The earthing system (TN, TT or IT) determines how the supply's neutral point and the installation's PE conductor are connected to earth — and it affects requirements for disconnection time, impedance and choice of protective equipment.
Introduction
The grounding system (TN, TT or IT) determines how the supply's neutral point and the installation's PE conductor are connected to ground – and it affects the requirements for breaking time, impedance and choice of protective equipment.
The three basic systems
| System | Supply | Installation | Typical DK use |
|---|---|---|---|
| TN-S | Zero point grounded | PE conductor separate all the way | Newer housing and business |
| TN-C | Zero point grounded | PEN conductor (N and PE combined) | Older installations — not allowed in new ones |
| TN-C-S | Zero point grounded | PEN to a point, then separate PE/N | Transition in existing systems |
| TT | Zero point grounded | Exposed parts grounded locally (own electrode). | Outdoor/allotment garden/campground |
| IT | Zero point isolated or via impedance | Exposed parts grounded | Hospitals, critical facilities |
Afbrydelsestider (DS/HD 60364-4-41, 230 V)
RCD in TT system
In a TT system, the size of the fault current is limited by the resistance of the earth electrode. Automatic disconnection via a circuit breaker alone is therefore typically not possible, and an RCD is required. Choice of rated tripping current:
Leakage current limit per RCD group
To avoid unwanted disconnection due to accumulated leakage current from many consumers applies:
Earth electrode — types and execution
The earthing system (TN/TT/IT) describes the connection to earth — but the actual earth connection must be physically established with an earth electrode. The common types:
- 01Foundation earth (reinforcing earth) — round or flat bar iron laid in the foundation's concrete and connected to the reinforcement. The most durable and preferred electrode in new construction.
- 02Ring earth — a conductor laid in a ring around the building at frost-free depth.
- 03Ground spike (deep ground) — one or more spikes driven vertically into the ground; effective where the deeper layer is moist.
- 04Band drain — a band laid horizontally in a trench, suitable where spikes cannot be driven down.
The electrodes must be corrosion-resistant (hot-dip galvanised steel, copper or stainless steel) and have sufficient cross-section. The specific minimum dimensions and materials for grounding electrodes and ground conductors are given in DS/HD 60364-5-54 and — for lightning protection — DS/EN 62305-3. Look up the dimensions in the current standard.
Ground resistance — requirements and measurement
The most important property of a grounding electrode is its ground resistance RA — the lower, the better a fault current can be diverted. RA depends on the soil's resistivity (moist clay is low, dry sand and rock are high), the electrode's size and depth. It is measured using the fall-of-potential method or a ground loop method.
There is no one universal RA requirement: in TT systems, the allowed RA follows from which RCD you want to use (see the calculation example below).
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