Purpose

Choose rated residual operating current and time delay from the protective purpose, leakage environment, earthing system, and selectivity design.

Learning objectives

• Distinguish additional/fault/fire purposes
• Interpret IΔn
• Plan leakage margin
• Coordinate time-delayed devices

Values such as 10, 30, 100, and 300 mA are product ratings, not universal prescriptions. The required value follows the protective objective and circuit context.

Core theory

Additional protection commonly uses devices not exceeding the applicable low residual-current threshold, but it supplements rather than replaces basic and fault protection.

Fault protection may use an RCD coordinated with RA/Zs and required disconnection time; fire-risk provisions can have different sensitivity and scope.

Normal protective-conductor current should be managed well below the operating threshold to reduce unwanted operation; upstream selectivity needs compatible sensitivity/time-delay and manufacturer evidence.

Assumptions: Leakages may coincide; No fault current.

1. Aggregate: 5×4=20 mA normal leakage.
2. Margin: 20 mA leaves limited margin for tolerances, transients, and additional loads.
3. Design: Improve circuit division or equipment allocation using current leakage guidance and device data.

The 20 mA aggregate is a continuity-of-service concern requiring redesign assessment.

Reasonableness check: Healthy leakage contributions add at a shared sensing device.

Knowledge check

Answer: No. Sensitivity must match the required protective purpose and design.

Practical exercise

Create a protection-purpose matrix for four circuits, leaving the final IΔn blank until current requirements and leakage data are supplied.

Summary

• Rating follows purpose
• Normal leakage needs margin
• Delay and sensitivity require coordination