Earthing (Grounding) (Ec Version) - Mitsubishi Electric FR-F500 Series Instruction Manual

2.3.9 Earthing (Grounding) (EC version)

( 1 ) Earthing (Grounding) and earth (ground) leakage current
(a) Purpose of earthing (grounding)
Electrical equipment usually has an earthing (grounding) terminal, which must be connected to the
earth (ground) before using equipment.
For protection, electric circuits are normally housed inside an insulated case. However, it is impossible
to manufacture insulating materials that prevent all current from leaking across them. Therefore it is
the function of earth (ground) (safety earth (ground)) to prevent electric shocks when touching the
case.
There is, however, another important earthing (grounding) function, which is to prevent equipment that
uses very weak signals (Audio equipment, sensors, transducers, etc.) or microprocessors from being
affected by Radio Frequency Interference, (RFI) from external sources.
(b) Points to remember when earthing (grounding)
As detailed above there are two entirely different types of earthing (grounding) and to attempt to use
the same earth (ground) for both will lead to problems. It is necessary to separate the "safety" earthing
(grounding) (a yellow/green wire to prevent electric shocks) from the "FRI" earthing (grounding) (a
braided wire strap to counter radio noise).
The inverter output voltage does not take the form of a sine wave but of a modulated pulse waveform
causing "noisy" leakage current due to the capacitance of the insulation.
The same type of leakage current will occur in the motor due to the charging and discharging of the
insulation from the high frequency waveform. This trend becomes more pronounced with higher carrier
frequencies.
To solve this problem it is necessary to use separate "dirty" earthing (grounding) for inverter and motor
installations an "clean" earting (grounding) for equipment such as sensors, computers and audio
equipment.
( 2 ) Earthing (Grounding) methods
Two main types of earth (ground)
1-To prevent electrical shocks
Yellow and green cable
2-To prevent RFI induced malfunction
Braided strap
It is important to make a clear distinction between these two, and to keep them separate by following the
measures below.
(a) When possible earth (ground) the inverter independently of other equipment.
If independent earthing (grounding) is not possible, use a common earthing (grounding) point.
Avoid connecting earthing (grounding) wires together particularly on high power equipment such as
motors and inverters.
Independent earthing (grounding) should always be used between sensitive equipment and inverters.
Inverter
(a) Independent earthing
(grounding)
(b) Safety earths (grounds) should be...
For 400V duty - Special class 3, 10Ω or less
For 200V duty - Class 3, 100Ω or less
(c) The safety earth (ground) should be as thick as possible, minimum thickness as stated in below table.
(d) The earthing (grounding) point should be as close to the inverter as possible, and the wire as short as
possible.
(e) The RFI earth (ground) should be a braided strap with a 10mm
short as possible.
(f) The earths (grounds) should be as far away from input and output cables (particularly to equipment
sensitive to RFI) as possible, and any distance where they are parallel should be kept to a minimum.
(g) Design in RFI prevention before installation.
Motor Size
3.7kW (5HP) or less
5.5kW, 7.5kW (7.5HP, 10HP)
11kW to 15kW (15 to 20HP)
18.5kW to 37kW (25 to 50HP)
45kW, 55kW (60HP, 75HP)
Other
Inverter
equipment
(b) common earthing (grounding)
(Unit: mm
Earth (Ground) Wire Size
400V Class
2
3.5
8
14
22
44
INSTALLATION AND WIRING
Other
Inverter
equipment
(c) Earthing (Grounding) wire of
other equipment
2
minimum cross sectional area, and as
2
)
Other
equipment
2