Читать книгу Electronics All-in-One For Dummies - Lowe Doug, Doug Lowe - Страница 58

Too many people are under the false impression that the 120 volts of alternating current running through household electrical wires isn’t enough to kill. So let’s start by getting one fact straight:

The electricity in your home wiring system is more than strong enough to kill you.

You’re exposed to household electrical current primarily in two places: in electrical outlets and in the lamp sockets within light fixtures. As a result, you should be extra careful whenever you plug or unplug something into or from an electrical outlet, and you should be careful whenever you change a light bulb. Specifically, you should follow these precautions:

 Never change a light bulb when the light is turned on. If the light is controlled by a switch, turn the switch off. If the light isn’t controlled by a switch, unplug the light from the wall outlet.

 If an extension cord becomes frayed or damaged in any way, discard it. When the insulation begins to rub off of an extension cord, the shock hazard is very real.

 Never perform electrical wiring work while the circuit is energized. If you insist on changing your own light switches or electrical outlets, always turn off the power to the circuit by turning off the circuit breaker that controls the circuit before you begin. Many people die every year because they think that they can be careful enough to safely work with live power.IS IT TRUE THAT CURRENT, NOT VOLTAGE, KILLS?There is an old adage that “it’s the current that kills, not the voltage.” Although this statement may be technically true, it’s also dangerously misleading. In fact, it stems from a fundamental misunderstanding of what current and voltage are. It can cause you to take dangerous risks if you don’t understand the relationship between current and voltage.The danger from electric shock occurs as current passes through vital parts of your body — specifically, your heart. It takes only a few milliamperes of current to stop your heart. At somewhere around 10 mA, muscles seize up, making it impossible to let go if you’re holding a live wire. At around 15 mA, the muscles in your chest can seize up, making it impossible to breath. And at around 60 mA, your heart can stop. It takes only a few moments of exposure for these effects to occur.So yes, it is current passing through your body that can kill you.But current is inseparable from voltage. Current can’t happen without voltage, and all other things being equal, the greater the voltage, the greater the current. As a result, it’s very difficult to receive a lethal shock from three volts even if you’re dripping wet and standing on bare concrete. But under those conditions, 30 volts may be enough to create a painful and damaging shock.Saying “it’s the current that kills, not the voltage” is kind of like saying “it’s lack of oxygen, not water” that causes drowning. Although it may be technically true, isn’t it the water that causes the lack of oxygen?

 Never work on an AC-powered appliance when it has power applied. Simply turning the appliance off isn’t enough to be safe. If the appliance has a power cord, unplug it before you work on it. If it doesn’t have a power cord, turn off the power to the appliance by throwing the circuit breaker on your home’s electrical panel.

 Take extra precautions when you’re working with your own AC circuits. In Book 4, I tell you more about working with AC circuits; I say more about AC safety then.