Sunday, January 27, 2008

Grounding

First we will talk about 120 volt branch circuits in your house, and why they must be grounded. Here is where a circuit originates:



Notice that the neutral wire and grounding wire are connected to the same buss bar.
Let’s follow the power to show you why.



The power comes from the breaker box to a receptacle as shown. When you plug in something like a clock or television, the power goes in through the hot wire, through the appliance and leaves on the neutral wire.

The grounding wire does not normally carry current. But it is needed when you plug in a metal chassis appliance. This could be a computer, refrigerator, or such. The grounding conductor is used to ground appliances for safety. Here is a demonstration:



This is a standard grounding plug with the third “prong”. This prong connects to the metal chassis. This connects the entire appliance to the neutral buss bar, (which connects to the ground) when plugged into a receptacle.



If something goes wrong (like a broken or bare wire) and the power makes it to the chassis, it (or anyone touching it) will become a conductor, unless the chassis is grounded.




If the circuit was not grounded here is the scenario: The electricity enters the black wire and to the chassis (bare wire touching chassis). Instead of going to the motor it now goes to you if you touch the appliance.

You could be injured or killed, depending on your connection to the ground. Here is what happens if the appliance is grounded:




The power enters from the black wire and to the chassis. However, the appliance is grounded by the grounding wire (green). A short circuit will result. Have you ever seen what happens when a live wire is touched to a grounded wire?

There is a loud pop and a great deal of current is used. The current now flows through the green wire and back to the breaker. This huge current rush will trip the breaker and turn off the power to the appliance.

Do you see why it is not a good idea to use one of those 3 prong to 2 prong adapters to plug in a metal appliance? If you do, your appliance will not be grounded.

You may live in an older house without grounded outlets. There are 2 ways to correct that. You can run a ground wire and use a new outlet. Or you can install a GFCI receptacle. The GFCI stands for “ground fault circuit interrupter”. They are also called “shock savers”.

Shock savers monitor the electric current coming and going. If there is a slight difference it will kill the power in a fraction of a second. Your outlet may be grounded, but if you are wet and in a situation to trip the breaker, it may be too late.

It may take the breaker, a second or so to trip. Electricity travels close to the speed of light. It could pass through you many times in one second. The GFCI can be a lifesaver.

GFCI outlets are important to have in bathrooms and kitchens. You should use one anytime there is an outlet within six feet of a water source. They should always be used outside, too.


At a later date I will write a section on installing GFCI outlets. Next is series number 8 "Parallel Circuits".

1 comment:

raeb said...

consider this:

with the current on, the ground wire is safe to touch, and is safe if its bare.

since both the neutral wire and ground wire come from the same bus, is the neutral wire also safe to touch?