Ungrounded three prongs: What you need to know.
The bottom prong on an electrical plug allows the ground wire to bond electrical components when an issue occurs. In normal conditions, there should never be a current on the ground wire, but it’s necessary on an outlet to allow surge protectors to do their jobs and prevent electrocution. When a three-prong outlet is installed with wires only connected to the top two prongs, it’s called an “ungrounded three-prong outlet”. It is a common issue in old houses that can be corrected easily (by professionals only!). In the 2014 National Electric Code (NEC), there are several options listed to repair this defect under section 406.4(D).
Option 1: Existing ground path
If a ground path exists, it must be used. Ground paths have been required in every home built after 1962, meaning that the only repair for an ungrounded outlet in a home built after 1962 is to ground it. A two-lead tester can be used to see if the electrical box is grounded. If it is, all that needs to be done is connect the outlet to the box with a wire. This is known as an equipment bonding jumper.
Option 2: No existing ground path
There are three options to replace an outlet in a home with no ground path. The first is to install a two-prong outlet, but this is not a recommended method because using a three-prong adapter to connect a three-prong device with a two-prong outlet is unsafe if the outlet is not grounded properly. The second option is to install a GFCI outlet, which will help prevent electrocution, but will still not allow a surge protector to do its job. If this method is used, a sticker stating “No Equipment Ground” needs to be attached to the face of the outlet.
The third option is to install a three-prong outlet but add GFCI protection at either the breaker panel or somewhere between the outlet and the breaker panel. Labels must be applied to the outlet in this case as well, stating “GFCI Protected” and “No Equipment Ground”.