Installers search for “time delay RCD”, “S‑type RCD”, and “100 mA RCD trip time” for one reason: selective coordination. A time‑delayed RCD makes sure the right device trips first - keeping the rest of the installation alive.
Quick pick: Need an upstream selector now? See our A‑Type 100 mA Time Delay RCD (63A/80A/100A).
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.
- 30 mA - Additional protection at final circuits. Rarely time‑delayed.
- 100 mA (time‑delay) - Upstream fault protection where discrimination with 30 mA devices is required (EV/PV distribution).
- 300 mA (time‑delay) - Fire protection / main distribution, coordinated with downstream devices.
Shop upstream selectors: A‑Type 100 mA Time Delay RCD. Pair with downstream Type A RCBOs for clean selectivity.
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 device 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.
Need a ready‑to‑fit upstream unit? See our Metal Surge Arrestor Unit (100 A switch + SPD) or add selectivity with the A‑Type 100 mA Time Delay RCD.
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.
- Application: Upstream selectivity? Choose S‑type (time‑delay).
- Sensitivity: 100 mA for upstream fault protection; 300 mA if you’re targeting fire protection.
- Poles: 2P (single‑phase) or 4P (three‑phase / full conductor disconnection requirements).
- Rated current: Match to the feeder (63A / 80A / 100A available).
- Device types downstream: 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 (Time‑Delay / S‑Type).
FAQs
Snappy answers to the queries we see in search and on site.
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 will trip 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. Pair an EV board with upstream selectivity: A‑Type 100 mA Time Delay RCD.
What’s the rated trip time of a 100 mA time delay RCD?
Manufacturer/type‑test dependent, but expect a deliberate delay (~130–500 ms at test points) to ensure selectivity with downstream 30 mA devices. Always verify against the datasheet and BS EN 61008‑1 testing.
Can I get a 30 mA time delay RCD?
30 mA is intended for additional protection and is normally non‑delayed. If you need selectivity, coordinate a 100/300 mA time‑delay upstream with 30 mA downstream, rather than delaying the 30 mA device.
Is 300 mA time delay just for fire protection?
It’s commonly used for fire protection at distribution level and to coordinate with downstream devices. Choose 300 mA time‑delay where that protection strategy applies and discrimination is required.
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 the EVSE includes RDC‑DD, a Type A path is acceptable. See downstream Type A RCBOs.
Final Word
Selective, not 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? Grab the installer‑favourite A‑Type 100 mA Time Delay RCD (63A/80A/100A), built for clean discrimination on real‑world boards.