Which measurement is used in a TT system test to verify earth resistance and what is the goal?

• A. The value must be exact 1 ohm
• B. The measured earth resistance must be low enough to ensure protective devices operate promptly
• C. The testing must be done with the live conductor energized
• D. The measurement is not needed if a TT system is used

Answer: (B)

In a TT system, the safety path for earth faults goes through the local earth electrode and the soil, not back through the supply earth. Because of that, you need to verify that the installation’s earth resistance is low enough to allow a fault current large enough to cause protective devices to operate promptly. The measurement used is the earth resistance of the installation’s electrode (often measured with a fall-of-potential method or a dedicated earth tester). The goal is to ensure the measured resistance is sufficiently low so that, if a fault occurs, the fault current will trigger the protective device quickly, minimizing shock risk. Values aren’t fixed to a single number like 1 ohm; they depend on the protective device and the circuit, but the key point is keeping the earth resistance low enough for prompt disconnection. Testing with the live conductor energized is not appropriate for this purpose, and TT systems do require this earth resistance check.