There are several possibile causes for your problem, but we have to divide them into two groups, depending on whether your house was built before or after around 1962. The reason is that the 1962 edition of the National Electrical Code (NEC) was the first one that made grounded (three-slot) receptacles required. And we added “around” to 1962 because the NEC gets revised every three years, but local jurisdictions don’t always adopt it right away. So it may not have been code-mandated in your area until a few years later, and also some builders started grounding their receptacles earlier. 

POSSIBLE CAUSES IF BUILT BEFORE AROUND 1962

1) All the receptacles in the home were originally 2-slot ungrounded. At some point they were changed to 3-slot, but the round ground slot is not connected to ground because there is no ground wire or metal conduit in the box for a connection. This often happens when an older house is “flipped” by an investor and new receptacles, usually with designer face plates, are installed. 

   Sometimes the most critical receptacles for grounding, at the kitchen and bathrooms, have been grounded or GFCI-protected, but the rest only look like they have a ground. There may also be a few old 2-slot receptacles remaining at a location that were overlooked, such as a furnace closet. 

2) There is a way that a remodeler can wire a receptacle to fool a three-light circuit tester into indicating there is ground when there is none. It’s called a bootleg ground, and you can read about it at What is a false ground, bootleg ground, or cheated ground receptacle?

3) Houses were once allowed to ground their electrical system to a metal water pipe that extended into the ground. This usually happens at a metal collar around a water faucet outside and near the electrical service drop. If the pipes were replaced with CPVC or other plastic pipes and the system was not reconnected to a new ground, then that’s your problem.

POSSIBLE CAUSES IF BUILT BEFORE OR AFTER 1962

4) Most home electrical systems are grounded by a metal rod penetrating deep into the soil and located near where the electrical service enters the home. The top few inches of the rod sticks out of the soil and has a clamp that holds a bare copper grounding wire against the rod. Because the wire is exposed to damage—especially if it is not secured to the wall as it runs down to the ground—and it can be accidentally knocked by a mower, kicked, or tripped over—and come loose or be totally disconnected from the ground rod.

5) The ground rod itself can go bad over time due to corrosion, but it takes anywhere from 20 to 50 years or more. Copper clad rods last longer but, as they deteriorate, the resistance to current flow as measured in ohms goes up. Extremely dry soil also increases the electrical resistance.

Here’s links to a collection of our blog posts about ELECTRICAL GROUNDING:

• Why is it unsafe to bond neutral and ground wiring at subpanels?

• When was it first required that neutrals and grounds be separated (not bonded) on any panel past the main service panel?

• What is the difference between "grounded" and "grounding" electrical conductors? 

• What is a false ground, bootleg ground, or cheated ground receptacle? 

• Can multiple neutral or ground wires be secured under the same terminal in an electric panel? 

• Is an ungrounded electric receptacle outlet dangerous?

• Is there an adapter that can be placed on a two-slot (ungrounded) receptacle to make it safe? 

• How can I tell if the electric receptacle outlets are grounded? 

• Are light switches required to be grounded? 

• What is the main bonding jumper and where do it find it in an electric panel? 

• When was metal water pipe banned as the sole ground electrode for a house electrical system? 

• What do ground wires look like? 

• When was the three-slot (grounding) outlet/receptacle first required?

• When was it first required that 240-volt receptacles for a clothes dryer and range be four slot (equipment ground connection)? 

• When was pool bonding first required by code?

• What is the difference between "grounded" and "grounding" electrical conductors? 

• What is a ground wire? 

• What is the difference between neutral and ground wires? 

• Should the ground slot of a receptacle outlet be on the top or bottom?