Quantcast Figure 4-1. Grounding for Fault Protection

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MIL-HDBK-419A
Figure 4-1 illustrates how personnel hazards are developed by improper installation and fault conditions.
Suppose, that one phase of the 230-volt line accidentally contacts the motor frame. If the motor is not
grounded, its frame will rise to 133 volts, and anyone coming in contact with it would be subject to a lethal
shock if simultaneous contact is made with a grounded object. To prevent this situation from arising, the motor
frame must be grounded via the green wire. The resistance of the fault path must be low enough to permit the
fault current to trip the overload protector and interrupt the fault. If the resistance of the fault path is too
large, the fault current will not be enough to trip the overload protectors. Thus to minimize both shock and fire
hazards, the resistance of the fault path must be as low as possible. However, the fault protection subsystem
normally does not depend on the earth electrode subsystem to trip overcurrent devices.  The fault current
normally flows through the green wire (grounding conductor) to the source side of the first service disconnect
means where the green wire and the neutral are tied together. The fault current then flows through the neutral
to the transformer to complete the circuit. This path functions completely independent of the connection to
the earth electrode subsystem.
Figure 4-1. Grounding for Fault Protection
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