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Electrical Safety

Introduction

This page is all about working safely with electrical circuits. Not only is this more than desirable in a practical situation, but it can also part of the Electronics curriculum. In the vast majority of cases, a good dose of common sense is all that is necessary.

Electrical current has two main effects on the human body:

1. A large current can cause burns due to ohmic heating. In normal domestic situations it is unlikely that large currents will flow through the body due to the high resistance of the skin. High frequency currents tend to travel over the surface of a conductor (called the skin effect) and so skin burns can be caused by contact with high power transmitting antennae at radio frequencies - not very likely in a school setting but worth being aware of, none the less. However, burns can easily occur when conductors, such as wires, pass large currents and get hot. Indirect burns due to other components getting hot is a genuine hazard.

2. A small current flowing through the body can interfere with the nervous system that controls muscle response. The muscles in the body can cease to respond meaning that it can be hard to let go of a live connection and then, fairly quickly, the heart muscles may cease to function leading to death. A current of only a few tens of milliamps can be lethal, the actual value depending on the individual concerned. A.C. is also more dangerous than D.C.

Note: An electric shock is what you get when current passes through the body. Electrocution is when the effect of the electric shock is fatal (either deliberately or accidentally) being derived from "electric" and "execution". Electrocution is often used incorrectly to describe an electric shock that is not fatal.


Electric Shock

As stated above, an electric shock can result in:


Ohmic Heating

Filament LampOhmic heating occurs when a current passes through any conductor that has resistance. Electrical energy is transferred to thermal energy and the conductor has to dissipate this energy. In simple terms, when current flows through a wire, the wire gets hot. The energy transferred each second depends on the potential difference across the conductor and the current through the conductor. However, current and potential difference are related and so it is usual to just talk about the current. The energy transferred per second when a current (I) passes through a conductor of resistance (R) is given by I2R



Electric toasterOhmic heating is extremely useful in many domestic appliances such as toasters, electric cookers, kettles, hair-dryers, hot water heaters, fan heaters, electric fires etc ... and of course, conventional light bulbs where a thin tungsten filament glows white hot. Ohmic heating, particularly in light bulbs is not very efficient but it is very simple. In other cases, ohmic heating is undesirable, leading to an unacceptable 'loss' of electrical energy. In power distribution networks, low currents of just a few amps are used to reduce the heating effect in the power cables - this requires the use of very high voltages.



Electrical fireIn a domestic situation, wiring inside the walls of a house can get hot and cause a house fire. The picture shows the wiring in a ceiling space that was overloaded and caused a fire damaging roof beams etc - luckily this fire was controlled and the damage repaired

The following short video demonstrates the effect of ohmic heating on domestic wiring: [Click HERE for Video] Seriously, don't try this at home!



Lightning damageAn extreme example of ohmic heating is when lightning strikes a conductor and a massive current flows for a very short time. The picture shows a Eucalyptus tree that has been blown apart by a lightning strike. The heating effect of the current flowing in the tree vapourised the water in the wood turning it in to high pressure steam, the force of which was enough to blow the tree apart. It is the rapid heating of the air through which a lightning strike flows that causes the thunder we hear. People have survived lightning strikes, often because they were soaking wet and the current travelled over the wet surface causing skin burns but not the sort of damage the tree suffered.




Dangers of Capacitors

High Voltage CapacitorCapacitors are electrical components that store electrical charge and electrical energy.

They can store energy and charge, at potentially a very high voltage, even after the power supply has been disconnected. The energy stored can be lethal and so a large value capacitor should always be treated with caution! The picture shows a 25µF 1000v capacitor. It would probably kill you if you touched it when it was fully charged ... can you tell if it is charged?

Electrolytic capacitors can explode if connected the wrong way round or if the supply voltage exceeds the maximum working voltage of the capacitor. In extreme cases the explosion can be very sudden and quite catastrophic.
The following video shows what happens to an electrolytic capacitor when it is connected incorrectly. [Click HERE for Video] Seriously, don't try this at home either!



Transformers

Isolating transformerTransformers are used to convert a high a.c. voltage to a lower, safer a.c. voltage. Transformers provide a safe, lower voltage for project work. Transformers are covered in more detail in the section about power supplies.

A slightly different use of transformers is in the bathroom. An isolating transformer is used to provide a safe mains voltage for toothbrushes and shavers etc. The voltage from isolating transformer is not reduced, it is still 230v, but it is isolated from the actual mains ... there is no actual electrical connection to the live and neutral. The result is that touching either of the contacts of an isolating transformer is not hazardous ... touching both contacts is still dangerous though. This makes use in the bathroom safer because one contact could be connected to the user, via moisture etc, and the user would not get an electric shock. However, if no isolating transformer were used, any contact with the live wire would always result in an electric shock.



Hazards in the Electronics Lab

There is not a definitive list of hazards in the Electronics lab, or any science lab, and so common sense is always necessary. Some of the possible hazards include:


Working Safely

Electronics at school level is essentially a safe subject as high voltages and large currents are not used. However, there is always the potential for things to go wrong and safety in the working area can be improved by:


In an Emergency

If somebody is suffering an electric shock: