6 Earthing and shielding
| Potential ratios
34
2484950000/00/03.2016
Manual UR67-PROIFBUS
A high-impedance earth connection is also another option.
In the control panel, the shield is then connected to the earth
potential, and the shield has a high-impedance connection to
earth at the measurement location via a gas discharge tube.
This solves the problem of a potential transfer and 50-Hz
humming.
For non-isolated measurement locations, two gas discharge
tubes must be installed. One connects the shield to earth,
and the other connects it to the non-isolated measurement
location. This method prevents a galvanic coupling between
the measurement circuit and the earthed measurement
location.
Shield connection
Earth
Measuring point
1
4
5
TS
2
3
IN
OUT
Summary
Earthing is a key element for the reliable functioning of an
electrical system in the event of interference. In this regard,
HF-related aspects must be taken into consideration. Only
the proper use of materials and a well thought-out circuit
design will lead to success.
6.2 Potential ratios
The reference potentials of system/input supply and output
supply are galvanically isolated from each other.
6.3 Electromagnetic compatibility (EMC)
u-remote products completely meet EMC requirements. EMC
planning, however, is necessary prior to installation.
Aspects to consider include all potential interference sources
such as galvanic, inductive and capacitive couplings, as well
as radiation couplings.
Ensuring EMC
To ensure EMC, the following basic principles must be ob-
served during installation of the u-remote modules:
–
Proper, extensive earthing of inactive metal parts
–
Correct shielding of cables and equipment
–
Proper layout of wires – cabling
–
Creation of a uniform reference potential and earthing
of all electrical equipment
–
Special EMC measures for special applications
(e.g. frequency converters, servo drives)
–
Contactors and relay coils must be equipped with the
corresponding interference suppressors.
–
Devices and function units with higher interferance poten-
tial have to be capsuled if necessary.
Earthing of inactive metal parts
The earthing of all inactive metal parts reduces the influence
of coupled interference. For this purpose, all inactive metal
parts (such as switch cabinets, cabinet doors, support beams,
mounting plates, DIN rails, etc.) must be con nected to each
other over a large surface area with low impedance, whereby
a uniform reference potential is ensured for all control unit
elements.
Required measures:
–
Removal of the insulating layer around screw connec-
tions. Protection of connection points against corrosion
–
Connection of moving earthed components (cabinet
doors, separated mounting plates, etc.) through short
earthing straps with large surfaces
–
When possible, avoid using aluminium parts, because alu-
minium oxidises easily and in this respect is unsuited for
earthing
PE connection
The connection from earth to the PE (protective earth) con-
nection must be done centrally.
WARNING
Possible danger to life!
In the event of a fault, the earth must never
take on a dangerous contact voltage, which is
why it must be connected to a PE conductor.
Unearthed operation
In the event of unearthed operation, the corresponding safe-
ty regulations must be observed.
