Once you know you need a Type B RCD, the next question is usually not whether it should be Type A or Type B. It is which sensitivity is actually right for the job.
That is where plenty of buyers get stuck. A 30mA Type B RCD is commonly used where additional protection for people is required on the final circuit, while 100mA and 300mA devices are more often used upstream for selectivity and fire-risk reduction. Bigger is not automatically better here. It depends on where the device sits in the protection strategy.
For wider product options, browse our Type B RCDs, 3-Phase RCDs, Heat Pump Boards and Three Phase ranges.
Quick Answer: As a rule of thumb, use a 30mA Type B RCD where the circuit needs additional personal protection, such as a final circuit feeding EV charging or inverter-driven equipment. Use 100mA or 300mA Type B devices more carefully for upstream discrimination, distribution-level protection or fire-risk reduction where the design calls for it. They do not replace a 30mA final-circuit device for personal protection.
| Sensitivity | Main Purpose | Typical Use Case |
|---|---|---|
| 30mA | Additional protection / personal protection | Final circuits for EV chargers, heat pumps, inverter-driven equipment |
| 100mA | Upstream selectivity / discrimination | Submains or upstream protection where coordinated with downstream 30mA devices |
| 300mA | Fire-risk reduction / upstream protection | Main incoming or distribution-level protection where design requires it |
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The Quick Answer
- 30mA Type B RCD = usually the right choice where the final circuit needs additional personal protection.
- 100mA Type B RCD = more often used upstream where discrimination with downstream 30mA devices is needed.
- 300mA Type B RCD = generally an upstream / fire-risk reduction choice, not a substitute for 30mA final-circuit protection.
- Always match the sensitivity to the position in the installation, not just the equipment type.
Related reads: What Is a Type B RCD? · Type A vs Type B RCD · What Is a Time Delay RCD?
What Does RCD Sensitivity Actually Mean?
The sensitivity rating on an RCD tells you the residual current level at which the device is designed to trip. On Type B devices, that sensitivity still sits alongside the important point that the unit can detect AC, pulsating DC and smooth DC residual current.
So when you compare 30mA, 100mA and 300mA, you are not comparing different “types” of Type B. You are comparing trip thresholds and therefore different jobs within the protection scheme.
That is why the same buyer can correctly need a 30mA Type B on one circuit and a 100mA or 300mA device upstream elsewhere in the same installation.
When a 30mA Type B RCD Is the Right Choice
A 30mA Type B RCD is the go-to choice where you need additional protection for people on the final circuit and the connected equipment can produce DC-sensitive fault conditions.
- EV charging circuits where standalone Type B protection is required
- Heat pump circuits where the equipment spec or protection design points to Type B
- Single-phase inverter-driven loads where smooth DC fault current is a concern
- Three-phase final circuits where Type B sensitivity is needed at the outgoing way
This is why most of the product demand around Type B RCDs tends to centre on 30mA variants. They are closest to the point of use and the point where additional protection is typically required.
Installer’s Pick: For standard single-phase Type B protection, our WEV240B-030 40A 30mA Type B RCD and WEV263B-030 63A 30mA Type B RCD are strong options for EV, inverter and specialist circuits.
When a 100mA Type B RCD Makes More Sense
A 100mA Type B RCD is usually more relevant where the device is being used upstream and the aim is not direct personal protection on the final circuit, but selectivity or coordination with downstream devices.
That might include:
- Sub-distribution arrangements
- Installations with multiple downstream 30mA devices
- Systems where you need a cleaner discrimination strategy
- Layouts where time delay / selectivity are part of the wider design approach
The important point is that a 100mA device is not just a “stronger” version of 30mA. It is usually doing a different job in the board design.
When a 300mA Type B RCD Is Used
A 300mA Type B RCD is generally the kind of device you see higher up the installation where the design intent is more about fire-risk reduction or main incoming / distribution-level protection than additional protection on the final circuit.
- Main incoming protection on specialist distribution setups
- Industrial or commercial boards
- Upstream arrangements where the final circuits are already separately protected
- Installations needing a more strategic discrimination layout
For most installers looking at a single EV charger, heat pump or similar final circuit, 300mA is usually not the first answer. It is more likely to be part of a broader board-level strategy.
Regulation Reminder: Time-delayed and higher-sensitivity upstream devices have their place for selectivity, but they are not a substitute for the correct 30mA additional protection where that is required on the final circuit.
Common Type B RCD Sensitivity Setups
These are the kinds of real-world patterns buyers usually work through:
| Scenario | Likely Direction | Why |
|---|---|---|
| Single EV charger final circuit | 30mA Type B | Additional protection at final circuit level |
| Heat pump final circuit | Usually 30mA Type B where Type B is required | Final-circuit equipment protection |
| Upstream protection feeding multiple outgoing ways | 100mA Type B | Better coordination with downstream 30mA devices |
| Main incoming / distribution-level protection | 300mA Type B | Fire-risk reduction / upstream role |
For many Power & Data buyers, the sweet spot remains the 30mA device, because that is what matches the final circuit where the job is actually being quoted.
Common Mistakes When Choosing Type B RCD Sensitivity
- Assuming 100mA or 300mA is “better” than 30mA – it usually means a different protection role, not better protection.
- Using an upstream device as a substitute for final-circuit protection – that is not what it is there for.
- Ignoring selectivity – the whole point of higher sensitivities upstream is often to coordinate properly with downstream devices.
- Forgetting the circuit position – the same equipment can need different sensitivity choices depending on whether you are protecting the outgoing circuit or the incoming distribution point.
- Only checking the current rating – sensitivity and device type both matter.
Products & Related Ranges
- 2-Pole 30mA Type B RCDs: WEV240B-030 · WEV263B-030
- 4-Pole 30mA Type B RCDs: WEV463B-030 · WEV4100B-030
- Heat Pump Type B Protection: WHP240-030 · 8 Way Heat Pump Board · 12 Way Heat Pump Board
- Browse collections: Type B RCDs · Heat Pump Boards · Three Phase · Consumer Units
FAQs
Is 30mA always the right Type B RCD sensitivity?
Not always, but it is usually the correct choice where the final circuit needs additional personal protection. Higher sensitivities are more often used upstream.
Can I use a 100mA Type B RCD instead of 30mA?
Not where the circuit requires 30mA additional protection. A 100mA device is usually serving a different role in the installation.
What is a 300mA Type B RCD mainly for?
Usually upstream protection, distribution-level coordination or fire-risk reduction, rather than personal protection at the final circuit.
Do EV chargers usually need 30mA or 100mA Type B protection?
Where a standalone Type B is required on the final EV circuit, 30mA is the usual direction. The exact setup still depends on the charger spec and the wider design.
Does sensitivity change the fact that it is still a Type B RCD?
No. The device is still Type B because of the fault-current waveforms it can detect. The sensitivity just changes the trip threshold and therefore its role in the protection scheme.
Final Word
If the Type B RCD is protecting a final circuit and additional protection is required, 30mA is usually the answer. If the device is sitting upstream and the design is about discrimination or fire-risk reduction, 100mA or 300mA may be the better fit.
The mistake is treating them like interchangeable versions of the same thing. They are not. The right sensitivity depends on where the device sits in the installation and what job it is actually doing.
👉 Ready to spec? Compare our Type B RCD range, choose a 30mA 2-pole Type B RCD, or explore three-phase Type B protection for more complex installs.