As elementary school children we learn in science class the basic electrical principals of conductors and insulators. The strands of wire used in class show this concept in practice. The copper wire conductor is wrapped in a plastic insulator. The same is found in our homes, conductive wires wrapped in an insulating material of either cloth or plastic.
As adults we understand this fundamental electrical law. We know that sticking a fork into an outlet will give us a electrified wallop we won't soon forget.
If that fork is metal. A plastic fork will yield not the tiniest slap.
Another fundamental, but confusing and in my experience not well understood electrical principal is grounding.
Grounding is mostly thought of as a safety component of an electrical system. Most people I meet while inspecting homes believe that an electrical ground will prevent them from being "zapped". The fact is grounding will minimize the voltage to a sting, not eliminate it. Put simply, this is because electricity takes all paths. A solid and properly sized ground connection provides the path of least resistance for errant voltage. Meaning it dissipates a good deal of any wayward current within the system. Additionally grounding stabilizes line to ground voltage and limits voltage during abnormal surges such as lightning or accidental contact with higher voltage lines.
For the home inspector, checking the electrical grounding is a basic part of inspecting the electric system of the dwelling. One of the most common ground connections is made to the water supply pipe on public water systems. The copper pipe makes an ideal ground connection point. Other common means of system grounding are metal rods driven into the earth. The rods can be outside or inside the house. Another type of residential system ground is the Ufer. A Ufer is a metal rod, often the rebar encased in the concrete of the foundation.
While inspecting the electrical system of a relatively new (about 20 years) condominium recently, I found a curious ground connection. The ground wiring appears to be the original connection made when the system was installed. I followed the single wire from the panel box to locate the connection point in order to confirm that a good connection exists. I found the wire clamped to a copper water pipe up in the floor joists near the panel.
Because I usually check the electric panel last, I knew this connection was a problem. The issue wasn't with the clamp or the wire, it was with the pipe itself.
As I had mentioned earlier, on publicly connected water systems, the main water pipe is often used as a ground connection. You will not see a system ground connection on a house with well water. The reason being the pipe coming from the well is plastic. What is becoming more and more common in modern construction is the use of plastic water pipes. Not just inside, but also outside to bring the water into the house. 
Thie water supply for this house is brought in by a plastic pipe.
Unless this plastic pipe possesses an until now unknown conductivity, the electrical system in this house is not, nor likely has ever been grounded.
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