what are two potential dangers of mains electricity?
electrocution (can cause death)
can start a fire (can cause death)
what are some hazards (5) of domestic electricity?
touching frayed electrical cables
long or overheating cables
damaged or incorrectly wired plugs
allowing water or wet objects to enter plug sockets or touch frayed cables
pushing metal objects into plug sockets
label this diagram of a 3-pin plug
what is a conductor?
a material which allows charge to move easily through it
what is current?
moving electric charges, for example, electrons moving through a metal wire
cause injury or death by an electric shock
what is a fuse?
an electrical component that protects circuits and electrical devices from overload by melting when the current becomes too high
what is an insulator?
material that does not allow charge or heat to pass through it easily
define potential difference
the potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit, units volts (V)
the opposition in an electrical component to the movement of electrical charge through it
resistance is measured in ohms
what are the three wires in a plug?
the neutral wire
the live wire
the earth wire
what colour is the live wire?
what colour is the neutral wire?
what colour is the earth wire?
green and yellow stripes
what is the function on the live wire?
provides the path along which the electrical energy from the power station travels
this wire is alternately positive and negative causing alternating current (ac) to flow along it
held at a voltage of 230 V
what is the function on the neutral wire?
completes the circuit
what is the function on the earth wire?
usually has no current flowing through it
a safety wire to stop the appliance becoming live - protects you if an appliance develops a fault
What do each of the three wires have at their core?
each have a core of copper as it is a good conductor of electricity
what do each of the three wires have as their outer layers?
each has an outer layer of flexible plastic as plastic is a good electrical insulator
what position is the live wire in the plug?
on the right
what position is the neutral wire in the plug?
on the left
what position is the earth wire in the plug?
on the top
how do you remember the position of the three wires in the plug?
blue (neutral) goes left
brown (live) goes right
striped (earth) goes to the top
what are the feautures (5) of a plug?
the case is made from tough plastic or rubber, because these materials are good electrical insulators
the three pins are made from brass, which is a good conductor of electricity
there is a fuse between the live terminal and the live pin
the fuse breaks the circuit if too much current flows
the cable is secured in the plug by a cable grip - this should grip the cable itself, and not the individual wires inside it
what are the pins in the plug made of? why?
it is a good conductor of electricity
what is the case of the plug, the cable grip and cable insulation made of? why?
tough plastic or rubber
these materials are good electrical insulators
what position is the fuse in the plug?
between the live terminal and the live pin
what is the purpose of a fuse?
fuses protect electrical circuits and appliances
works in conjunction with the earth wire to ensure safety as the fuse is a wire that melts and breaks the circuit when too much current flows through it
what is double insulation?
a casing made from an insulator such as plastic
why don't appliances with double insulation require earthing?
they have plastic casings, or they have been designed so that the live wire can not touch the casing. As a result, the casing cannot give an electric shock, even if the wires inside become loose
use a two-wire flex so they do not need an earth wire
what is the symbol for double insulation?
give some examples (2) of double insulated appliances
is there an "outer metal casing" in double insulation?
is there an "outer metal casing" in single insulation?
how many layers of insulation are there in double insulation?
give some examples (3) of electrical appliances with an "outer metal casing"
live wire touches the "outer metal casing", the ... wire provides ...
live wire touches the "outer metal casing", the earth wire provides safe route for the current to flow through
what will happen if the live wire inside an appliance comes loose and touched the metal casing?
you will get an electric shock
why does a strong current surges through the earth wire when the live wire inside an appliance comes loose and touches the metal casing?
the earth wire because it has a very low resistance
what is the earth wire in your home in contact with?
what is the purpose of insulation?
covers the live wire with a material that won't conduct the electricity
what is the danger of a damaged plug?
an cause a spark which can cause a fire
the exposed wires could also expose you to an electric shock
what is the danger of water around sockets?
water can conduct electricity and it provides a conducting path for a large current to flow
it could electrocute you if you were to touch it
what is the danger of pushing metal objects into sockets?
metal conducts electricity
how does the earth wire work together with the fuse to make sure faulty appliances are safe?
if a fault occurs in the live wire, the live wire touches the outer metal casing of theapplaince
the current now flows from the live wire through the outer metal casing and down the earth wire
the earth wire is at OV and the current, which has a high voltage, tried to find the path of low resistance to the ground to be earthed. this cicuit has a low resistance as there is no heating element (has a high resistance). the other circuit has a higher resistance
the current increases dramatically so the fuse wire melts (heating effect) and breaks the cicuit so you cannot be electrocuted
the appliance no longer works
what is the heating effect?
if current is too high the temperature increases and the fuse melts
what is the equation for power?
power = current x voltage
P = IV
rearrange the power equation so that current is the subject
curent = power ÷ voltage
I = P ÷ V
rearrange the power equation so that voltage is the subject
voltage = power ÷ current
V = P ÷ I
what is power measured in?
what is voltage measured in?
what is current measured in?
Use V = E ÷ Q and Q = It to prove E=ItV
V = E÷It so E = ItV
what equation links energy, current, time and voltage?
energy = current x time x voltage
E = I x t x v
what is an easy way to remember the equation that links energy, current, time and voltage?
Itv the tv channel
E = ItV
if E = ItV, how can P = IV?
E = ItV
power = work/energy (E) ÷ time
power = ItV ÷ t
power = IV
A kettle has a power rating of 2000W and is plugged into the mains (230V)
Calculate the current flowing through the kettle
P = IV so I = P ÷ V
= 2000 ÷ 230
= 8.66 (2 d.p) amps
Which size fuse should be fitted if the current is 8.66 (2 d.p) amps
choice: 3A, 5A, 13A
which size fuse should you always choose?
the bigger size
e.g. 1A = choose 3A
e.g. 4A = 5A
e.g. 9A = 13A
what are the three standard choices of fuses?
what size fuse should you choose if the current is 5A ?
A lamp has 60W bulb and is plugged into the mains (230V) Calculate the curent flowing through the lamp
P = IV so I = P ÷ V
= 60 ÷ 230
= 0.26 (2 d.p) amps
what is one of the most powerful devices in the home?
in a home, there is nothing bigger than ... that you can plug in
in a home, there is nothing bigger than 3000W that you can plug in
the fuse would break
if the power of one of your appliances exceeds 3000W, e.g. an electrical oven (7000W), where do you connect it to?
connect it directly to the mains
what is the purpose of circuit breakers?
protect electrical circuits and appliances
what do circuit breakers do to fulfil their purpose?
they detect faults and then stop the flow of electricity
residual current circuit breakers (RCCBs) protect ... circuits
residual current circuit breakers (RCCBs) protect some circuits
how do residual current circuit breakers protect some circuits?
they detect a difference in the current between the live and neutral wires
what workes faster, a fuse or a residual current circuit?
a residual current circuit
what could happen if the plug is damaged?
the live parts could be exposed and you could get a shock
what could happen if the cable is frayed?
the live parts might be uncovered
wht is the danger of the cable being too long for the appliance?
they could be a trip hazard
why should you never push a metal into a plug socket (unless its the pins of a plug)?
metals conduct electricity so you could get a shock
why should you check that there's no water near electrical objects?
water is a very good conducter of electricity so you could get a shock
Briefly: how does earthing and fuses prevent fire and shocks?
If a fault develops in which the live wire somehow touches the metal case, then because the case is earthed, a big current flows in through the live wire, through the case and out the earth wire
This surge in current 'blows' (melts) the fuse (or trips the circuit breaker) which cuts off the live supply
This isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of fire caused by the heating effect of a large current
what are circuit breakers?
Circuit breakers are an electrical safety device used in some circuits. Like fuses, they protect the circuit fro, damage if too much current flows
How do circuit breakers work?
When circuit breakers detect a surge in current in a circuit, they break the cicuit by opening a switch
circuit breakers act as ... fuses
circuit breakers act as resettable fuses
How do Residual Current Circuit Breakers work?
Normally exactly the same current flows through the live and neutral wires
If somebody touches the live wire, a small but deadly current will flow through them to the earth. This means the neutral wire carries less current than the live wire
The RCCB detects this difference in current and quickly cuts off the power by opening a switch
What are advantages of circuit breakers (including Residual Current Circuit Breakers) over fuses?
A circuit breaker (and the circuit they're in) can easily be reset by flicking a switch on the device. This makes them more convenient than fuses - which have to be replaced once they've melted
Residual Current Circuit Breakers (RCCBs) operates much faster than fuses - they break the circuit as soon as there is a current surge - no time is wasted waiting for the current to melt a fuse. This makes them safer
RCCBs even work for small current changes that might not be large enough to melt a fuse. Since even small current changes could be fatal, this means RCCBs are more effective at protecting against electrocution