In a TN-S or TN-C-S system, what is the main purpose of the protective conductor?

• A. To carry normal operating current
• B. To carry fault current safely to earth and keep exposed parts at earth potential
• C. To connect to the neutral
• D. To provide lightning protection

Answer: (B)

The protective conductor provides a low-impedance path for fault current to earth and keeps exposed conductive parts at earth potential. In TN-S and TN-C-S systems, this protective path is connected to the earth electrode and bonded to all exposed metalwork. If a fault occurs—such as a live conductor contacting a metal case—the fault current travels via the protective conductor to earth. This high current causes the protective device (fuse or circuit breaker) to operate quickly, disconnecting the supply and greatly reducing the risk of electric shock. The protective conductor’s main job is safety, not carrying normal operating current—that’s the role of the live and neutral conductors. In TN-C-S, a PEN conductor may carry both neutral and earth up to the main distribution, but once the installation is fed, the system uses separate protective conductors to maintain low-impedance fault paths. It’s not primarily for lightning protection, though a solid earth connection also helps with surge currents.