II - DCE FEL ČVUT v Praze

Mechanical Configuration of an Automation System with SIMATIC S7 Ex Modules
1-32
If you install your controller in a terminal box, use a cast metal or sheet
metal housing if possible. Plastic housings should always have a metallized
surface.
Doors or covers of housings should be connected to the grounded body of the
housing with ground strips or contact springs.
If you are working on the system with the cabinet open, observe the
guidelines for protective measures for electrostatically sensitive devices
(ESDs).
Electrical systems must be installed such that the risk of ignition by
electrostatic charges cannot be expected. Refer to ”Guidelines for avoiding
the risk of ignition resulting from electrostatic charges” laid down by the
main association of Industrial Employers’ Liability Insurance.
If electrostatic charges cannot be avoided, a charge should be kept as low as
possible or safe discharge should be provided. The following measures, in
particular, should be applied:
– Electrostatic grounding of all conductive elements. Solid materials
can be considered as being electrostatically grounded if their leakage
resistance at any point is not greater than 106 . Under favorable
conditions, 108 is satisfactory, particularly for small equipment of
low capacitance.
– Reducing the electrical resistance of the material moved or parts
moved with respect to each other.
– Incorporation of grounded metal elements in material subject to
electrostatic charging.
– Increasing the relative air humidity. By increasing the relative air
humidity to about 65 % with air conditioning, sprays or by hanging
moist cloths, the surface resistance of most non-conductive materials
can be adequately reduced. However, if the surface of plastic material
is water-repellent, this measure will not succeed.
– Ionization of the air.
1.8.4 The Most Important Basic Rules for Ensuring EMC
To ensure EMC, it is often sufficient to observe some elementary rules.
When assembling the control system, observe the five following basic rules.
1. When installing the programmable controllers, ensure high quality
surface grounding of the inactive metal elements
Connect all inactive metal elements over a large area and at low
impedance.
On painted and anodized metal elements, make screwed connections with
special contact washers or remove the insulating protective layers.
Provide a central connection between chassis ground and the ground/
protective conductor system.
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04