The
first way: working grounding.
Due to the need of
operation and safety of power system, the neutral point is often grounded,
which is called working grounding. Working grounding has the following
purposes:
Reduce electric shock
voltage. In systems with ungrounded neutral point, when one phase is grounded,
and other two phases touch the human body, electric shock voltage is 1.732
times of phase voltage. While in systems with grounded neutral point, electric
shock voltage is reduced to equal or close to the phase voltage.
Cut off faulty equipment
quickly. In systems with ungrounded neutral point, when one phase is grounded,
the grounding current is very small( because there is electric capacity and
insulation resistance between conducting wire and ground, it can also form a
current path). It is not enough to make the protection device act and cut off
the power supply. Faulty grounding can not be found easily, it will maintain
for a long time and not safe for human body. While in systems with grounded
neutral point, the grounding current is bigger( close to single phase short
circuit), the protection devices act quickly and cut off faulty place.
Reduce the insulation
level of electrical equipment to the ground. In systems with ungrounded neutral
point, when one phase connect to ground, will let ground voltage of other two
phases increase to line voltage. While in systems with grounded neutral point,
it is close to phase voltage, so it can reduce the insulation level of
electrical equipment and transmission lines and save investment outlay. At the
same time, ungrounded neutral point also has benefit. First, one phase
grounding is usually instantaneous and can be eliminated automatically. In the
system with ungrounded neutral point, there will be no power failure due to
tripping. Second, one phase to ground fault can be allowed to exist for a short
time, so as to find and repair the fault.
The
second way: protective grounding.
Protective grounding is
to ground the metal shell of electrical equipment (which is uncharged under
normal conditions), which is suitable for low-voltage system with ungrounded
neutral point. We can analyze the protective grounding of the motor.
When the insulation of
one phase winding of the motor is damaged, the shell is charged and ungrounded,
the human body touches the shell, it is equivalent to single-phase electric
shock. At this time, the grounding current capacity (the current flowing into
the earth through the fault point) depends on human body resistance and
insulation resistance. When the insulation of the system decreases, there is a
risk of electric shock.
When insulation of one
phase winding of generator is damaged, which making the shell charge, while the
shell is grounded and human body touches the shell. Because the resistance of
human body is in parallel with the grounding resistance, and usually the resistance
of human body is far greater than the grounding resistance, the current passing
through human body is very small, so there is no danger. This is the role of
protective grounding to ensure personal safety.
The
third way: protective zero connection.
Protective zero
connection is to connect the metal shell of electrical equipment to the zero
line, which is suitable for the low-voltage system of neutral point grounding.
Let us take motor as an
example again, when the insulation of one phase winding of motor is damaged,
which causing to touch the shell, a single-phase short circuit is formed, which
quickly fuses the fuse in this phase, so the shell is no longer electrified.
Even when the human body touches the shell before the fuse is fused, the current
passing through the human body is extremely small because the human body
resistance is far greater than the line voltage.
At the same time, it
should be noted that the protective grounding is not used in the system of
generator set neutral grounding.
Starlight company is a
manufacturer of diesel generator set, more supports or product details, please
feel free to contact us by email sales@dieselgeneratortech.com.
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