2391-52 Inspection and Testing Qualification Practice Test

The maximum earth fault loop impedance for a TT system is correctly identified as 200 ohms. This value is crucial because it is tied to the safety of the electrical installation. In a TT system, the earth fault loop impedance determines whether a protective device, such as an RCD (Residual Current Device), can detect a fault condition and disconnect the supply promptly to prevent electric shock or fire hazards.

The 200 ohm limit ensures that in the event of a fault, the earth fault loop impedance is low enough for sufficient fault current to flow and be detected, allowing the protective device to operate within the required time frame. This safety threshold is established to maintain adequate performance of protective devices under fault conditions. Thus, a maximum impedance of 200 ohms is necessary to ensure the effectiveness of the safety mechanisms in place within a TT system, ensuring both compliance with safety standards and the protection of individuals and property from electrical faults.