Communications devices – especially those used in broadband communications systems – generate capacitive and inductive electrical phenomena that lead to electromagnetic disturbances.
Communications networks are generally vast, connecting equipment located in rooms whose electrical installations potentially have different earthing systems. What’s more, if the different rooms do not have the same electrical potential, high transitory currents and differences in electrical potential between the equipment hooked up to the installation can occur. This is the case for insulation faults or lighting strikes, for instance. The communications cards in computers and automation devices do not have high dielectric withstanding voltages (between the active conductors and mechanical parts). In general, the withstanding voltage for such equipment is around 500 V and can be as high as 1.5 kV for the most robust communications equipment. For complex, meshed electrical networks with TN-S earthing systems, these withstanding voltages are acceptable. Whatever the case may be, surge protectors (common and differential modes) are recommended.
Choosing the right cabling
The type of communications cabling used is an important variable. The cable chosen must be suitable for the type of transmission. Cable characteristics – such as impedance twisted pairs or not, linear capacitance-resistance, and the type of shielding – should be taken into consideration to ensure reliable, robust communications across the network.
Implementation best practices
Whatever the communications protocol (Modbus, Profibus, CanOpen, DeviceNet, Ethernet, or LON), it is important to follow best practices for effective implementation.
The following factors should be taken into consideration:
- The communications cable impedance is compatible with the bus used
- End of line resistance is compatible with the impedance of the cabling used
- Line polarization if recommended for bus
- Maximum bus length including drops
- Metal connectors appropriate to the type of bus (avoid open-style connectors)
- Ensure 360° shielding (no pigtails/patch cords)
Selecting the best transmission links
Symmetrical (differential) transmission links – the most robust in terms of electromagnetic behavior – are also crucial. However, in harsh electromagnetic environments or for wide communications networks with installations with differences in electrical potential and with IT, TT, or TN-C earthing systems, fiber optic connections are strongly recommended. To protect people, the fiber must not have metal components (which can create an electric shock hazard if the fiber connects two areas with different earths).