Type B RCDs and time-delay RCDs get mixed up all the time, especially when installers are trying to solve a tripping issue or work out what belongs upstream of an inverter-driven circuit. But these two devices do not do the same job.
A Type B RCD is chosen because of the kind of fault current it can detect, including smooth DC. A time-delay RCD, often called an S-type RCD, is chosen because of how it trips - more slowly, so downstream devices can clear the fault first.
For related protection options, browse our Type B RCDs, 100mA Time Delay RCDs, Three Phase and RCDs ranges.
Quick Answer: A Type B RCD is used where the circuit may produce smooth DC residual current, such as EV chargers, solar inverters, battery storage, heat pumps and drives. A time-delay (S-type) RCD is used upstream where you need selectivity so a downstream device trips first. They are not alternatives to each other. One is about fault-current type. The other is about trip timing.
| Feature | Type B RCD | Time Delay (S-Type) RCD |
|---|---|---|
| Main job | Detects AC, pulsating DC and smooth DC residual current | Trips more slowly for upstream selectivity |
| Used where | Inverter-driven / DC-sensitive circuits | Upstream positions with downstream RCD protection |
| Additional protection? | Often yes, depending on rating and setup | No – not used as a substitute for 30mA additional protection |
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The Quick Answer
- Type B RCD = chosen for the kind of fault current it can detect.
- Time-delay RCD = chosen for the way it trips, more slowly, to allow selectivity.
- Type B is relevant for inverter-driven and DC-sensitive circuits.
- S-type / time-delay is relevant for upstream positions where a downstream device should trip first.
- They solve different problems and should not be treated as interchangeable.
Related reads: What Is a Time Delay RCD? · What Is a Type B RCD? · Type B RCD Sensitivity: 30mA vs 100mA vs 300mA
What a Type B RCD Actually Does
A Type B RCD is all about detection capability. It is designed for circuits where the fault-current waveform can include smooth DC residual current as well as AC and pulsating DC.
That is why Type B is used on circuits involving:
- EV chargers
- Solar inverters
- Battery storage systems
- Heat pumps
- Variable speed drives
- Three-phase converter-based equipment
So when someone asks whether Type B is “better” than another RCD, the real answer is that it is better suited where DC-sensitive fault conditions may be present.
Installer’s Pick: For inverter-driven circuits that need standalone DC-sensitive protection, our WEV240B-030 2 Pole Type B RCD and WEV463B-030 4 Pole Type B RCD are built for exactly that job.
What a Time Delay RCD Actually Does
A time-delay RCD, often called an S-type RCD, is not about detecting a special fault-current waveform. It is about delaying operation so that a downstream protective device has the chance to trip first.
That matters in installations where you have RCDs in series and need discrimination or selectivity. The point is to stop the whole installation dropping out when the fault is really only on one final circuit.
So if Type B is about “what kind of fault current can this RCD handle?”, time delay is about “when should this RCD trip compared with the one below it?”
The Main Difference Between Type B and Time Delay RCDs
The easiest way to think about it is this:
- Type B answers the question: what waveform can this RCD detect?
- Time-delay / S-type answers the question: how quickly should this RCD operate?
That means you should never treat them as like-for-like alternatives. A time-delay device does not replace Type B on a DC-sensitive circuit. And a standard Type B device does not automatically solve an upstream selectivity requirement just because it is more specialist.
| Question | Type B RCD | Time Delay RCD |
|---|---|---|
| What problem does it solve? | DC-sensitive / inverter-driven fault-current detection | Upstream discrimination / selectivity |
| Can it replace the other? | No | No |
When You Need a Type B RCD
You are usually into Type B territory where the connected equipment can produce smooth DC residual current or where the manufacturer’s instructions specifically call for Type B protection.
- EV charging
- Solar PV
- Battery storage
- Heat pumps
- VFDs and drives
- Three-phase converter-based equipment
On those circuits, a time-delay RCD does not solve the real issue if the device still cannot handle the expected fault-current profile properly.
When You Need a Time Delay RCD Instead
You are looking at a time-delay RCD where the real problem is selectivity, not waveform detection.
- Upstream RCD protection feeding downstream 30mA devices
- Installations where the final circuit should trip before the whole board drops out
- Distribution arrangements needing clearer discrimination between devices
That is why time-delay RCDs often appear at upstream positions, while 30mA devices sit downstream on the final circuits.
Regulation Reminder: S-type / time-delay RCDs are not used for additional protection. Their role is upstream selectivity. If a final circuit needs 30mA additional protection or DC-sensitive detection, that requirement still has to be met by the correct downstream device.
Common Mistakes People Make With This Comparison
- Thinking time-delay means “better” – it just means slower operation for a specific upstream purpose.
- Using a time-delay RCD on a circuit that really needs Type B detection – this does not fix the waveform issue.
- Using Type B where the real problem is selectivity – that does not automatically solve series coordination.
- Mixing up “Type B” with “time delayed” – they describe different characteristics entirely.
- Assuming upstream selectivity devices can replace final-circuit additional protection – they cannot.
Products & Related Ranges
- Type B RCDs: WEV240B-030 40A 30mA · WEV263B-030 63A 30mA · WEV463B-030 4 Pole
- Time Delay RCD Range: 100mA Time Delay RCDs
- Browse collections: Type B RCDs · Time Delay RCDs · Three Phase · RCDs
FAQs
Is a Type B RCD the same as a time-delay RCD?
No. A Type B RCD is about what it can detect. A time-delay RCD is about how it trips.
Can I use a time-delay RCD instead of a Type B?
No. If the circuit needs DC-sensitive detection, a time-delay RCD does not replace Type B protection.
Can I use a Type B RCD instead of an S-type RCD?
Not automatically. If the real requirement is upstream selectivity, a standard Type B device does not automatically replace a time-delay arrangement.
Are time-delay RCDs suitable for additional protection?
No. S-type / time-delay RCDs are not used as a substitute for the final-circuit additional protection requirement.
When does Type B matter most?
On circuits involving EV chargers, solar PV, battery storage, heat pumps, drives and other inverter-driven equipment where smooth DC residual current may be present.
Final Word
Type B and time-delay RCDs are not competing options. They solve different protection problems.
If the circuit is DC-sensitive or inverter-driven, you are thinking about Type B. If the issue is upstream discrimination and letting the downstream device trip first, you are thinking about time delay / S-type. Get those two ideas mixed up, and it becomes very easy to spec the wrong device.
👉 Ready to spec? Compare our Type B RCD range, browse time-delay RCDs, or explore our wider RCD range to match the right device to the job.