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1.1 Grounding as Countermeasure against Noise
Grounding of a power magnetics cabinet or device is very important to prevent electric shock and
effects of noise. This manual explains a method of grounding that suppresses effects of noise.
1.1.1 Grounding types
There are three types of grounding for CNC systems as shown below.
(1) Signal ground
Gives the standard potential (0 V) for an electric signal system.
(2) Frame ground
Aims at assuring safety and suppression of external and internal noise. For example, the frame
ground is performed through shielding of the interface cables which connect between device
frames, panels, and devices.
(3) System ground (PE)
Connects the frame ground provided between devices or units to the earth at one point as the
1.1.2 Grounding methods
Generally, high frequency noise causes a malfunction in devices. To suppress high frequency
noise, ground devices at low impedance
. The grounding methods to address the problem are
(1) Multi-point grounding method
In this method, when the metal sheet of a cabinet is grounded at sufficiently low impedance,
the metal sheet of the cabinet is used as a ground plate to ground each device locally. It is
possible to ground each device to the metal sheet of the cabinet with low impedance in the
shortest distance, so the effect of suppressing high frequency noise is remarkable, but the
suppression effect depends on the structure of the cabinet because the metal sheet of the
cabinet is used as the ground plate. Refer to Section 1.1.4 about the cabinet. The outline of
wiring is shown in Fig. 1.
If multi-point grounding is used, units can be grounded at low impedance and ground lines can
be shortened, thereby simplifying wiring.
(2) One-point grounding method
In this grounding method, effects of noise caused by the power system on the signal system
can be suppressed by separating grounding of the signal system from that of the power system
and establishing the grounding at one point. Since the connection wire for grounding devices
tends to become long, use a thick wire or multiple wires to obtain sufficient effects against high
frequency noise. The outline of wiring is shown in Fig. 2.
Note) Impedance includes a reactance element in addition to a resistance element, which changes current into
heat, and interferes the flow of alternating current at certain frequency.