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 earthing: IT, TT, TN

The type of system earthing must be selected carefully as it essentially determines the behaviour 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 earth (functional earthing). The exposed-conductive parts of the electrical installation are connected to earth electrodes which are electrically isolated from the earth electrode for earthing the system.

Permissible protective devices:

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

TN system

In TN systems, one point is connected directly to earth and the ex-posed-conductive parts of the electrical installation are connected to this point via protective earth conductors.

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

  • TN-S: The protective earth conductor is separate throughout the system.
  • TN-C: The neutral and protective earth conductors are com-bined 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 earth or one point is connected to earth via an impedance. On the occurrence of an insulation 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 earth faults.

The exposed-conductive parts of the electrical installation either

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

The following protective devices are permitted:

  • Insulation monitoring devices (IMDs)
  • Overcurrent protective devices
  • Residual current protective devices (RCDs), also known as residual current circuit breakers (RCCBs).

Characteristic features

  • A first insulation fault will not cause a fuse or RCD/RCCB to trip.
  • An insulation monitoring device will detect and signal an imper-missible deterioration in insulation.
  • An insulation fault should be eliminated as quickly as possible before a second insulation 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 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 earth leakage current in small systems
• Influence on neighbouring installations is reduced, this in turn makes earthing 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 earth 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 RCDs
• Regular functional test required
• Operational earthing 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 earthing going unnoticed

Evaluation and comparison

Criterion TT TN-C TN-S IT
Safety of personens *** *** *** ***
Safety against fire hazard *** * ** ***
Safety for 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.