Essential Q&A on Electrical Earthing and Protection Systems

Posted on Oct 14, 2025 in Electrical Engineering

Fundamental Questions on Electrical Protection and Distribution

1. What device eliminates fault currents in a TT system?

Fault currents in a TT system are preferably eliminated by a Residual Current Device (RCD).

2. Which earthing system arrangement is used in public distribution facilities?

In public distribution facilities, the earthing system commonly used is the TT system.

4. Which are the three basic types of earthing (neutral) systems?

The three basic types of earthing systems are:

  • TT
  • TN (TN-C, TN-S, TN-C-S)
  • IT

5. Other ways to refer to “star” topology

Star topology is also known as radial distribution or centralized distribution.

6. Where is comb distribution used?

Comb distribution (often referring to busbar or trunking systems) is used for purposes of centralized control, management, maintenance, and monitoring of a system or process dedicated to a specific application.

7. What device guarantees interruption when a fault current occurs in a TN system?

When a fault current occurs, resulting in a short-circuit, it will be eliminated by an overcurrent protection device (e.g., circuit breaker or fuse).

8. What is a PEN conductor?

A PEN conductor is a conductor in which the neutral (N) and the protective earth (PE) functions are combined into a single conductor.

9. Which letter in the neutral system indicates the situation for the metal masses?

The second letter indicates the connection status of the exposed conductive parts (metal masses) of the installation relative to the earth:

  • TT: Masses connected to earth.
  • TN: Masses connected to the Neutral conductor.
  • IT: Masses connected to earth.

10. What percentage of harmonics is considered tolerable in a network?

Generally, a facility cannot tolerate a significant percentage of harmonics. The most common usage limits are:

  • Maximum 5% for voltage harmonics (Total Harmonic Distortion, THDV).
  • Maximum 10% for current harmonics (THDI).

11. What is understood as a “fault loop”?

The fault loop is the circuit path through which the current flows when a fault occurs. This path typically includes the equipment, the protective conductor, sometimes a person, the transformer, the electrodes, the earth, and other conductive elements.

12. Harmonics: How to reduce them

Harmonics can be reduced or mitigated by:

  • The installation of specialized transformers (e.g., low voltage/low voltage delta/star zigzag) to isolate the third harmonic and its odd multiples.
  • The installation of passive or active filters.

13. CPA, what is it?

CPA stands for Control Permanente de Aislamiento (Permanent Insulation Control). This system monitors insulation failure in electrical systems and allows the circuit to continue working until a second failure occurs (typically used in IT systems).

14. DDR: Another way to recall this initialism

DDR means “Dispositivo Diferencial Residual” (Residual Differential Device). It is commonly referred to simply as a “Differential” or Residual Current Device (RCD).

15. Define the following terms: Mass, Earth, Connection Conductor

  • Mass (Exposed Conductive Part): It is a conductive part of the equipment that can be touched and which is not normally under voltage, but which might become energized due to an insulation fault.
  • Earth: The conductive mass of the earth, whose electric potential at each point is taken by convention to be equal to zero.
  • Connection Conductor (Equipotential Bonding Conductor): It is a protective conductor that provides equipotential bonding. It is connected to metallic parts that are not intended to be part of the electric circuits.

16. Difference between TNC and TNS systems

  • TN-C: The Neutral (N) and Protective Earth (PE) conductors are combined into a single PEN conductor throughout the entire circuit.
  • TN-S: The Neutral (N) and Protective Earth (PE) conductors are separate conductors throughout the entire circuit.

17. The UPS system: What does the acronym mean?

UPS means Uninterruptible Power Supply. This system ensures the continuity of the electrical supply by providing backup power during outages or voltage fluctuations.

18. In the IT system, what happens when two insulation failures occur?

Upon the second insulation failure, the circuit operates like a TT or a TN system, requiring immediate disconnection by a protective device.

19. List the three levels of distribution used in Low Voltage (LV)

The three levels of distribution used in Low Voltage (BT) are:

  1. Distribution from the General LV Board (CGBT).
  2. Secondary Distribution, used to distribute electricity within specific zones.
  3. Terminal Distribution, used to supply the various final loads and outlets.

20. Explain the meaning of the 1st letter when defining neutral arrangements

The first letter denotes the feeding situation (usually the transformer secondary neutral) relative to the earth:

  • T (Terra): The neutral point is directly connected to the earth.
  • I (Isolated): The neutral point is isolated from the earth or connected to the earth through a high impedance.

21. Explain what the following letters mean: TT, TN, and IT

  • TT: T (Neutral connected to earth) T (Masses connected to earth).
  • TN: T (Neutral connected to earth) N (Masses connected to Neutral).
  • IT: I (Isolated or impedance-earthed neutral) T (Masses connected to earth).

22. Make a list of facilities that cannot tolerate an untimely power outage

Facilities requiring high continuity of service include:

  • Locations with risk of fire or explosion.
  • Control installations with numerous sensors and critical monitoring equipment.
  • Installations with strict continuity requirements (e.g., surgery rooms, critical ventilation, pumps).
  • Appliances sensitive to leakage currents (risk of destruction of coils/windings).

23. Make a list of the sensitivity of Residual Current Devices (RCD/DDR)

Common RCD sensitivity levels include:

  • 30 mA: Used for normal protection in homes and general use areas (protection against direct contact).
  • 300 mA: Used for protection against fire risk or when supplying equipment that requires more power, such as large motors or industrial applications (protection against indirect contact).

24. Computer equipment: What type of earthing system do they require?

Computer equipment often requires the IT system due to its high tolerance for the first insulation fault and the need for continuous operation.

25. Voltage levels used in Low Voltage (BT)

There are two primary voltage levels used in Low Voltage (BT) distribution:

  • Three-Phase: 380 V, 400 V, or 415 V (or 480 V in exceptional cases), mainly used for motors and process applications.
  • Single-Phase: 220 V, 230 V, or 240 V (or 277 V in exceptional cases), used for lighting circuits and general outlets.