EMC behaviour of different electrical distribution architecturing
The system earthing arrangement must be properly selected to ensure the safety of life and property. The behaviour of the different systems with respect to EMC considerations must be taken into account.Figure R1 below presents a summary of their main characteristics.
European standards (see EN 50174-2 § 6.4 and EN 50310 § 6.3) recommend the TN-S system which causes the fewest EMC problems for installations comprising information-technology equipment (including telecom equipment).
| TT | TN-S | IT | TN-C | |
| Safety of persons | Good RCD mandatory |
Good Continuity of the PE conductor must be ensured throughout the installation | ||
| Safety of property | Good | Poor | Good | Poor |
| Medium fault current (< a few dozen amperes) |
High fault current (around 1 kA) |
Low current for first fault (< a few dozen mA), but high for second fault |
High fault current (around 1 kA) | |
| Availability of energy | Good | Good | Excellent | Good |
| EMC behaviour | Good - Risk of overvoltages - Equipotential problems - Need to manage devices with high leakage currents |
Excellent - Few equipotential problems - Need to manage devices with high leakage currents - High fault currents (transient disturbances) |
Poor (to be avoided) - Risk of overvoltages - Common-mode filters and surge arrestors must handle the phase- to-phase voltages - RCDs subject to nuisance tripping if common-mode capacitors are present - Equivalent to TN system for second fault |
Poor (should never be used) - Neutral and PE are the same - Circulation of disturbed currents in exposed conductive parts (high magnetic-field radiation) - High fault currents(transient disturbances) |
Fig. R1: Main characteristics of system earthing
When an installation includes high-power equipment (motors, air-conditioning, lifts, power electronics, etc.), it is advised to install one or more transformers specifically for these systems. Electrical distribution must be organised in a star system and all outgoing circuits must exit the main low-voltage switchboard (MLVS).
Electronic systems (control/monitoring, regulation, measurement instruments, etc.) must be supplied by a dedicated transformer in a TN-S system.
Figure R2 below illustrate these recommendations.
Fig. R2: Recommendations of separated distributions
