One of the basic prerequisites for guaranteeing safety on any residential or industrial electrical is to provide an earth electrode. If there is no earth/ground electrode, people’s lives may be endangered and electrical installations and other property may be damaged. An earth/ground electrode alone, however, is not enough to guarantee total safety. Only regular inspections can prove that the electrical installation is operating correctly. There are many earth resistance measurement methods available, depending on the type of neutral system, the type of installation (residential, industrial, urban environment, rural environment, etc), thBefore starting any earth resistance measurements, the first thing you need to find out is the acceptable maximum value for correct earthing.e possibility of cutting off the power supply, etc.

Why is earthing necessary?
Earthing means setting up an electrical bond between a given point in a network, installation or machine and an earth electrode. This earth electrode is a conductive part which may be inserted in the ground or in a conductive medium, in electrical contact with the Earth (see definition in NFC 15-100). Earthing thus involves using a wire conductor to connect an earth electrode to the metal chassis earths which might accidentally come into contact with the electric current due to an insulation fault on an electrical device. In this way, there will be no danger for people because the fault cur- rent will have drained into the earth. If there is no earthing, any person involved will be subjected to an electric current which may kill them, depending on its level.

Earthing therefore enables leakage currents to flow away safely and, if it is linked to an automatic cut-off device, can ensure that the power supply to the electrical installation is switched off. So correct earthing keeps people safe while also protecting installations and property if there are fault currents or lightning strikes. It should always be linked to a cut-off system.

Example:
If the insulation on the load is faulty, the fault current is drained to earth via the protective conductor (PE). Depending on its value, the fault current may cause the installation to be cut off when the residual current device (RCD) is tripped.

What should the value of the earth resistance be?
Before starting any earth resistance measurements, the first thing you need to find out is the acceptable maximum value for correct earthing.The earth resistance requirements vary according to the country, the neutral systems used and the type of install- lation. For example, a power distributor such as EDF will require an extremely low earth resistance, often of only a few ohms. So it is important to check beforehand on the standards applicable to the installation to be tested

As an example, let’s take a TT residential installation in France:
To keep people safe, an installation must be equipped with protective devices which trip as soon as a “fault voltage” flowing in the installation exceeds the threshold voltage liable to harm the human body. Studies by a working party of doctors and safety experts have determined a permanent contact voltage accepted as safe for people: 50 V AC in dry premises (the limit may be lower for humid or immersed environments). Furthermore, in residential installations in France, the residual current device (RCD) linked to the earth electrode usually allows a current up to 500 mA.

According to Ohm’s Law:               U = R I
In this case:         R = 50 V / 0.5 A = 100 Ω

To make sure there is no danger for people or property, the resistance of the earth electrode must be less than 100 Ω.The calculation above clearly shows that the value depends on the rated current of the RCD controlling the installation.
For example, the correlation between the earth resistance and the RCD rated current is specified by the NF C 15-100 standard, as shown in the following table

Maximum resistance of earth electrode according to RCD rated current