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, EN 50310 and HD 60364-4-444) recommend the TN-S eathing 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	Correct Use of residual current devices (circuit breaker)is mandatory	Correct Continuity of the PE conductor must be ensured throughout the installation
Safety of property	Correct	Poor	Correct	Poor
	Medium ground-fault current (< about ten amps)	High ground-fault current (about 1 kA)	Low ground-fault current for first fault (< about ten mA), but high for second fault	High ground-fault current (about 1 kA)
Availability of energy	Correct	Correct	Excellent	Correct
EMC performance	Correct - Risk of overvoltages - Equipotential problems - Need to manage devices with high leakage currents	Excellent - Good equipotential situation - Need to manage devices with high leakage currents - High ground- 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 (not recommended) - Neutral and PE are combined - 50/60 Hz and harmonics currents circulate in the earthing and grounding structures - High ground-fault currents (transient disturbances)

Fig. R1: Main characteristics of the different earthing sytems

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 illustrates these recommendations.

Fig. R2: Recommendations of separated distributions