What Is a Time-Delay (S-Type) RCD?

In one line: it’s an RCD with a short, intentional delay so upstream protection doesn’t trip before the downstream device has a chance to clear the fault.

Also known as S-type, a time-delay RCD sits upstream of your 30 mA devices. In a fault, the downstream RCBO/RCD operates first; the S-type only trips if the fault persists. That’s how you avoid full-site blackouts and needless call-backs.

Installer favourite: A-Type 100 mA Time-Delay RCD (WARTD series) — built for EV, PV and discrimination boards.

Related guides: How to Install an EV Consumer Unit · What Is an EV Consumer Unit?

30 mA vs 100 mA vs 300 mA — Which Sensitivity When?

Rule of thumb: 30 mA protects people at the final circuit; 100/300 mA are for upstream selectivity and fire protection.

Shop upstream selectors: A-Type 100 mA Time-Delay RCD. Pair with downstream Type A RCBOs for clean selectivity.

Also read: What Surge Protection Does an EV Charger Need?

Where Do Time-Delay RCDs Make Sense?

Use them whenever a nuisance upstream trip would take out too much of the installation.

• Dual-RCD / split-load boards — Keep the main RCD selective to avoid whole-board trips.
• EV charger installs — Upstream time-delay feeding a dedicated EV board with 30 mA Type A RCBO.
• Solar PV / inverter DBs — Coordinate upstream with inverter-fed sub-circuits.
• Industrial DBs — Multiple outgoing RCBOs where continuity is critical.

Trip Times & Selectivity — The Fast Version

This is what stops the “all-off” scenario. A standard 30 mA device must clear quickly (typically <300 ms at rated residual). An upstream S-type introduces a measured delay (typ. ~130–500 ms depending on test point and device) so the downstream device wins the race.

Outcome: the final-circuit RCBO trips, the main stays on, and your client doesn’t lose the whole house/board.

How to Choose the Right Time-Delay RCD

Five checks and you’re sorted.

1. Application: Upstream selectivity? Choose S-type (time-delay).
2. Sensitivity: 100 mA for upstream fault protection; 300 mA where fire protection/greater selectivity is required.
3. Poles: 2P (single-phase) or 4P (three-phase / full conductor disconnection).
4. Rated current: Match to the feeder (63A / 80A / 100A available).
5. Downstream devices: Coordinate with Type A RCBOs (or Type B paths where EVSE lacks 6 mA DC detection).

Spec it in minutes: A-Type 100 mA Time-Delay RCD.

EV reading list: Do You Still Need an Earth Rod with PME Protection? · EV Charger Surge Protection · EV Consumer Unit Checklist

FAQs

Snappy answers to on-site queries.

What’s the difference between a time-delay (S-type) RCD and a normal RCD?

An S-type adds a short intentional delay so the downstream 30 mA device trips first. A normal 30 mA device has no intentional delay and trips as fast as possible for additional protection.

Do EV chargers need a time-delay RCD?

Not at the charger. Use 30 mA at the final circuit (typically a Type A RCBO). Use a time-delay 100 mA upstream if you need discrimination with other 30 mA devices.

What’s the rated trip time of a 100 mA time-delay RCD?

Manufacturer/type-test dependent; expect a deliberate delay (~130–500 ms at test points). Verify against the datasheet and BS EN 61008-1 testing.

Can I get a 30 mA time-delay RCD?

30 mA is for additional protection and is normally non-delayed. For selectivity, use 100/300 mA time-delay upstream with 30 mA downstream.

Do I need Type B if my EVSE doesn’t have 6 mA DC protection?

Yes — if the EVSE lacks RDC-DD (6 mA DC detection), use a Type B path. If EVSE includes RDC-DD, a Type A path is acceptable.

Final Word

Selective, not just sensitive. Time-delay (S-type) RCDs don’t replace 30 mA protection — they make your board more resilient by ensuring the right device trips first.

• Use 100 mA time-delay upstream for EV/PV distribution.
• Coordinate with 30 mA Type A RCBOs downstream.
• Add surge protection where required: SPDs.

👉 Ready to spec? Order the A-Type 100 mA Time-Delay RCD or browse our full RCBO range.