Notes on earthing
Leakage currents flow in the inverter or the EMC filter respectively. To prevent an
electric shock, the inverter, input filter and motor must be earthed. (This inverter must
be earthed. Earthing must conform to the requirements of national and local safety
regulations and electrical codes. (JIS, NEC section 250, IEC 536 class 1 and other
Use the dedicated earth terminal to earth the inverter. (Do not use the screw in the casing, chas-
Use the thickest possible earth cable. Use the cable whose size is equal to or greater than that
indicated in Tab. 3-4, and minimize the cable length. The earthing point should be as near as
possible to the inverter.
Always earth the motor and inverter
● Purpose of earthing
Generally, an electrical apparatus has an earth 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 flow into the case. The purpose of earthing the case
of an electrical apparatus is to prevent operator from getting an electric shock from this
leakage current when touching it.
To avoid the influence of external noises, this earthing is important to audio equipment,
sensors, computers and other apparatuses that handle low-level signals or operate very fast.
● Earthing methods and earthing work
As described previously, earthing is roughly classified into an electrical shock prevention
type and a noise affected malfunction prevention type. Therefore, these two types should
be discriminated clearly, and the following work must be done to prevent the leakage current
having the inverter's high frequency components from entering the malfunction prevention
– Where possible, use independent earthing for the inverter. If independent earthing (I) is
impossible, use joint earthing (II) where the inverter is connected with the other equip-
ment at an earthing point. Joint earthing as in (III) must be avoided as the inverter is
connected with the other equipment by a common earth cable.
Also a leakage current including many high frequency components flows in the earth
cables of the inverter and inverter-driven motor. Therefore, they must use the independ-
ent earthing method and be separated from the earthing of equipment sensitive to the
In a tall building, it will be a good policy to use the noise malfunction prevention type
earthing with steel frames and carry out electric shock prevention type earthing in the
independent earthing method.
– This inverter must be earthed. Earthing must conform to the requirements of national
and local safety regulations and electrical codes. (JIS, NEC section 250, IEC 536 class
1 and other applicable standards).
– Use the thickest possible earth cable. The earth cable should be of not less than the size
indicated in Tab. 3-4.
– The grounding point should be as near as possible to the inverter, and the ground wire
length should be as short as possible.
– Run the earth cable as far away as possible from the I/O wiring of equipment sensitive
to noises and run them in parallel in the minimum distance.
(I) Independent earthing
Fig. 3-4: Earthing the drive
Main circuit connection
(III) Common earthing
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