All About Homes Residential Inspection Service

I have an older home with the old style, two pronged outlets that are not grounded. I’m concerned about having my electrical system properly grounded, but I don’t really understand how a grounding system works. All I know is that it is supposed to prevent you from getting shocked. Can you tell me a little bit about it?

R.P., San Mateo

Grounding an electrical system is more than just driving a grounding rod and connecting it to the main panel. It is a matter of providing a path of least resistance for electricity to return to its source, to prevent it from traveling back through our bodies, outlets, and appliances. To understand grounding, it is important to know how electricity works, and how it travels through a house’s electrical system.

Electricity can basically be broken down into two elements, voltage and amps. A good analogy for electricity is pressurized water in a pipe, like the water lines in your house. Water is similar to the electron flow in an electrical current. As water flows through a pipe, the electrons in an electrical current flow through a wire, or any similar conductive material. Voltage is similar to the water pressure in the pipe, and amperage is similar to the volume of water that comes out the end of the pipe. The common voltage for residential construction is 120/240 volts, and the common amperage is 100 amps.

The one golden rule about electricity is that it always wants to get back to its source. That is why electrical systems need to utilize at least two wires, a hot and a neutral. The hot wire is the one that carries the current to house, and the neutral wire is to provide a return path to the utility or source of the power. Unfortunately, this is not a completely safe system because electricity has the ability to travel through other conductive objects as well as the ground we stand on. The grounding portion of an electrical system is designed to provide a safe, alternate path for electricity to travel back to its source, without the possibility of traveling through a person or an object.

The grounding system for a house, starts at the main panel. In the main panel, there are usually two hot conductors, each 120 volts, that supply the electrical power to the panel, and there is also a neutral conductor that acts as the return path back to the utility. Since the neutral conductor is the dedicated source back to the utility, all potential paths that an electrical current can travel on, need to be connected or “bonded” to this neutral wire.

This is accomplished by connecting a “grounding wire (usually just a bare copper wire), to all of the panel boxes, outlets, appliances and metal pipes in the house, and connecting the other end to the neutral in the main panel. The interesting thing about this connection to the neutral line is that this is only done in a main panel, and not in a sub panel. In a sub panel, the neutral wires and grounding wires must be kept separate, and not connected together. This is why the neutral buss bar located in a sub panel, is isolated from the ground connections by plastic insulators.

If the neutrals and grounds are connected together in a sub panel, and there was a short to the ground, the electrical current would have the ability to travel through other circuits in the house, potentially injuring a person in another area.

Many accidents occur ever year where someone is electrocuted by touching the metal housing of an appliance, metal water line or metal furnace ducting, because an electrical current from a short, was running though it. The current in these items was just waiting for a path back to ground, and in each case, the person touching the item, provided that path. A person’s safety is the most important reason to have a properly grounded electrical system.

To properly ground an electrical outlet, you need to connect one end of a ground wire to the base of the outlet, to the green screw, and the other end to grounded surface. If your house wiring contains a grounding (bare copper) wire, it is probably attached to the metal wall box that houses the outlet. In this case, you can connect the ground wire from the outlet directly to the box. This can be done with either a special screw or clip.

If there is no grounding wire in the wiring, you must run a ground wire to another grounded surface such as the electrical sub panel, or a metal water pipe that has been connected at some point, to the grounding system. This is the only way to guarantee that there is an alternate path for electricity to flow back to its source.

Whenever you are dealing with electricity, it is always best if you consult with a competent electrical contractor. Properly grounding an electrical system is not difficult, but it can be very technical, and there are a lot of specialized fittings and hardware. If electrical work is done incorrectly, or carelessly, it will not provide the protection it should.

John R. Schneider is a licensed general building contractor and a certified residential code specialist. He is president of All About Homes, a residential inspection company, and has been performing code and construction consultations since 1985. Readers may address their comments to John Schneider, 24326 Mission Blvd. Suite 7, Hayward, Ca 94544. Fax number: 510 537-8666. Please include your phone number. Schneider will answer questions of general interest in the paper. He reserves the right to edit the letter for brevity and clarity. Readers are encouraged to contact a competent contractor or code consultant for specific information regarding questions they may have about their homes.