What is a non-grounding type receptacle?

Non-grounding type receptacles are not a great hazard by themselves (if installed properly and used only to connect equipment that is not broken), but their presence is an indication of very old wiring inside the walls behind the receptacle, and the likelihood that the old wiring is shared by many locations originally designed to provide low power to mostly floor lamps (100W each vs. 300W and up for computers, vacuums, air conditioners, etc.).

By contrast, modern grounding-type receptacles typically connect to modern wiring that includes an equipment grounding conductor that bonds the receptacle and any cord-and-plug connected load to the building system ground (it is literally a single, chemically-bonded piece of metal running from the receptacle all the way to the dirt under your building). This creates a safe path for fault current in the event a ‘hot’ wire improperly contacts and energizes the accessible surfaces of the receptacle or connected loads. The fault path carries high current (100′s of Amps) sufficient to trip the 15A or 20A circuit breaker in fractions of a second. A non-grounding-type receptacle can only trip its circuit breaker if there is a direct short between the supply conductors (‘hot’ and neutral), if the connected load draws current greater than the breaker rating for several minutes, or if there is a short between ‘hot’ and neutral in the connected cord or cord-supplied equipment.

Receptacle grounding also minimizes physical damage and shock hazard arising from surge voltage induced on metal in the home by nearby (or direct) lighting strikes.

Newly-installed receptacles under the 2008 edition of the National Electric Code (NEC), enforced in Philadelphia since 2011, must be Tamper Resistant (TR) type, if located within 6′ above finished floor. An exception is made where non-grounding type receptacles must be installed because no ground facility exists in the underlying wiring method, because TR receptacles are required only if available, and there are no TR-type non-grounding receptacles.

Mechanical considerations also play a role in evaluating the relative safety of receptacles. All receptacles rely on spring tension in their sockets to maintain electrical contact with cord-and-plug connected appliances. As the receptacles age, springs may become fatigued and provide poor contact even with full insertion, and/or allow plugs to slip partially out, further impeding good electrical contact. Poor contact can result in arcing, which heats up the termination and may ignite accumulated dust/hair, or overheat the attached cords or the branch circuit wires connected to the receptacle device to the point of igniting the plastic insulation (this is one reason AFCI protection has been mandated for all newly-installed outlet locations in residences, that are not elsewhere required to be GFCI-protected).

Because of the circuits’ tendency to be overloaded due to the number of locations served, the age of the wires and the receptacle devices, and the lack of an equipment grounding conductor that provides added safety in the event of circuit faults, and the likelihood that the receptacles have fatigued spring sockets, high-power appliances including SPACE HEATERS, HAIR DRYERS and AIR CONDITIONERS SHOULD NEVER BE ATTACHED TO THESE RECEPTACLES.

See also:

• What is a receptacle?
• Learning to live with old wiring in old Philadelphia homes