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

• A. 0.35 Ω
• B. 200 Ω
• C. 0.8 Ω
• D. 0.5 Ω

Answer: (C)

In a TN-S system, Zs is the earth fault loop impedance from the live conductor back to the source via the protective conductor. Keeping this impedance low is essential so that, in the event of a line-to-earth fault, enough current can flow to energize and trip the protective device quickly. For most standard TN-S circuits, the designated maximum Zs is 0.8 ohms, because this limit provides a fault current high enough to ensure disconnection within the required time across typical protective devices. Values much lower (like 0.35 or 0.5 ohms) are stricter limits, but the common maximum used in practice is 0.8 ohms. A value as high as 200 ohms would not allow enough fault current to guarantee timely disconnection. Therefore, the maximum Zs value for a TN-S system is 0.8 Ω.