Oceanmaiden specifically asked me about a 30 year house and rigging a whole house GFI system but didn't specify a geographic location. Codes could vary place to place depending on soil and weather conditions but generally follow the NEC.
.......A 30 year old house anywhere in this country is probably not GFI protected. GFI's came around in the early 80's and haven't been code since the late 80's and early 90's in some areas. Municipalities (cities) usually have stringent electrical codes that follow the NEC (National Electric Code). In Texas anyway, many rural areas don't even have codes. Codes are a good thing. They add cost to a building but make them far safer. A long time ago there were many deaths from electrical home fires from faulty wiring. Modern electrical codes began to evolve in the 1950's, 60's and 70's. But a good thing is a 30 year old house is more than likely wired with a 3 wire system, the third wire being an earth ground. If your recepticles have a small round hole under the two skinny rectangular shaped holes you PROBABLY, but not necessarily, have an earth ground system.
Rigging the whole house for GFI protection can be problematic, expensive and unecessary. You really want GFI where there is the possibilty of an electrical appliance coming into contact with water like around a kitchen sink, anywhere in the bathroom or any outside plug. You do not need a GFI on every recepticle. This will be a little hard to explain but I'll try. You have a main circuit breaker box that is fed 230 VAC single phase (can also on occasion be 3 phase) from your power company meter. The distribution panel (breaker box) feeds individual circuit breakers that in turn feed your lights and wall plugs. Most residential circuits and breakers are designed to carry 20 amperes maximum. Here is the important distinction between an ordinary circuit breaker and a GFI circuit breaker. Pay attention here. A breaker in your breaker box is designed to react to an over amperage situation, it shuts off if the current draw exceeds 20 amperes. It has a momemtary amp rating around 300% higher than full load amps which means it can tolerate a high amp draw for a 1/4 of a second or so but will open and stop the current flow if the amp draw remains high. A GFI senses for faults to ground. It looks for a rapid current discharge to your earth ground and reacts and opens the circuit in a millionth of a second. Electrons flow through wires at 186,000 miles per second, the same as an electromagnetic radio waves. To put this in perspective a radio wave can travel around the earths 24,000 circumference in about 7.5 seconds. GFI's and their extremely fast switching were developed to protect against, well, ground faults.
Now, next point. Think of the circuits and wall plugs in your house as a string of Christmas tree lights. A GFI circuit on the very first light protects all the other lights down stream. A GFI on the last light only protects the last light. You don't need a GFI on every plug. Let's say you have 2 plugs in you bathroom and the first plug feeds the second one. Install a GFI on the first plug and both plugs are protected. Install a GFI on the last plug and only the last plug is protected. In other words GFI don't protect anything upstream of their physical installtion.
I wouldn't suggest a whole house GFI system - but - I could be out my league here, maybe there is something new I'm not familiar with. I'm very good friends with the chief electrical inspector in my city of 110,000. This guy is a guru and I'll call and pick his brain. He told me a true story about a local master electrician who added a hot tub to his own house. He's sitting there one night with a little AM radio plugged into a non GFI plug. The radio fell in the water and the guy was electrocuted and killed. He knew better but didn't take the time to change the plug out.