When evaluating fault loop impedance (Zs) in a circuit, how should the measured value be assessed?

• A. Compare the measured Zs with the maximum allowable Zs for the protective device rating; lower values indicate safer protection.
• B. Zs has no bearing on protection.
• C. A higher Zs ensures faster tripping.
• D. Zs only affects voltage drop under normal operation.

Answer: (A)

The important idea here is that fault loop impedance (Zs) determines how much fault current will flow if a live part comes into contact with earth. When a fault occurs, the current that can flow is roughly the supply voltage divided by Zs. Protective devices are designed to trip within a specified time when the fault current exceeds a threshold. For safety, each protective device rating has a maximum allowable Zs value. To judge a measured Zs, you compare it to that maximum. If the measured Zs is at or below the maximum, the fault current is large enough to drive the protective device to trip quickly, providing safe disconnection. If Zs is higher than the maximum, the fault current may be insufficient to trip promptly, leaving a dangerous fault present. Lower Zs means a higher fault current and faster disconnection, hence safer protection.