Protective Grounding & Functional Grounding; Chapter 3 Solution To Emi: Grounding - Delta C2000 series User Manual

The leakage current of an electronic equipment is conducted to ground via the grounding wire and the
ground electrode. According to Ohm's law, potential differences may arise when the electrode's ground
and the ground's ground resistance are different.
According to Ohm's law, the earth resistance for electrode and the ground are different,
in this case potential differences may arise.
3.1 Protective Grounding & Functional Grounding
Please carefully read the following instruction if two types of grounding are applied at the same time.
Protective grounding is applied outside buildings and must have low resistance. On the other hand,
functional grounding can be applied inside buildings and must have low impedance.
The goal of EMC is to avoid any interference effects. Grounding for EMC can be distinguished by
frequency. For frequencies lower than 10kHz, a single-point ground system should be used and for
frequencies higher than 10 kHz, a multiple point ground system should be used.

Single Point Grounding: all signal grounds of all IT equipment are connected in series to form a single
reference point. This point can be grounded directly to earth; to the designated grounding point or to
the safety point that is already grounded.

Multiple Point Grounding: all signals of all IT equipment are grounded independently.

Hybrid Grounding: this type of grounding behaves differently for low and high frequencies. When two
pieces of IT equipment (A and B) are connected via a shielded cable, one end is connected directly to
ground while the other end is connected to ground via a capacitor. This type of grounding system
fulfils the criteria for high and low frequency grounding.

Floating grounding: the signals of all IT equipment are isolated from each other and are not grounded.
DC current flows evenly throughout the conductor section. But AC current flows towards the conductor's
surface as frequency increases; this is called the "skin effect". It causes the effective cross-section area to
be reduced with increasing frequency. Therefore it is suggested to increase the effective ground cross-
section area for high frequencies by replacing pigtail grounding by braided conductors or strip conductors.
Refer to the following figure.
This is why a thick short ground wire must be implemented for connecting to the common grounding path
or the ground busbar. Especially when a controller (e.g. PLC) is connected to an AC motor drive, it must
be grounded by a short and thick conducting wire. It is suggested to use a flat braided conductor (ex:
metal mesh) with a lower impedance at high frequencies.
If the grounding wire is too long, its inductance may interfere structure of the building or the control
cabinet and form mutual inductance and stray capacitance. As shown in the following figure, a long
grounding wire could become a vertical antenna and turn into a source of noise.

Chapter 3 Solution to EMI: Grounding

1
2
3
Pigtail
1
Braided
strapl
4
HF
LF-HF