Download Electrical Safety -- and its implications for designing products

Electrical Safety -- and its implications for designing products
1. How can electricity be unsafe ?
Basically two ways.
a) It can flow through living tissue (including ours !) and cause damage either
through resistive heating or through upsetting the rhythm of the heart. The latter
unfortunately requires less current (under 50 mA can be enough !) and the upset
rhythm can stop the heart beating and therefore be fatal.
b) The heating effect can cause a fire either in the product or elsewhere.
2. How can we make it safe (or at least as safe as possible) ?
a) By making it as difficult as possible for the user to come into contact with any
voltages high enough to produce enough current in the user to cause injury.
240 V
This user will suffer
an unpleasant electric
shock .. may be fatal.
240 V
This user will
probably not suffer a
shock ... why not ?
There is very little we can do to prevent a user who simultaneously touches both
terminals of a mains supply from getting an electric shock. One thing we can do is
to provide a FUSE, which is a piece of low-melting-point wire which heats up and
melts if it is carrying more current than it should. The fuse is now often (usually ?)
contained in a small "cartridge" so that it can be changed more easily if it blows.
But the fuse will only protect against a higher current than the circuit should be
taking .. and the "normal" current is almost certainly enough to cause the user a
fatal shock. There are other ways of protecting against other forms of malfunction,
as we will see in due course.
The product designer can do much to minimise the risk of an electrocuted user.
The parts of the appliance which are at mains voltage are often placed in a metal
enclosure which is difficult to dismantle or insert implements into and is earthed.
The incoming cable is securely clamped near its point of entry to prevent it from
pulling out (perhaps with live wires protruding at the end) accidentally. We will see,
both today and in future lab/tut sessions, how this requirement is met in actual
products.
Earthing the enclosure serves two purposes. The first one is that a current through
the user cannot result if everything touchable is at 0 V. To understand the second
one,we need to examine how mains supplies are arranged.
Line
Our Appliance
"The Works"
Neutral
Earth
The socket in our home etc.
We notice that the 'line' and 'neutral' wires are the ones actually carrying the
supply current; the 'earth' wire is there for safety purposes and it carries no current
if the appliance is working properly. The neutral wire is connected to earth at the
supply transformer but NOT at the user's premises. The earth connections in the
sockets in our home/Uni/firm etc. are literally that; they are all wired to a spike
driven into the ground (a more old-fashioned solution was to connect them to a
copper water-supply pipe, but with the trend towards plastic pipes we cannot
guarantee the pipe to conduct throughout). We will assume that both the inner and
outer appliance enclosures are connected to earth for this purpose (there may only
be one in practice). In normal circumstances, therefore, the user can only touch
the earthed 'box' and therefore cannot get a shock.
If a fault causes the line input to make contact with the 'box', the circuit will be
completed via earth and a heavy current will flow. The fuse will blow, often with a
bang, and the appliance will be left 'dead' but safe.
We still have the problem of a less serious fault in which only mA flow from the line
to earth (e.g. a washing machine or a lawnmower on a damp morning where a few
drops of water have found their way where they should not have done). We can
guard against that eventuality by using a Residual Current Breaker or RCB. This
device is placed between the incoming mains supply and the appliance and it
incorporates a circuit breaker (a form of switch) which opens automatically when
the line and neutral currents are not equal, i.e. when some current is leaking away
to earth (the device used to be known as an Earth Leakage Circuit Breaker).
These devices are normally set to operate at a current imbalance of between 10
mA and 25 mA. 10 mA is better at more or less completely preventing shocks for
sensible users but in situations like washing machines or lawnmowers it frequently
opens the breaker on momentary low-level leakage which can quite annoying !
Serious injury is very unlikely at 25 mA.
An alternative to the earthed metal enclosure is one made of insulating material
such as some form of plastic. In that situation, the user would be protected from
harm by the fact that the enclosure cannot conduct current, but danger could still
be present in damp conditions unless the enclosure is hermetically sealed (which
may be difficult if cooling is needed, as electrical insulators often do not conduct
heat well either).
3. Low-voltage equipment having a mains power supply
This category includes most electronic equipment., as electronic circuitry normally
requires a supply of a few volts d.c. Unfortunately, the supply needs to be steady;
significant fluctuation in the voltage is usually not tolerable. So we have to do three
things with our incoming mains voltage; transform it down to just a few volts, rectify
it (make it unidirectional instead of alternating), and smooth it and regulate it.
Rectification is done with DIODES or a DIODE BRIDGE. A diode is a 'one-way
street' for current.
This circuit is a HALF-WAVE RECTIFIER.
a.c.
supply
Our product (shown as
a resistor)
We can do better if we utilise the negative half-cycles as well.
a.c.
supply
Our product (shown as
a resistor)
Unfortunately, though the current will all now go in the same direction, the voltage
will fluctuate considerably. The graph on the next page shows how the rectified
voltage will vary with time.
In electronic systems, the usual method of smoothing the voltage is to place a
capacitor in parallel with the output. The capacitor acts as a 'reservoir'; it fills up
with electric charge when the rectified voltage is near its peak, and then supplies
the load whilst the rectified voltage is lower. (Just like an actual reservoir with our
water supply). Where we do not have any electronic circuits in our product but
only, for example, a small d.c. electric motor, we may not need to do any
smoothing at all.
400
The rectified version
300
200
Voltage
100
0
-100
-200
-300
The supply voltage
-400
0
0.01
0.02
0.03
Time in seconds
0.04
0.05
A complete power supply arrangement
Transformer
Electronic
Turns n1:n2
Circuit
Some possibly useful information
Rectified average voltage (full-wave unsmoothed) = VPeak x 0.637 = r.m.s. x 0.9
(The capacitor-smoothed version is almost equal to VPeak).
Transformer:
V1/V2 = n1/n2 = I2/I1
0.06
Some Points to Think About
1. Why do we often see birds land on overhead power lines, stand there
nonchalantly, and then fly off again (rather than chirping "Ouch !" and flying off
immediately, or just falling off electrocuted) ?
2. Comment on the safety or otherwise of a user of an unearthed washing
machine with no r.c.b. in use.
3. Comment on the steps the designer has taken to make the appliances you will
see dismantled electrically safe.
4. We often hear of appliances being "PAT Tested". What does the term mean and
what tests are done ? Why ?