B curve RCBOs are one of the most common protective devices used across domestic and light commercial circuits. They trip reliably on standard overloads and short circuits, making them ideal for lighting, socket circuits, small power loads and most general-purpose wiring in UK homes.
Quick Answer: B curve RCBOs are designed for circuits with low inrush current - typically lighting, sockets, immersion heaters and small appliances. They trip between 3–5× the rated current, making them perfect for standard domestic loads that don’t generate a sharp startup surge.
You can browse all available models here: B Curve RCBOs.
This guide breaks down what a B curve RCBO is, where it’s used, how it compares with C curve devices, and when installers choose one over the other.
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What Is a B Curve RCBO?
A B curve RCBO is a combined overcurrent and earth leakage protective device that trips at 3–5× its rated current. This makes it ideal for the vast majority of domestic circuits that have low or normal startup current.
RCBO benefits include:
- Combined overload + earth leakage protection on a single device
- Better fault isolation than RCD + MCB setups
- Cleaner board layouts for modern Amendment 2 installs
- Improved selectivity compared with shared RCDs
All B curve variants can be found in the dedicated range: B Curve RCBOs.
Where Are B Curve RCBOs Used?
Most domestic and small commercial circuits fall neatly into B curve territory. They’re designed for loads that don’t produce high inrush current.
Common examples include:
- Lighting circuits (LED, halogen, mixed loads)
- Socket circuits / ring mains
- Immersion heaters
- Small domestic appliances
- Bathroom fans and extractor systems
- General 13A radial circuits
B curve devices remain the “default” choice unless a load clearly demands a different curve.
B Curve vs C Curve RCBOs - What’s the Difference?
The key difference is how much inrush current each device expects before tripping.
| Curve | Trips At | Typical Loads |
|---|---|---|
| B Curve | 3–5 × rated current | Lighting, sockets, immersion, small appliances |
| C Curve | 5–10 × rated current | Motors, compressors, EV chargers, inrush loads |
If a device has a motor, transformer, inverter or high inrush, it usually needs a C curve or even a D curve - never a B.
When Do Installers Choose B Curve RCBOs?
Most sparkies choose a B curve when:
- The circuit supplies standard domestic loads
- No motor or heavy electronics are involved
- You want clean RCBO-based protection under Amendment 2
- The board is being upgraded and existing circuits are low-inrush
- You’re wiring new-build or rental properties
Compliance note: Most modern RCBO installations require Type A sensitivity under the 18th Edition. Always check the device’s earth leakage type as well as its curve.
View all B curve options here: B Curve RCBOs
FAQs
Here are the most common questions installers ask about B curve RCBOs.
Can I use a B curve RCBO on a circuit with a motor?
Usually no. Motors create inrush that can trip a B curve device. A C curve is normally more suitable.
Are B curve RCBOs compliant under the 18th Edition?
Yes - as long as the device is Type A (or higher) for most modern circuits.
Can I mix B and C curves in the same board?
Yes, as long as the curves match the load characteristics of each circuit.
Is a B curve suitable for kitchen appliances?
Generally yes, unless the appliance has a motor or heavy inductive load that causes high startup current.
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