Electricity Basics

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Voltage and Current

Electricity has 2 fundamental properties,

  • Current in amps (A)
  • Potential difference in volts (v)

They are often thought of as being like water in a pipe, where pressure is voltage, and flow rate is current.

These can also be understood by visualising them as food in a sieve. If you put currants in a sieve and press, the force is equivalent to the voltage, and the rate of flow of currants is equivalent to the electrical current flow.

AC, DC & Rectified AC

Electricity supplies can be ac, dc or rectified ac.

DC (direct current) means a voltage that is nominally constant and stays the same polarity. Batteries produce dc.

AC means the voltage repeatedly varies between + some_value and - some_value, in other words it keeps alternating between + and -. This has practical advantages and disadvantages, and the choice was much argued over a century ago.

UK mains is 230 or 240v ac 50Hz. 50Hz means that the voltage cycles 50 times per second.

For UK mains, the peak voltage is around 330v, and the overall ac waveform gives the same heating effect as 240v dc would It is called 240v because it is equivalent to 240v dc for heating and lighting.

Rectified ac is often confused with dc, but is not the same thing. Wallwarts are the main source of rectified ac, most of them produce this, but not all. As far as wallwarts go it can be treated as if it were dc for most applications. Further detail is outside the scope of a basics article.


  • ac: ~
  • dc: =
  • rectified ac: like = but the lower line is dotted.

Power and phase

Power = voltage x current x power factor

For resistive loads, power factor = 1.

However not all loads are resistive. Motors, transformers, electronics, fluorescent lights and so on have power factors of less than 1.

Resistive loads include: Filament lightbulbs, heating elements,

Also handy: P = V2 / R


Resistance is measured in ohms. Ohm simply means volts per amp.

V = I R

or Volts = Current in Amps x Ohms

Note this only holds true for resistive loads. Motors for example do not obey this equation.

Also beware of the filament bulb gotcha. Cold resistance is around 1/8th of hot resistance with these, so running current will be miscalculated if you use a cold resistance figure.

A similar situation occurs with some load types, so measuring resistance doesnt always tell you what current something takes.

230v or 240v

The UK has been 240v since the 1950s, but in recent times harmonisation was introduced to make 240v UK and 220v Europe more compatible. In practice the UK is still 240v, and France is still 220v, but the upper and lower permitted voltage limits have been changed for both countries so that both now cover the same voltage range. Consequently modern appliances are generally compatible with both British and French mains supplies.

However the supplies have stayed at real voltages of approx 220 and 240v to satisfy the large amount of pre-harmonisation equipment still in use.

Filament bulbs are affected by voltage too much to have any one type of bulb work satisfactorily on both 220v and 240v. 240v filament bulbs used in France will be dim, and 220v bulbs used here run bright and have short lives.

So are we 230v or 240? To some extent its a game of words. Really we're still 240v with 240v filament bulbs, but our 240v with the new asymmetric tolerance limits is now known as 230v, even though in real voltage terms it isnt. France's 220v with revised tolerance limits is also known as 230v.

There are always people who will argue for 230v and those who will argue for 240v, but the argument is moot.

Note that older appliances built before harmonisation were usually not designed to run on the full voltage range the other side of the channel, so may not always be fully UK/Fr compatible.


Rated fuse current is the current the fuse will pass indefinitely. Significantly more current flow is needed to blow the fuse.

Why didn't the fuse blow?

A fuse needs a certain amount of temperature rise to blow, and it is a finite lump of metal, albeit a small one, so it needs a certain amount of i x t to get that temp rise. The higher the fuse current rating, the thicker it is, and thus the more thermal capacity it has, and the more i x t it needs to blow. This is why high current wires such as cooker feeds can short momentarily but not blow the fuse.

How safe is electricity?

20 to 30 people die from electrocution each year in the UK. Of these, most are due to faulty appliances, with a minority due to faulty fixed wiring installations and a small number due to darwinism.

In 2001 there were 69,000 house fires in the UK, of which 18,800 were repored as electrical fires, caused mostly by misuse of appliances. 486 people perished and 13,900 were non-fatally injured in the 69,000 fires.

Thus fire is a bigger killer than electric shock.

A smoke alarm raised the alarm in 17,700 of these fires, resulting in less damage, less injuries and fewer casualties.

Source: Fire Statistics United Kingdom, 2001, 2003, ODPM.

See Also