What are the three types of earthing systems defined in IET Wiring Regulations?

The correct answer is grounded in the definitions set forth in the IET Wiring Regulations, specifically those dealing with earthing systems. The IET (Institution of Engineering and Technology) Wiring Regulations categorize earthing systems for electrical installations into three main types: TN, TT, and IT.

The TN system, which stands for "Terre Neutral," connects the neutral point of the supply system directly to earth and has a metal connection back to the source. This provides a low impedance path for fault currents, facilitating effective operation of protective devices during faults.

The TT system, which means "Terre Terre," features a direct earth connection for the consumer's installation but relies on the earth for the supply source rather than direct connection back to the source. Each installation's earthing system is independent, which can be beneficial for certain applications, especially in rural and isolated areas.

Finally, the IT system, or "Isolated Terre," involves a system where the supply conductors are isolated from the earth. It allows for the continuity of supply during the first earth fault, with appropriate measures in place for protection.

Thus, the classification of these three earthing systems reflects their design principles and operational characteristics, which are essential for ensuring safety in electrical installations. The other options introduce