Ever wonder why the older electrical receptacles have only two slots, and don’t have the hole below the slots to allow three prongs? Think they can just be switched out? Sometimes they can, but other times that would be a potentially dangerous mistake.
Understanding the implications of two prongs or three can be one of the more bewildering situations a home buyer has to deal with.
Let’s review a little of the history of residential wiring to see how this came about. Knob-and-tube (KNT) wiring was phased out in the 1930s, as both nonmetallic sheathed cable (NM for short, commonly referred to as Romex) and armored cable (AC, commonly referred to as BX) became prevalent. Until the early 1960s, most NM and AC cable for residential use did not have a grounding conductor. The two-prong receptacles normally used until this time have only a “neutral” slot, and a smaller “hot” slot, without the additional hole below for the ground. As houses were wired with the grounding conductor starting in the early 1960s, three-prong receptacles were generally installed.
Why should an electrical receptacle be grounded? The lack of a ground path could lead to getting an electrical shock. If there were a short circuit, then an ungrounded metal object could become energized, and the circuit breaker would not trip. The ground protects you from getting a shock, as it directs a large amount of current back to the circuit breaker, causing the breaker to trip.
A properly wired three-prong receptacle is both more convenient and safer than an ungrounded two-prong, so how and when can they be switched? If a house is wired with BX without the ground wire but metal boxes, then the metal armor and box will still generally be grounded. A three-prong receptacle can be grounded to the box with a grounding wire.
It is more complicated, and normally not worth the trouble to install a three prong receptacle when the wiring is Romex cable without the ground wire. That would require running a separate ground wire or a completely new cable with ground wire.
Often the solution is to install GFCI circuit breakers and three-prong receptacles, or, what is more commonly done, to replace the two prong receptacles with GFCI receptacles. The receptacles should be labeled “No Equipment Ground”. These labels come in the box when purchasing GFCI.
The GFCI function is not the same as grounding, but in most cases, it is even safer. GFCI outlets and breakers trip, disconnecting the circuit when it detects leakage current that the electric current is not balanced between the energized conductor and the return neutral conductor. And it trips quicker than a regular circuit breaker.
Unfortunately, too many homeowners and contractors, either through ignorance or to save money, have taken the easy solution, by simply replacing the two-prong receptacle with the three-prong. And some have even gone a step further: They create a “bootleg” or false ground by making a connection between the ground and neutral on a receptacle. This is not detectable with a simple receptacle tester used by most inspectors. If it is suspected, then there is more sophisticated equipment, although the simple solution is to remove the electric plate and check whether the wire was added between the neutral and ground screws on the receptacle.
Submitted by Ken Roleke, NPI Franchise Owner, Tucson, Arizona