An open ground receptacle is an electrical receptacle that is not showing a ground wire connected. This could be because the ground wire disconnected on the receptacle or somewhere in the circuit or series or because there simply isn’t a ground wire (because it is an old 2 wire electrical system).

The reason this is unsafe is that although the receptacle appears to be grounded, it is not. Thus plugging in a three-prong cord (which requires a grounding conductor for safety) creates a potentially unsafe condition.

Modern homes now have three-wire receptacles that accommodate electrical cords with three-prong plugs. The third prong provides a path to ground in which the electric current travels. Most major appliances, such as stoves, refrigerators, and computers, have three-prong plugs, meaning they must be grounded through the receptacle. If, for whatever reason, there is no grounding conductor at the receptacle, there is a danger of possible shock or electrocution and damage to the equipment. Basically, the equipment could become energized leading to a dangerous situation for anyone in contact with that equipment.

A missing grounding conductor, or open ground, often occurs in older homes that previously had ungrounded 2-prong outlets (2 wire system with no ground wire) that were replaced with 3- prong receptacles without the presence of grounding conductors. In newer homes the grounding conductor could have been mistakenly not connected or has come off the terminal, hence showing open ground on a tester. Although three-prong adapters are available and can be purchased, they are unsafe and not recommended.

How are they fixed?

• First you can check whether the ground wire was left disconnected by mistake. If that’s the case then you just need to connect it in the receptacle. Only do this if you are experienced in this and make certain power is shut off to the areas you are working on. Keep in mind that this will not be the case if the receptacle was originally installed as an ungrounded (2-prong) receptacle back in the day.
• If you don’t need a ground at this receptacle because you will only be plugging in lamps or other two-prong cords, then you can change the three-prong receptacle to a two-prong receptacle. Most things, such as lights don’t need a grounded-receptacle.
• If you want to plug in three-prong cords (that is, appliances that require a ground at the receptacle to be safe), but you are not concerned about protecting the equipment from damage, just with protecting people from possible shock, then you could install a ground fault circuit interrupter (GFCI) receptacle. This will protect people from possible shock, but the equipment  can still be damaged.
• If you want to protect people from shock, electrocution, and protect sensitive electronic equipment from lightning strikes or electrical surge damage, (i.e. you want a true equipment ground), then you must run a new ground wire from the receptacle back to the main panel.
• Note that a surge protector plugged into an ungrounded outlet will not do anything and your equipment will still get damaged.

For reference:

3 Prong Receptacle

2 Prong Receptacle