The third hole in a three-prong outlet connects your grounding sheet to earth. How the system works, and why some three-prong outlets are not actually grounded.
Jen Angela·
A three-prong outlet (also called a grounded outlet or NEMA 5-15R in the US) has three slots: a hot slot, a neutral slot, and a ground slot. The ground slot connects through the home's electrical system to a true earth ground reference, typically via a grounding rod or grounded water pipe at the building's electrical service entry.
For grounding sheets, the third (ground) hole is what connects you to earth. The grounding cord plugs into this hole, providing the conductive path from your sheet to actual earth. Without a properly wired ground hole, a grounding sheet doesn't ground you.
Three-prong outlets became standard in US residential construction starting in 1962. Houses built before this date often have two-prong outlets that have been replaced with three-prong outlets without adding actual ground wires behind the wall. This is a real and common problem that affects whether grounding products work.
The visible appearance of a three-prong outlet is identical regardless of whether the ground slot is actually connected to anything. Walking up to an outlet and seeing three holes doesn't tell you whether it's properly grounded. The ground slot might connect to a wire that goes to a building ground reference, or it might connect to nothing at all.
The way to verify whether a three-prong outlet is actually grounded is a $7 plug-in outlet tester. The tester has three lights that indicate ground present, hot/neutral correct, and other wiring conditions. About 1 in 7 outlets in older US homes test as ungrounded despite three-prong appearance.
Other electrical systems use different outlet styles with different prong configurations. UK three-prong (BS 1363), EU three-prong (CEE 7), Japanese two-prong, and various country-specific standards each have their own ground hole locations. Grounding sheet brands that ship internationally typically include the correct plug style for the destination market.
Related terms: ground rod, GFCI, body voltage, multimeter.
Frequently Asked Questions
What is a three-prong outlet?
A grounded outlet, called NEMA 5-15R in the US, with hot, neutral, and ground slots. The ground slot is meant to connect through the home's wiring to a true earth reference, usually a ground rod or grounded water pipe at the electrical service entry.
Does a three-prong outlet mean my grounding sheet is grounded?
Not necessarily. The outlet looks identical whether the ground slot connects to earth or to nothing at all. Houses built before 1962 often had two-prong outlets swapped for three-prong ones without a ground wire ever being run, and about 1 in 7 outlets in older US homes test as ungrounded.
How do I check whether an outlet is actually grounded?
Use a $7 plug-in outlet tester. Its three lights show whether ground is present and whether hot and neutral are wired correctly.
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