English
The method of connecting the shielding of the analog input and output cables and the cables of the serial communication lines significantly affects the resistance of the systems built in the cabinet to the effects of electromagnetic interference from the external and internal space. The following principles apply to connecting shielding:
- the shielding of the external and internal cables of the switchboard connects to the earthed chassis of the switchboard on one side of the cable
- in metal cabinets, the shielding of the external cables is connected at the entrance to the cabinet to the earthed casing of the cabinet
- for plastic cabinets, the shielding of the external cables is connected as close as possible to the cabinet entrance to the grounded mounting plate
- the shielding is connected with the largest possible area directly to the grounded surfaces of the switchboard, in the case of using clamps, the untwisted and twisted shielding is always connected directly
- the shield is not connected using additional wires
Fig. 1 shows three ways of connecting the cable shield.
Fig. 1 Example of connection of shielding of analog inputs and outputs and cables of serial communication lines in the cabinet
In case a), the shielding of the outer cable is connected to earth by means of a metal grommet designed to connect the shielded cables, the outer casing of the switchboard and the protective clamp. This method is the most efficient because it minimizes the interference radiated into the switchboard. Suitable grommets are supplied, for example, by the company IES (Progress MS EMV cable glands).
The shield of the inner cable is connected to the ground by means of a grounding clamp, a mounting plate and a protective clamp.
In case b), the shielding of the external cables is connected to the ground by means of a metal clip, a mounting plate and a protective clamp. The shield of the inner cable is connected to the ground by means of a grounding clamp, a mounting plate and a protective clamp. This or another similar method is especially suitable for plastic switchboards with a metal mounting plate.
In case c) an inappropriate connection method is indicated. Although the cable shielding is connected to the protective terminal, the cable connection degrades the shielding efficiency, and electromagnetic interference is introduced and radiated into the switchboard through a long loop.