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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 ?