What Is Grounding?
Generally speaking, the difference between grounding and bonding is: Grounding is a direct connection to the earth to aid in removing damaging transient over-voltages due to lightning. The purpose of bonding is to ensure the electrical continuity of the fault current path, to provide the capacity and ability to conduct safely any fault current likely to be imposed, and to aid in the operation of the over-current protection device. Properly bonding all metal parts within an electrical system helps ensure a low-impedance fault current path.
The issue of grounding and bonding confuses many inspectors. Due to its complexity, in this section, we will try to break it down to its fundamentals, and look at the basic requirements and common failures that can lead to unsafe conditions around the home.
Grounding
To go back to the beginning, the last stop on the utilities distribution chain, before the supply goes to the home, is the transformer. This steps the high-voltage primary distribution down to the neighborhood, to the 240/120-volt feeds to the homes.
This transformer has a winding known as a phase coil that is center-tapped to provide voltage stabilization, and a return path for the higher voltage system to aid in clearing primary side faults. As discussed earlier, on a typical 240/120-volt service drop, we will have two ungrounded conductors and a single grounded conductor. This means that we have to establish our own grounding electrode system at the dwelling. It is vital in removing dangerous voltages imposed on the system via lightning strikes and over-voltage surges from higher voltages on power lines. If ground-rod, pipe or plate electrodes are used, they must have a rating of 25 Ohms or less; otherwise, an additional electrode must be added, per Section 250.56 of the NEC.
Grounding Electrodes
There are several methods of connecting the grounding system to the ground, with a driven rod being the most common in most areas. Most residential construction requires two separate grounding electrodes in any combination of the following (which need to be at least 6 feet apart):
driven rods (rod-type grounding electrode rods);
metal water pipes;
well casings;
Ufer grounds;
ground plates;
steel framing; and
ground rings.
Historically, the grounding system had just one connection to ground, and this was nearly always made on the water supply pipe. However, two connections are now required by most jurisdictions to ensure a low-impedance ground (one with little resistance).
Because most utility companies now install plastic potable water supply lines, a water pipe can NOT be used as a grounding means, so one of the other electrodes listed must be used. It is also important to note that all electrodes that are present in the dwelling must be bonded together to form a single and complete grounding electrode system. Typically, the two required grounding electrodes need to be at least 6 feet apart. If one is the water pipe ground and the supplemental is a ground rod, another ground rod may need to be added in order to meet the requirements of section 250.56 of the NEC. Gas piping should not be used as a grounding electrode for safety reasons, but, in most areas, gas lines are required to be bonded to the grounding system if they are likely to become energized. Provided by InterNachi