Types of distribution systems for power supply

Power supply systems

Electrical systems differ on the basis of:

  • Current type: AC, DC, 3(N)AC
  • The type and number of live conductors in the system: L1, L2, L3, N resp. L+, L-
  • The type of system grounding: IT, TT, TN

The type of system grounding must be selected carefully as it essentially determines the behavior and properties of the supply system. It is also a contributing factor to issues associated with system usage, such as:

  • Supply reliability and/or availability of power
  • Installation outlay
  • Maintenance, downtimes
  • Electromagnetic compatibility

TT system

In TT systems, one point is connected directly to ground (functional grounding). The exposed-conductive parts of the electrical installation are connected to ground electrodes which are electrically isolated from the ground electrode for grounding the system.

Permissible protective devices:

  • Overcurrent protective device
  • Fault current protective devices (GFCIs)
Figure: TT system
TT system

TN system

In TN systems, one point is connected directly to ground and the exposed conductive parts of the electrical installation are connected to this point via protective ground conductors.

There are three types of TN systems, differentiated on the basis of the arrangement of the neutral and protective ground conductors:

  • TN-S: The protective ground conductor is separate throughout the system.
  • TN-C: The neutral and protective ground conductors are combined in a single conductor throughout the system.
  • TN-C-S: Neutral and protective functions are combined in a single conductor in a part of the system.
Figure: TN-S system
TN-S system
Figure: TN-C system
TN-C system

IT system

In IT systems, all live conductors are isolated from ground or one point is connected to ground via an impedance. On the occurrence of a ground fault, therefore, only a small leakage current, essentially caused by system leakage capacitances, can flow. The upstream fuses do not trip. The voltage supply is also maintained in the event of single-pole direct ground faults.

The exposed-conductive parts of the electrical installation either

  • have separate connections to ground or
  • share a common connection to ground or
  • share a common connection to the system ground.

The following protective devices are permitted:

  • Ground-fault monitoring devices (IMDs)
  • Overcurrent protective devices
  • Residual current protective devices (RCDs), also known as ground-fault circuit interrupters (GFCIs).

Characteristic features

  • A first ground fault will not cause a fuse or RCD/GFCI to trip.
  • A ground-fault monitoring device will detect and signal an impermissible deterioration in insulation.
  • A ground fault should be eliminated as quickly as possible before a second ground fault can occur on a different live conductor, as this would cause the system to fail.
Figure: IT system
IT system

Advantages and disadvantages of the system types

Type of supply system Your benefits Disadvantages
SELV or PELV (safety extra-low voltage or protective extra low voltage) • No hazard potential on contact • Limited power if deployment of equipment is to be cost-effective • Specific requirements on current circuits
Protective insulation • Maximum level of safety
• Can be combined with other types of system
• Double insulation of equipment
• Only cost-effective for small loads
• Insulating material pose fire hazard on thermal loads
IT system • EMC friendly
• Increased availability: 1st fault is simply reported Disconnection in the event of a 2nd fault
• Low ground leakage current in small systems
• Influence on neighboring installations is reduced, this in turn makes grounding easier
• Little technical effort for cable and conductor installation
• Use of appropriate devices facilitates fault location
• Equipment has to be insulated universally for the voltage between external conductors.
• An overvoltage protective device is required for N conductors
• Potential problems with going offline on second ground fault
TT system • EMC friendly
• Protection is dependent on the system's short circuit power
• Little technical effort for cable and conductor installation
• Touch voltage can vary from one area to another
• Can be combined with a TN system
• Only compatible with low power ratings due to the use of GFCIs
• Regular functional test required
• Operational grounding is complex (≤ 2 Ω).
• Equipotential bonding compulsory for every building
TN-C system • Easy to set up
• Low material expenses
• Not EMC friendly
• Building stray currents and low frequency magnetic fields make the system incompatible for use in buildings housing information technology equipment
• Risk to life and limb in the event of PEN break
• Increased risk of electrical fires
TN-C-S system •A cost-effective compromise for buildings which do not house information technology equipment. • Not EMC friendly
• Low-frequency magnetic fields possible
TN-S system • EMC friendly • Low voltage rise in the healthy phases
• Increased safety engineering outlay for remote multiple infeeds
• Risk of multiple grounding going unnoticed

Evaluation and comparison

Criterion TT TN-C TN-S IT
Safety of persons *** *** *** ***
Safety against fire hazard *** * ** ***
Machine protection *** * * ***
Availability ** ** ** ****
Electromagnetic compatibility ** * ** ***
Maintenance ** **** **** ***
Installation * ** ** ***
Overall result * 16 * 14 * 16 * 22

Signs and symbols

*Weak
**Average
***Good
****Excellent

The IT system convinces with its many advantages.