The cascade problem
Many panels have a main (incomer) RCD plus group RCDs. If all of them have the same sensitivity and speed, a leak in the bathroom can trip the main RCD — and the whole home goes dark, not just one group.
How type S solves it
A selective RCD (marked S) trips with a deliberate delay and usually a higher threshold (typically 100–300 mA). The fast 30 mA group RCD clears the fault first; the main RCD stays on. Protection becomes layered:
- Group RCD, 30 mA, instant — protects people.
- Main RCD, 100–300 mA, type S — backup and fire protection.
The rule
If a panel has RCDs in cascade, the upstream one must be selective. Our rule engine checks this automatically.
Cascades and RCD types
Selectivity is not only about timing — the type matters too. An EV charger is protected by a type B RCD because it can leak smooth DC current. That same DC current magnetizes and blinds an ordinary type A or AC RCD upstream (IEC 60364-5-53). So if any group RCD is type B, the incoming RCD must also be type B; the rule engine reports an error otherwise.