In older homes the original wiring did not have a ground wire connected to the outlets. These ungrounded outlets are easily distinguished by their two hole / slot configuration verses the newer grounded type of outlet that has three holes / slots. Ungrounded outlets that have two holes / slots used in a home that was originally wired in this manner and has not been rewired are considered acceptable.
The problems for the owners of older homes start when grounded type outlets are substituted for the ungrounded type without the necessary rewiring that adds a ground wire to the new three prong grounded type outlet. Grounded type (three hole / slot) outlets may not be substituted for ungrounded outlets unless a ground wire is connected.An exception to this rule is allowed by the National Electric Code, when the outlet is protected by a ground fault interrupter (GFI or GFCI).
There is a correction available for those home owners who do not want to rewire the entire house. Replace the outlets with Ground Fault Interrupters. There are two types of GFI available, one takes the place of the regular circuit breaker in the load center. The second type that is available takes the place of the standard outlet and replaces it with a special GFI protected outlet. These are commonly used in the kitchens and bathrooms of newer homes. Most people know them for their black and red reset buttons.
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.