Difference between revisions of "Electricity Basics"
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Also handy: '''P = V<sup>2</sup> / R ; P = I<sup>2</sup> × R'''
Also handy: '''P = V<sup>2</sup> / R ; P = I<sup>2</sup> × R'''
Latest revision as of 16:01, 19 October 2015
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.
Plugs & Sockets
The mains plug is the one with the 3 pins sticking out. What it plugs into is a socket (shown). There's no such a thing as a plug socket or a socket plug.
Cable and Flex
'Cable' generally refers to electrical wire with single solid copper conductors. If flexed repeatedly it cracks and breaks.
Fixed wiring uses cable, as it dos not sag and is slightly cheaper than flex. If however the wire will experience movement, flex must be used.
'Flex' means electrical wire using stranded conductors. This is used for appliance leads, and any other applicaton where the wire will be moved. Cable is not suitable for this job.
Cable may not be used on portable appliances, as movement will cause the wire to fail.
Flex can be used for fixed wiring, as long as the connectors grip it properly, and it is sufficiently rated for the job. However it is deprecated as flex found in domestic fixed wiring all too often means under rated appliance flex, and who knows what other bodges hidden away. Also not all cable connectors grip flex satisfactorily.
Flex is short for 'flexible cable,' so technically flexes are a type of cable, but the 2 words are generally used with the meanings above.
AC, DC & Rectified AC
Electricity sources 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. 240v is the 'rms' value.
Rectified ac is often confused with dc, but is not the same thing. Some wallwarts produce rectified ac. It can be treated as if it were dc for most applications, as only a minority of devices will fail to work on rectified ac. Further detail is outside the scope of a basics article.
Power and phase
With simple resistive loads, current is proportional to voltage at any instant, so current and voltage waveforms are in phase. Power factor is defined as 1.0 in such a situation, and Power = voltage × current
Some loads are more complex, and the current waveform is not in phase with the voltage waveform, is distorted, or is both. For those,
Power = voltage × current × power factor
Power Factor is a measure of how ill-matched the 2 waveforms are. For resistive loads, current & voltage waveforms are the same shape and in phase, and power factor = 1.0.
- Resistive loads include: filament lightbulbs, heating elements,
- Inductive loads include: motors, iron ballasted fluorescent lights, etc.; these have a PF of less than 1.0,
- Capacitive loads have PF less than 1.0, but are seldom encountered in practice,
- Loads that are neither inductive or capacitive, but have a PF less than 1.0, include: CFLs, rectifier-based power supplies, switched mode supplies and appliances with switched mode supplies in them.
Also handy: P = V2 / R ; P = I2 × R
For more, see Power_factor in this Wiki.
Resistance is measured in ohms. Ohms simply means volts per amp.
Ohm's law: V = I × R
or Volts = Current in Amps × Ohms
Note this only holds true for resistive loads. Motors for example don't 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 doesn't always tell you what current something takes.
The equations for inductive loads (motors etc.) are more complex, and outside the scope of a basics article. For more info, see Droppers.
230v or 240v
The UK has been 240v since 1960, but in recent times harmonisation was introduced to make 240v UK and 220v Europe more compatible. In practice the target voltage in the UK is still 240v, and in 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 normally compatible with both British and French mains supplies.
The supplies have stayed at target voltages of 220v (fr) and 240v (uk) to satisfy the large amount of pre-harmonisation equipment still in use, a percentage of which is not able to work or run safely & reliably on the wrong voltage.
This means modern 230v appliances will work fine on either UK or FR supplies, but old equipment marked 220v or 240v is not always cross-compatible.
Filament bulbs are an exception to the above situation, as they're affected by voltage too much to have one type of bulb work satisfactorily on both 220v and 240v. 240v filament bulbs used in France would be too dim, and 220v French 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 isn't normally 230v. The spec is 230v -5% +10%, or 218.5v to 253v. France's 220v with revised tolerance limits is also known as 230v.
There are always people who will argue whether we're 230v or 240v, but really the argument is a bit moot.
Before 1960 there were multiple standards in use in Britain. Most places were anything from 200v to 250v 50Hz, but some were 110v. Its common to see transformer tappings for 200,220,230,240,250v on 1950s & 60s appliances.
Rated fuse current is the current the fuse will pass indefinitely. A lot more current flow is needed to blow the fuse. For example a 13A mains plug fuse can pass over 20A for half an hour without blowing.
Why didn't the fuse blow?
A fuse is a thin wire link, and needs a certain amount of temperature rise to blow (melt). Its a lump of metal, albeit a small one, so it needs a certain amount of current and time to get that temp rise. The amount of temp rise is calculated by i squared times t. The higher the fuse current rating, the thicker it is, and thus the more thermal capacity it has, and the more i squared t it needs to blow. This is why high current wires such as cooker feeds can short momentarily but not blow the fuse.
Effective cord grips stop a few causes of accidents happening:
- Flex with bare live conductors pulled out by tripping over the flex
- Flex ends move in use, gradually fracturing the copper conductors leading to overheating and sometimes fire
- Flex ends move in use, gradually undoing screw terminations, leading to overheating and sometimes fire
To be properly effective, a cord grip needs to completely stop the flex moving.
Fixed wiring generally doesn't use cord grips as its fixed still, whether by cable clips or due to inaccessibility.
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.
The appliance that kills the most people is the electric blanket.