Residual current devices (RCDs) in secondary distribution boxes are classified by type, rated residual current, sensitivity, and selectivity to ensure protection against electric shock and fire hazards.

Types of RCDs

1. Type AC – Detects only sinusoidal alternating residual currents. Suitable for general-purpose circuits without electronic loads .
2. Type A – Detects both AC and pulsating DC residual currents. Commonly used in circuits with electronic devices like computers or LED lighting .
3. Type F – Designed for circuits with frequency-converting equipment, such as variable-speed drives, where residual currents may have mixed frequencies .
4. Type B / B+ – Detects AC, pulsating DC, and smooth DC residual currents. Required for circuits with electric vehicle chargers, photovoltaic inverters, or industrial equipment .

Sensitivity and Rated Residual Current

• Standard sensitivity: 30 mA for personal protection against electric shock .
• High sensitivity: 10 mA for enhanced protection in special areas like bathrooms or medical facilities .
• Fire protection: 100 mA or 300 mA for preventing fire hazards due to leakage currents .

Selectivity and Coordination

• Upstream and downstream RCDs must be coordinated to avoid unnecessary tripping. This can be achieved by:
  • Using time-delayed (selective) RCDs upstream.
  • Subdividing circuits so that each downstream RCD protects a specific group of loads .
• The upstream RCD's non-tripping time should exceed the maximum tripping time of the downstream RCD for all fault currents to ensure total selectivity .
• Manufacturers may provide RCDs with improved measurement accuracy, allowing selectivity ratios lower than the standard 3:1 between upstream and downstream devices .

Installation Considerations

• RCBOs combine RCD and overcurrent protection in a single device, useful for individual circuit protection .
• Placement: RCDs should be installed in secondary distribution boxes according to the load type, sensitivity requirements, and selectivity rules .
• Testing: Regular testing using the test button ensures proper operation and safety compliance .

Summary

To classify RCDs in secondary distribution boxes, consider:

1. Type (AC, A, F, B/B+)
2. Rated residual current (10 mA, 30 mA, 100 mA, 300 mA)
3. Selectivity (time-delayed upstream, coordinated downstream)
4. Application (personal protection, fire protection, industrial or electronic loads) Proper classification and coordination ensure effective protection against electric shock, leakage currents, and fire hazards while minimizing nuisance tripping in secondary distribution systems .