Until about 1950 most electrical receptacles were ungrounded, which means that the receptacles had only two slots; one connected to the black wire and one connected to the white? Typically the receptacles were installed in a metal box, and sometimes had the wiring run in EMT, Electrical Metallic Tubing. After 1960, grounded receptacles, those with three slots, became popular and are now required by code. This allows electrical current that would flow through a person in an ungrounded receptacle to flow through the ground wire. This is a safety path for electricity to follow in case of a malfunction in the system.
Unfortunately, there are many two slotted receptacles that have been upgraded to three-slot receptacles without providing a ground wire. This has no benefit, and may be misleading when plugging a three-pronged device into an upgraded but ungrounded receptacle.
To be 100% sure, that your receptacles are not grounded to the box you should take a voltage measurement from the hot wire, black wire, to the metal box, it should be about 120 volts. If your feel uncomfortable, you could pay for a service call from an electrician to come in and verify that your metal boxes are grounded.
Before making any repairs or replacements, TURN OFF THE POWER TO THE RECEPTACLES BY SHUTTING OFF THE BREAKER IN THE PANELBOARD. VERIFY THAT THE POWER IS OFF WITH A TESTER OR PLUGGING IN A LIGHT OR RADIO.
If the boxes are grounded, you can install a three-prong receptacle with the jumper connected from the green screw on the receptacle to a screw threaded into the metal box. This screw should be a machine screw made for electrical boxes, not a sheet metal or wood screw. There are also receptacles on the market that have a spring clip on one of the mounting screws that will insure grounding continuity through the mounting screw, if you use one of these, you can skip the wire jumper to the box. As per the National Electric Code (NEC), "250.146(B) Contact Devices or Yokes. Contact devices or yokes designed and listed as self-grounding shall be permitted in conjunction with the supporting screws to establish the grounding circuit between the device yoke and flush type boxes"
I replaced all of the receptacles in my home that was built in 1959 with three-prong receptacles. I chose to add a pigtail to the ground that was wrapped around one of the screws inside the box that secured a metal clip that held the wires.
An alternative to adding grounding pigtails to each outlet would be to replace all the 2-prong receptacles with GFCI receptacles. This can be done because the GFCI receptacle only uses the white and black wire to determine the protection. A GFCI and a grounded outlet do two different things and protect against two different scenarios, but one is not a replacement for the other. A grounded outlet protects against a short in the wiring, as when a wire comes loose and touches the metal electrical box or housing. If the box or housing is grounded, the fuse or breaker will blow and presumably you will know that there is a problem. If there is no ground anyone who touched the box or housing would get a shock and it would not matter if you have a GFCI outlet or not. A GFCI outlet protects against a short or current leakage involving you and an electrical device plugged into the outlet. If you chose to go this route, you must place a sticker that comes with the GFCI receptacle that says something like “GFCI Protected” and “No Equipment Ground.”
Usually the GFCI receptacle will continue to work because when a GFCI protection device fails, the switching contacts remain closed and the device will continue to provide power without GFCI protection. Several manufacturers now make a GFCI receptacle that de-energizes the circuit if the GFCI protection feature fails.
A GFCI Receptacle
You should purchase an inexpensive receptacle tester that has three-prongs and simply plugs into the outlet. There are three indicator lights that will tell you if the outlet is normal or if there is a problem. They also have testers with a button that will trip the GFCI receptacle to determine if it is functioning properly. Sever electric shock or death can occur if a person touches the energized (line or hot) and neutral conductor at the same time, even if the circuit is GFCI protected. This is because the current transformer within the GFCI protection device does not sense an imbalance between the departing and returning current and the switching contacts remain closed. According to a study based on data accumulated by the American Society of Home Inspectors that was published in the November/December, 1999 issue of the IAEI News, out of 1,583 GFCI circuit breakers tested, 21% had failed. Out of 4,585 GFCI receptacles tested, 19% had failed. The failures were primarily attributed to damage from short circuits and voltage surges such as those caused by lightning and other transient electrical surges to the metal oxide varistors that are used for built-in surge suppression. It was noted that in areas of high lighting activity such as Southwest Florida, the failure rate for GFCI circuit breakers was over 57%.
A tester for receptacles with or without GFCI test button
To protect all of your electronic equipment you should understand that a surge protector plugged into an ungrounded outlet will not operate as the manufacturer intended. When a large surge or spike hits, the surge protector uses the ground wire to take the "hit" away from the protected equipment and send it safely to ground. If the surge or spike is not sent to ground by the surge protector it will destroy the delicate electronics you were trying to protect. The warranty offered by the surge protectors manufacturer offer, is only valid if the surge protector is used in a properly grounded outlet.
Again, if you feel you can’t make the reapirs, contact a qualified electrician, but you can still test your outlets to determine what work needs to be done and you will have a better understanding of the reasons for it.
