expressions:

Watts divided by amps = volts
Watts divided by volts = amps
Amps multiplied by volts = watts