Whenever you build an electronics project that uses line voltage, you must take extra precautions to ensure your safety and the safety of anyone who may come in contact with your project. Line voltage is potentially deadly, so these precautions are absolutely mandatory.
Many people are under the mistaken belief that line voltage is not sufficient to cause serious injury or death. That simply isn't true; 120 VAC is more than enough voltage to kill, given the right conditions. In fact, you should treat any voltage above 50 V as potentially lethal.
When you work with line voltage, be sure to take the following precautions:
Never work on the circuit when the power plug is plugged in.
Never leave exposed line-voltage connections anywhere that you or anyone who comes into contact with your project might accidentally touch. All line-voltage connections must be completely insulated or contained within an insulated project box or a grounded metal project box.
Always enclose projects that use line voltage in a sealed project box so that stray hands can’t accidentally come in contact with bare wires or other components.
Always use grounded power cords if your project is contained in a metal box, and always connect the metal box itself to the power cord’s ground lead.
Always use the correct gauge of wire for the amount of current your circuit will be carrying.
Always ensure that all line-voltage connections are tight and secure. If you’re using stranded wire instead of solid wire, always check for stray strands at your connections.
Always provide some form of strain-relief for wires that carry line voltage. The most common way to do this is to pass the wire through a grommet-protected hole in the project box and tie the wire into a knot inside the box. The knot will prevent the cord from sliding through the hole in the case and possibly pulling loose.
Always incorporate a fuse in the primary side of your line-voltage circuit. The fuse will automatically detect when too much current is flowing and immediately break the circuit.
Never use a fuse that's rated for more than the maximum current your circuit is designed to bear. For example, if you’re using a relay that can switch 5 A of current, use a fuse rated for 5 A or less.
Never arrange the wiring for your project in a way that causes wires to move or rub against one another. The rubbing will eventually wear off the insulation and create a shock hazard.
Always be aware of heat sinks that may be hot.
Never use a three-prong to two-prong adapter to plug a three-prong power connector into a two-prong extension cord. This disables the safety provided by proper grounding and can result in a potentially fatal electric shock if a short circuit occurs.
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Source:http://www.dummies.com/how-to/content/electronics-safety-lesson-how-to-stay-safe-with-li.html
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