Comments On Electromagnetic Compatibility - Siemens NXAIR Installation Instructions Manual

Preparing installation

5.11 Comments on electromagnetic compatibility

To achieve appropriate electromagnetic compatibility (EMC), some basic requirements must be observed while
erecting the switchgear. This applies especially to the installation and connection of external cables and wires.
Basic measures for ensuring EMC are already taken during design and assembly of the switchgear panels.
Among other things, these measures include:
•
The low-voltage compartment is an integral part of the panel, which means that the protection and
control devices with the internal wiring are metal-enclosed.
•
Reliable earth connections of the frame parts via toothed contact washers or locking washers.
•
Inside the panel, wires are laid in metal ducts.
•
Spatial separation of sensitive signal wires from wires with high interference voltage levels.
•
Limitation of switching overvoltages of inductive loads (e.g. relay or contactor coils, motors) by means
of protective circuits with diode, varistor or RC element.
•
Within the low-voltage compartment, the secondary devices are mounted in defined zones.
•
Shortest possible connection between corresponding modules in subracks.
•
Consideration of the magnetic leakage fields of conductor bars and cables.
•
Protection of subracks and wiring backplanes against interference by perforated shielding plates.
•
Large surface bonding between all modules and devices as well as bonding to the earthing conductor
of the switchgear assembly.
These measures basically enable proper operation of the switchgear itself. The planner or operator of the
switchgear must decide whether additional measures are required depending on the electromagnetic
environment where the switchgear is installed. Such measures must be implemented by the installation
company in charge.
In an environment with heavy electromagnetic interference it may be necessary to use shielded cables and
wires for the external connections. This makes it possible to avoid interferences in the low-voltage compartment
and thus, undesired influences on the electronic protection and control or other automation devices.
Cable shields must be electrically bonded to be able to carry high frequencies, and contacted concentrically
at the cable ends.
The shields of cables and wires are connected and earthed in the low-voltage compartment.
Connect the shields to earth potential - with high electrical conductivity and all around as far as possible. Protect
the contact surfaces from corrosion in case of humidity (regular condensation).
When laying cables into the switchgear assembly, separate the control, signaling and data cables and other
lines with different signal and voltage levels by laying them on separate racks or riser cable routes.
Corresponding to the different shield designs, there is a number of methods to perform connection. The
planning department or site management determines which of the methods will be used, taking EMC
requirements into account. The preceding points should always be taken into account.
The shield is connected to cables or wires with clamps contacting all around. If low demands are placed on
EMC, it is also possible to connect the shield directly to earth potential (combine or twist the shield wires) or
via short cable connections. Use cable lugs or wire-end ferrules at the connecting points.
Always keep the connecting leads of the shields as short as possible (< 10 cm).
If shields are used as protective earth conductors at the same time, the connected plastic-insulated lead must
be marked green/yellow over its entire length. Non-insulated connections are inadmissible.
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