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
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 micro processors 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 wave form
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 wave form. This trend becomes more pronounced with higher
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
It is important to make a clear distinction between these two, and to keep them separate by following the
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
Independent earthing should always be used between sensitive equipment and inverters.
(a) Independent grounding
b) Safety earths should be...
For 400V duty - Special class 3, 10Ω or less
For 200V duty - Class 3, 100Ω or less
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(b) common grounding
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(c) Grounding wire of