Earth (Ground) - Mitsubishi Electric FATEC FR-A800 Manual

7
REVIEW OF INSTALLATION ENVIRONMENT
7.2.5

Earth (ground)

Generally, an electrical apparatus has an earth (ground) terminal, which must be connected to the
ground before use.
An electrical circuit is usually insulated by an insulating material and encased. However, it is
impossible to manufacture an insulating material that can shut off a leakage current completely, and
actually, a slight current flows into the case.
The original purpose of the earth (ground) terminal is earthing (grounding) the case of an electrical
apparatus to prevent operators from getting an electric shock from this leakage current when they
touch it. To avoid the influence of external noises, earthing (grounding) is important to machines that
handle low-level signals or operate in a significantly high speed such as audio equipment, sensors,
and computers.
As described above, earthing (grounding) has two completely different types, and a problem occurs
when these are earthed (grounded) together. Therefore, consider a dirty earth (ground) for electric
shock prevention and a clean earth (ground) for noise prevention separately.
When an inverter is used, its output voltage has not a sine waveform but a precipitous waveform.
Therefore, the charging/discharging current to the static capacitance in the isolated section flows as
the leakage current.
Furthermore, the same charging/discharging leakage current flows into the motor where the output
voltage of the inverter is applied. The leakage current contains more high frequency components and
the current value is larger compared to the operation with a commercial power supply as shown in
Fig. 7.27. The higher the carrier frequency of the inverter is, the stronger this tendency is.
(1) Earthing (grounding) methods and earthing (grounding) work
As described previously, earthing (grounding) is roughly classified into the electrical shock
prevention type and noise-influenced malfunction prevention type. These two types should be
clearly distinguished, and the following work must be done to prevent the leakage current having
high frequency components from entering the malfunction prevention type earthing (grounding):
(a) Make the separate earth (ground) connection for the inverter. (Refer to Fig. 7.26.)
If separate earthing (grounding) (I) is not available, use (II) common earthing (grounding)
where the inverter is connected with the other equipment at an earthing (grounding) point.
Do not use the common earthing (grounding) in which one earthing (grounding) cable is
shared between the inverter and the other devices as shown in (III).
Especially, the common earthing (grounding) with the high-power devices such as motors or
transformers must be avoided. As leakage current containing many high frequency
components flows into the earthing (grounding) cables of the inverter and the motor driven
by the inverter, the inverter must also be earthed (grounded) separately from the noise-
sensitive devices described above. In a high building, it may be effective to use its iron
structure frames as earthing (grounding) electrode for EMI prevention in order to separate
from the earth (ground) system for electric shock prevention.
Inverter
(I) Separate earthing (grounding): Good
7.2 Noise
114
Other
Inverter
equipment
(II) Common (single-point)
earthing (grounding): OK
Fig. 7.26 Earthing (grounding) methods
Other
Inverter
equipment
(III) Inadequate common (single-point)
earthing (grounding): Bad
Other
equipment