If the peripheral devices have metal housings and the cable shields are also in contact
with their housings, it is assumed that all devices involved in the installation have the
same ground potential
.
This is achieved by complying with the following conditions:
■
Mounting the devices on conductive metal surfaces
■
Correctly grounding the devices and metal surfaces in the system
■
If necessary: low-impedance and current-carrying equipotential bonding between
areas with different ground potentials
SICK
Device
7
4
6
Power Supply
U
=
8
=
9
1
2
3
I
5
System
Controller
Figure 3: Example: Occurrence of equipotential bonding currents in the system configuration
1
System controller
2
Device
3
Voltage supply
4
Grounding point 2
5
Closed current loop with equalizing currents via cable shield
6
Ground potential difference
7
Grounding point 1
8
Metal housing
9
Shielded electrical cable
If these conditions are not fulfilled, equipotential bonding currents can flow along the
cable shielding between the devices due to differing ground potentials and cause the
hazards specified. This is, for example, possible in cases where there are devices within
a widely distributed system covering several buildings.
Remedial measures
The most common solution to prevent equipotential bonding currents on cable shields
is to ensure low-impedance and current-carrying equipotential bonding. If this equipo‐
tential bonding is not possible, the following solution approaches serve as a suggestion.
NOTICE
We expressly advise against opening up the cable shields. This would mean that the
EMC limit values can no longer be complied with and that the safe operation of the
device data interfaces can no longer be guaranteed.
Measures for widely distributed system installations
On widely distributed system installations with correspondingly large potential differen‐
ces, the setting up of local islands and connecting them using commercially available
electro-optical signal isolators
is recommended. This measure achieves a high degree
of resistance to electromagnetic interference.
6
ELECTRICAL INSTALLATION
18
O P E R A T I N G I N S T R U C T I O N S | UC40
8027772//2022-08-11 | SICK
Subject to change without notice
