Article 250 of the NEC covers grounding. A good grounding system is essential to a safe electrical installation.
Do you wonder why a mobile home has a 4 wire service and your home has a 3 wire service? Well the mobile home is an excellent example of a subpanel and how a ground should be installed in all electrical systems. Mobile home services are expected to be connected to a main panel at the power service pole or some connecting outlet to that main panel. Since the panel in the mobile home is a subpanel, the ground system is continued through to the main panel at the source of power, which is where the ground is connected to the grounded conductor ( the neutral ). This provides a fault circuit from the fault all the way to the source of power. Also the neutral buss is not bonded at the mobile home panel but at the main panel it is to be bonded.
This brings up an interesting sutuation for the troubleshooting electrician. The trouble call...... My trailer house shocked me, please come fix it........ When he arrives he may find only 3 wires to the service ( 2 hots and a neutral ) or the fourth green wire run but not connected or simply loose at one of the terminals. Under either of these conditions the mobil home has no ground at all, because the neutral is not bonded to the panel in the mobile home. The ground is just an open wire with no ability to cause a breaker to trip or fuse to blow. This is true of any subpanel properly connected. Remember the ground goes all the way to the source of power.
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