A time-delay RCD, also known as an S-type RCD, is designed to sit upstream of downstream 30 mA protection so the final circuit trips first in a fault. That makes it useful where you need selectivity, reduced nuisance tripping, and better continuity across EV, PV and distribution setups.
Quick Answer: A time delay (S-type) RCD is designed for upstream selectivity, ensuring downstream 30 mA devices trip first. It’s essential on EV, PV, and distribution boards where a nuisance upstream trip could drop the whole install.
🧭 Jump to:
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.
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.
| Sensitivity | Purpose | Typical Use |
|---|---|---|
| 30 mA | Additional (personal) protection | Final circuits (non-delayed) |
| 100 mA (time-delay) | Discrimination with 30 mA | Upstream EV/PV boards |
| 300 mA (time-delay) | Fire protection & upstream selectivity | Main distribution |
Shop upstream selectors: A-Type 100 mA Time-Delay RCD. Pair with downstream Type A RCBOs for clean selectivity.
Regulation Reminder: Under BS 7671 (18th Edition), selective coordination is required when using upstream 100 mA and 300 mA RCDs. Always confirm trip times and discrimination; type-testing is to BS EN 61008-1 / 61009-1.
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.
Installer’s Pick: The A-Type 100 mA Time-Delay RCD is 18th Edition-ready and UK-stocked for next-day delivery (63A / 80A / 100A).
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.
| RCD Type | Rated Residual Current Test | Typical Operating Time Guidance |
|---|---|---|
| General non-delay RCD | At IΔn | Less than 300 ms |
| S-type time-delay RCD | At IΔn | 130 ms to 500 ms |
Outcome: the final-circuit RCBO trips, the main stays on, and your client doesn’t lose the whole house/board.
RCD Trip Times Explained
When electricians talk about RCD trip times, they usually mean how quickly the device disconnects at its rated residual operating current during testing. A standard non-delayed RCD should trip in under 300 ms at IΔn, while an S-type time-delay RCD is intentionally slower so downstream devices have time to clear the fault first.
That intentional delay is exactly why S-type RCDs are used for selectivity rather than additional protection at the final circuit.
How to Choose the Right Time-Delay RCD
Five checks and you’re sorted.
- Application: Upstream selectivity? Choose S-type (time-delay).
- Sensitivity: 100 mA for upstream fault protection; 300 mA where fire protection/greater selectivity is required.
- Poles: 2P (single-phase) or 4P (three-phase / full conductor disconnection).
- Rated current: Match to the feeder (63A / 80A / 100A available).
- 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.
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 devices are generally used for additional protection and are normally non-delayed. In practice, time-delay selectivity is usually achieved with 100 mA or 300 mA devices installed upstream of downstream 30 mA protection.
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.
What is the maximum trip time for a 30 mA RCD?
For a general non-delayed 30 mA RCD tested at its rated residual operating current, the expected maximum operating time is less than 300 ms. Always confirm against the current standard, the device type and the manufacturer’s data.
What is the operating time of an S-type RCD?
An S-type time-delay RCD is intentionally slower than a standard RCD. At rated residual current, the operating time is typically expected to fall between 130 ms and 500 ms.
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.