Why is There a "Live" Wire in Our Electrical Supplies?

3-pin plug. © Eugene Brennan

If you've ever wired a plug, you'll know there are three cores in the flex that need to be connected to the terminals: brown - live, blue - neutral and green/yellow - earth. The same goes for the wiring to all socket outlets and ceiling roses. The brown wire is the dangerous one because it's normally at a voltage of approximately 230 V when power is connected. So if AC voltages switch direction and there's no inherent difference between live and neutral, why is live at 230 V and neutral at 0V (assuming earthing is done properly in a premises)? Why if I use a phase tester screwdriver or Fluke VoltAlert type tester do I get a glow when I check the live wire? The reason is because all supply transformers have one point in the secondary winding earthed to the bulk of the ground surface at one or more points. The neutral supply to our homes is supplied from that point in the transformer. Also neutral in our homes is tied to the earth terminal at the electrical panel and connected to an earth rod to pull the potential of earth and neutral as close as possible to zero volts. (Known as TN-C-S earthing). Because neutral and earth are held at zero volts, live is consequentially at 230 volts with respect to earth. That's why a phase tester screwdriver lights up because current flows from live to the tip of the screwdriver, through the neon tube and high ohmic value resistance down through your body to the ground you're standing on. It then makes its way trough the bulk of the ground back to the supply transformer which is earthed.

 

Why is a supply transformer earthed? 

 

For safety reasons and to cater for dangerous voltages on the secondary that supplies homes. If it wasn't, several dangerous scenarios could arise: For instance, if the insulation broke down in the transformer between primary (which is supplied at 10,000 V) and secondary, a lethal voltage could end up being delivered to a home. Another possibility is that very high voltage lines (e.g. the ones carried by the pylons at Silliot Hill) could fall on output lines from the transformer and a third scenario is a lightning strike on a line. By connecting one point of the winding of a transformer to ground, rather than having it floating, it makes the system safer.

 

110 V Safety Transformers 

 

Cordless power tools are more common nowadays, but previously these transformers were used for safety reasons to reduce 230 V to 110 V. Firstly, the lower 110 V lessens the risk of electrocution because current is what kills when it passes through the heart. Lower voltages, reduce the risk because they "push" less current though a circuit. Another safety feature of these transformers is that the centre tap of the secondary winding is earthed (unlike neutral in a home). This means that there are effectively two "lives", but each is only at half of 110 V or 55 V wrt to earth. This is a secondary safety feature, if for instance contact is made with a cut flex.