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INSTALLATION AND WIRING
42
2.3.9 Earthing (EC version)
(
1
)
Earthing and Earth Leakage Current
(a) Purpose of Earthing
Electrical equipment usually has an Earthing Terminal, this must be connected to earth 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 the earth (safety earth) to prevent electric shocks when touching the case.
There is however, another important earthing 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
As detailed above there are two entirely different types of earthing and to attempt to use the same
earth for both will lead to problems. It is necessary to separate the “safety” earthing (a yellow/green
wire to prevent electric shocks) from the “FRI” earthing (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 for inverter and motor installations
an “clean” earting for equipment such as sensors, computers and audio equipment.
(
2
)
Earthing methods
Two main types of earth
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 the inverter independently of other equipment.
If independent earthing is not possible, use a common earthing point.
Avoid connecting earthing wires together particularly on high power equipment such as motors and
inverters.
Independent earthing should always be used between sensitive equipment and inverters.
Other
equipment
Inverter
(a) Independent grounding
Other
equipment
Inverter
(b) common grounding (c) Grounding wire of
other equipment
Other
equipment
Inverter
(b)Safety earths should be...
For 400V duty - Special class 3, 10
Ω
or less
For 200V duty - Class 3, 100
Ω
or less
(c) The safety earth should be as thick as possible, minimum thickness as stated in below table.
(d)The earthing point should be as close to the inverter as possible, and the wire as short as possible.
(e)The RFI earth should be a braided strap with a 10mm
2
minimum cross sectional area, and as short as
possible.
(f) The earths 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.
(Unit: mm
2
)
Earth Wire Size
Motor Size
400V Class
3.7kW (5HP) or less 2
5.5kW, 7.5kW
(7.5HP, 10HP)
3.5
11kW to 15kW (15 to 20HP) 8
18.5kW to 37kW
(25 to 50HP)
14
45kW, 55kW (60HP, 75HP) 22
2