Connecting up
3.4 Grounding
SCALANCE M874, M876
36
Operating Instructions, 06/2015, C79000-G8976-C331-05
3.4
Grounding
EMC disturbances are diverted to ground via ground. This increases the immunity of the
data transmission.
It is crucial for the correct operation of ground that the connection to the reference potential
surface always has low impedance. Such a connection of the functional ground of the device
does not go first through the cable channel and then to the mounting plate or DIN rail
terminal, but goes directly to the mounting plate or DIN rail terminal.
The SCALANCE S615 has a terminal for grounding, refer to the section "Power supply
(Page 34)".
The terminal is identified by the following symbol for the grounding.
1.
Connect the terminal of the device with as short a cable as possible
≤
150 mm and a
large cross-sectional area
≥
2.5 mm² to the functional ground of the system. In many
cases, the entire metallic construction of the system serves as ground.
2.
Also connect the standard rails of a system with ground.
Protective earth/functional ground
The connection of the reference potential surface with the protective earth system is
normally in the cabinet close to the power feed-in. This earth conducts fault currents to
ground safely and according to DIN/VDE 0100 is a protective earth to protect people,
animals and property from too high contact voltages.
Apart from the protective earth, there is functional grounding in the cabinet. According to
EN60204-1 (DIN/VDE 0113 T1) electrical circuits must be grounded. The chassis (0 V) is
grounded at one defined point. Here, once again the grounding is implemented with the
lowest leakage resistance to ground in the vicinity of the power feed-in.
With automation components, functional ground also ensures interference-free operation of
a controller. Via the functional ground, interference currents coupled in via the connecting
cables are discharged to ground.
In terms of the large-area and low impedance implementation, a functional ground set up for
this purpose generally also meets the requirements of protective earth. On the other hand,
protective earth does not always meet the requirements of functional ground. In general
while the connection for protective earth is always low resistance, it is not necessarily low
impedance.
The resistance of a connection for protective earth must always be as small as possible to
divert fault currents safely to ground. The length of a protective earth cable can therefore be
several meters (m) long, without seriously impairing this effect. For a functional ground for
diverting HF disturbances, this cable does however have impedance and is therefore not
suitable.
