What is TN circuit?

What is TN circuit?

Diagram tn-c

Differential current monitoring in grounded systemsIn a grounded network (TN-S/TT system), one or more points of the network, installation or electrical equipment are grounded to ensure electrical safety.

In TN systems, the neutral point of the power transformer is grounded with a negligible impedance and the electrical installation grounds are connected to the service ground of the network by means of a protective conductor. In TT systems, the neutral point is also grounded with negligible impedance, but the electrical installation grounds are grounded independently of the system ground.

Why is earth leakage current monitoring necessary? With earth leakage current monitoring, residual and residual currents are detected before the system has to be switched off in the event of a fault. In this way, deterioration of the insulation level is detected early and reliably.

What does tn-s mean

However, there were problems with this system, exemplified by the transcontinental telegraph line built in 1861 by the Western Union Company between St. Joseph (Missouri) and Sacramento (California). In dry weather, the ground connection often developed high resistance, making it necessary to pour water over the connecting rods to make the system work.

Later, when telephony began to replace telegraphy, it was found that currents induced in the ground by other apparatus, railroads, and lightning caused unacceptable interference, so the two-wire system was reintroduced.

To make the connection of this ground potential to an electrical circuit, a grounding electrode is used, which can be something as simple as a metal rod (usually copper) driven deep into the ground, which sometimes needs to be prepared for better conduction.

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It is a concept linked to the safety of people, because they are at the same potential because they are standing on the ground. If any device is at the same potential, there will be no difference between the device and the person, so there will be no dangerous electric shock.


First of all, a distinction must be made between the grounding conductor and the PE protective conductor. Although both are part of the installation, the grounding conductor is the one that connects the electrode and the main ground busbar, while the protective conductor PE is the one that runs throughout the installation and to which the different electrical masses and secondary ground busbars are connected.

For example, if there is a line conductor of section S of 16mm2, the protective conductor PE will also be of 16mm2. On the other hand, if the cross section of the line conductor is 70mm2, the cross section of the protective conductor will be half, i.e. 35mm2.

It should also be considered that the minimum cross section for grounding conductors shall not be less than 4mm2. For PE protective conductors, the section shall not be less than 2.5mm2 if they are mechanically protected or 4mm2 if they are not.

It is not correct to have independent grounding systems for lightning protection or electronic functional earths. Although a lightning protection system will have its own grounding system, as specified by the Occupational Health and Safety Law, it must be linked to the protective grounding of the electrical installation to obtain equipotentiality between them.

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Generally, “Earth” (for electrical circuits or systems) can be associated with an equipotential point or plane that serves as a potential reference and may or may not be at ground potential.

The use of electrical energy began in the early 1900’s. In order to reach the current knowledge and regulations on the subject of grounding, events such as the following had to happen:

1892 The New york Board of Fire Underwriters (NYBFU) determined that the practice of grounding was dangerous and these should be removed before 01/01/1892. It was based on the study of Professor Henry Morton.

1909 The American Insitute of Electrical Engineers (AIEE) and the NYBFU made grounding mandatory for systems of 150 V or less and optional for those operating at more than 250 V phase-ground.

Protective Earth (PE), avoids the risk of electric shock by maintaining the exposed conductive surfaces of a device at ground potential, under normal conditions no current flows through this conductor, in case of a fault, the fault current flows through this conductor in order to operate the protections and open the power circuit to the fault.

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