What is the maximum Zs value for a TN-C-S system?

• A. 1.0 Ω
• B. 2.0 Ω
• C. 0.35 Ω
• D. 0.8 Ω

Answer: (C)

Zs is the earth fault loop impedance—the total resistance the fault current must flow through from the live conductor to earth through the protective device. In a TN-C-S system, the protective path includes the PEN conductor up to the point where earth is separated, so keeping this impedance very low is key to getting enough fault current quickly to trip the protective device within the required time.

For TN-C-S installations, standard practice sets a strict limit on Zs to ensure disconnection within the specified time (typically a fraction of a second) under worst‑case conditions. The value 0.35 ohms is the commonly cited maximum to guarantee this fast tripping for final circuits using standard overcurrent devices in a 230 V system. With such a low Zs, the fault current is large (roughly 230 V divided by 0.35 Ω, giving about 650 A), which reliably drives the protective device to operate promptly.

The other numbers would not meet the required disconnection performance for typical protective devices in a TN-C-S system, hence they are not the correct maximum.