METREL MI 3399, Instruction Manual With Exercises

Page: 111 / 272

Electrical Installation Safety Trainer AD 1 - MI 3399
111
Figure 2-50: Lightning system testing
2.8.1 Background of measurement
2.8.1.1 Main earthing
The installation/distribution point or power source is earthed via the so-called main
earthing. The earthing is realized with metal electrode(s) depleted in the soil. The size
and complexity of the earthing arrangement depends on the application (size of object,
soil resistivity, maximum allowed earthing resistance etc.).
In TN installations the earthing is realized at the source and/or distribution points. The
earthing resistances are usually very low (below 1Ω).
TT installations have their own main earthing. The resistances are usually higher than in
TN systems (from few Ω up to several hundred Ω). Because of this dangerous fault
voltages and body currents can occur at relatively low fault currents. Therefore, TT
systems usually have additional RCD protection.
2.8.1.2 Lighting systems
Another application of earthing are lightning protection systems. The lightning rods of a
lighting system must have relatively low resistances (between 1 Ω and 10 Ω to prevent
the installation/buildings before a direct lighting struck. Lighting systems can be very
large.
2.8.1.3 Earth loop test, external source, no probes
In TT systems with the loop resistance test the following loop resistance is measured: