What is the maximum earth fault loop impedance (Zs) value for a TT system?

• A. 200 Ω
• B. 0.35 Ω
• C. 0.8 Ω
• D. 1.0 Ω

Answer: (A)

In a TT system the fault current returns to the source through the earth itself, not via the neutral conductor. That makes the earth fault loop impedance (Zs) highly dependent on the soil’s resistance, which can vary a lot and is often fairly large. To ensure a protective device can still disconnect quickly even with a high and variable earth path, standards set a practical upper limit on Zs for TT installations. For typical 230 V circuits using protective devices like an RCD (with or without a fuse), the maximum Zs is around 200 Ω. Values far smaller than this would not reflect real-world soil conditions and could give a false sense of safety. So, 200 Ω is the appropriate maximum for a TT system.