What is the key difference between TN-S and TN-C-S earthing arrangements?

• A. TN-S uses separate protective earth and neutral conductors; TN-C-S uses a combined PEN conductor up to a point where it is separated into PE and N
• B. TN-S uses a combined PEN conductor; TN-C-S uses separate PE and N
• C. TN-S uses no earth; TN-C-S uses earth
• D. TN-S uses earth and neutral combined; TN-C-S uses only earth

Answer: (A)

The main idea being tested is how earth and neutral are arranged in TN-S versus TN-C-S systems, focusing on the use of a PEN conductor in TN-C-S.

In a TN-S arrangement, the protective earth (PE) and the neutral (N) are kept separate from the source to the load throughout the installation. This means you have distinct conductors for earth and neutral all the way from the supply to outlets and equipment.

In a TN-C-S arrangement, the conductor from the supply to the consumer’s installation is a single combined PEN conductor that serves as both protective earth and neutral. At a defined point, usually at the main distribution board, this PEN conductor is separated into separate PE and N conductors. After separation, the PE provides the protective earth path for exposed metal parts, and the N carries the neutral return.

Choosing the correct description highlights the key difference: TN-S uses separate earth and neutral throughout; TN-C-S uses a single PEN conductor up to the separation point, then splits into PE and N. The PEN conductor concept is what distinguishes the two, and the separation point is where the system transitions from a combined to separate conductors.