Mitsumi electronic FR-F520-0.75K, 55K Earthing EC version, Earthing and Earth Leakage Current

INSTALLATION AND WIRING

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 cable2 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.

Inverter

Other

equipment

Inverter

Other

equipment

Inverter

Other

equipment

(a) Independent grounding		(b) common grounding	(c) Grounding wire of
			other equipment
(b) Safety earths should be...
For 400V duty - Special class 3, 10Ω		or less
For 200V duty - Class 3, 100Ω	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 10mm2 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: mm2)

Motor Size	Earth Wire Size
	400V Class
3.7kW (5HP) or less	2
5.5kW, 7.5kW	3.5
(7.5HP, 10HP)
11kW to 15kW (15 to 20HP)	8
18.5kW to 37kW	14
(25 to 50HP)
45kW, 55kW (60HP, 75HP)	22

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