Understanding Electrical Grounding and How It Works

Grounding is a principle of electricity often mentioned in wiring projects but can be confusing to homeowners and DIYers. To understand the importance of electrical grounding in a home wiring system, you'll need to understand the nature of electrical energy flow.

Grounding Basics

The electrical current in your home's wiring system carries a flow of electrons through its metal circuit wires. This polarized current comes in two forms: a negative and a positive charge.

The negative half of the charge is the "hot" current, which you'll typically see carried by black wires in your home. White neutral wires carry a positive charge. Both sets of wires enter your home through the utility's main service wires, running through your electrical service panel and side-by-side through every circuit in your home.

Electricity seeks to return its electrons to "ground"—discharging negative energy and returning to equilibrium—which would normally happen through the neutral wires in an electrical system. But if there are any breakdowns or short circuits along the pathway, the hot current may stray and try to take a shorter path to ground through other materials, such as metal or wood framing, metal pipes, or insulation. This is how most electrical fires and shocks start.

Your Home's Grounding System

To prevent this danger, your home's electrical system includes a backup plan: a system of grounding wires running parallel to the hot and neutral wires. This provides an alternate pathway for electrical current to follow in the event of a breakdown, loose wire connection, or damage.

A grounding pathway is formed by a system of bare copper wires connected to every device and metal electrical box in your home. In standard sheathed NM, or non-metallic cable, this bare copper wire is present alongside the insulated hot and neutral wires inside the cable. The grounding wires terminate in a grounding bar in your main service panel, which is connected to a rod deep underground outside your home.

In most home wiring systems, evidence of the grounding system can be seen at each outlet receptacle, with the third round slot in the "face" representing the grounding connection. When a grounded appliance is plugged in, its round grounding prong connects to the system of bare copper grounding wires inside the house's circuits.

While a complete grounding system is standard in homes with circuit breakers that are wired with sheathed NM cable, older wiring systems installed between 1940 and 1965 may be grounded through metal conduit or metal cable, not copper (and systems from pre-1940 may not have any form of grounding at all).

Built-In Protection

Your home's wiring system will also include other safety devices to help minimize the risks of shock and fires. Circuit breakers or fuse boxes are in place to protect and control each circuit. These serve two functions: protecting the wires against overheating if they are ever overloaded, and tripping or "blowing" to instantly stop the flow of current when they sense short circuits.

The metal plumbing pipes in your home may also be connected to the grounding pathway, and are typically protected by a grounding wire clamped to a metal water pipe near your water heater or where the public water line enters your home.

Appliance Grounding

Not only does your home wiring system have a grounding system for safety but many plug-in appliances and devices also do. High-power appliances such as power tools and vacuums are much safer when they have a third prong on the plug, which is shaped to fit the round grounding slot on an outlet receptacle.

The presence of this third prong shows an appliance has a grounding system, and it's essential to only plug these into grounded outlets.

Plugs and Adapters

You may have seen plug adapters that allow three-prong plugs to be inserted into two-slot outlet receptacles, but these can only give grounding protection under a very specific set of circumstances: if the adapter's metal loop or pigtail wire is properly attached to the mounting screw on the outlet's cover plate, the cover plate screw is connected to a metal box, and the metal box is properly grounded.

That's why it's safest to avoid using three-prong to two-slot adapters, and only plug three-prong devices into three-slot receptacles.

Of course, not all appliances and devices have a three-prong grounded plug—but don't worry. They're still safe since they normally have double-insulated construction to minimize the risk of short circuits.

If a grounded outlet isn't feasible in an older wiring system, some protection can still be offered by installing a GFCI (ground-fault circuit interrupter) receptacle. The GFCI will sense ground faults and shut off the power before straying current can cause problems. But using a GFCI doesn't create a grounding pathway, it just makes an ungrounded outlet a little safer.