B Curve RCBOs Explained: Uses, Ratings & B vs C Curve
B curve RCBOs are widely used for domestic and light-commercial final circuits where loads have relatively low starting current. They combine residual-current protection with overload and short-circuit protection in one device, allowing each circuit to be protected independently.
The most important thing to understand is that “B curve” and “Type B” do not mean the same thing. B curve describes the RCBO’s overcurrent trip characteristic. Type A, Type F and Type B describe its residual-current characteristics.
Quick answer: A B curve RCBO has an instantaneous magnetic trip characteristic of approximately 3–5 times its rated current. It is commonly used for circuits with low or moderate inrush such as general sockets, heating and many lighting circuits.
Do not confuse B curve with Type B. You can have a Type A B-curve RCBO: Type A describes its residual-current protection, while B curve describes its overcurrent response.
👉 Compare our B Curve RCBOs, or browse the wider RCBO range.
| Term | What It Describes | Example |
|---|---|---|
| B Curve | Overcurrent / magnetic trip characteristic | B16, B20, B32 RCBO |
| Type A | Residual-current detection characteristic | Type A 30mA B32 RCBO |
| Type B | Broader residual-current detection including smooth DC | Specialist inverter / EV / drive protection where required |
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What Is a B Curve RCBO?
An RCBO combines two types of protection in one device:
- Residual-current protection against earth leakage faults
- Overcurrent protection against overloads and short circuits
The letter B in “B curve” describes the instantaneous overcurrent characteristic of the circuit-breaker element.
A B curve device is designed to operate magnetically at around 3–5 times its rated current.
For example, a B32 RCBO has:
- 32A nominal current rating
- B curve overcurrent characteristic
- A separate residual-current characteristic such as Type A
- A residual operating current commonly specified separately, for example 30mA
This is why the full device specification matters more than simply calling something a “Type B RCBO”.
B Curve RCBO vs Type B RCBO: What’s the Difference?
This is one of the most common RCBO terminology mix-ups.
B curve and Type B describe completely different characteristics.
B Curve
B curve relates to the overcurrent protection part of the RCBO.
It tells you how the device responds to high current caused by faults or starting current.
Type B
Type B relates to the residual-current protection part of an RCD or RCBO.
It is designed to detect a wider range of residual currents, including smooth DC, and is used for particular applications involving power electronics where that protection is required.
Easy way to remember it:
B curve = overcurrent behaviour.
Type B = residual-current behaviour.
They are independent specifications.
A common domestic RCBO could therefore be described as:
32A Type A B Curve 30mA RCBO
Each part tells you something different:
- 32A = rated current
- Type A = residual-current characteristic
- B curve = overcurrent trip characteristic
- 30mA = rated residual operating current
How Does a B Curve RCBO Work?
An RCBO has separate mechanisms for residual-current faults and overcurrent faults.
Overload protection
If a circuit carries excessive current for a sustained period, the thermal element operates.
Short-circuit protection
If current rises sharply during a fault, the magnetic part of the device operates rapidly.
For a B curve circuit breaker characteristic, the instantaneous magnetic operating range is approximately 3–5 times the rated current.
Residual-current protection
The RCBO also monitors the balance of current flowing through the circuit conductors. Where residual current exceeds the device’s operating threshold, it disconnects the circuit.
This combination means one RCBO can provide:
- Overload protection
- Short-circuit protection
- Residual-current protection
- Individual circuit disconnection
Where Are B Curve RCBOs Used?
B curve devices are commonly used on circuits with relatively low or predictable starting current.
Typical applications can include:
- General socket circuits
- Ring final circuits
- Radial socket circuits
- Resistive heating
- Immersion heaters
- General domestic appliances
- Many lighting circuits
- Controls and small auxiliary circuits
However, the load type alone does not automatically determine the curve.
Modern equipment can have very different inrush characteristics even when two circuits appear similar.
For example, a large LED lighting installation may have significantly more startup current than a small conventional lighting circuit.
The protective device should therefore be selected from:
- Actual design current
- Starting / inrush current
- Cable characteristics
- Required disconnection time
- Earth fault loop impedance
- Manufacturer instructions
👉 Browse B Curve RCBOs.
B Curve vs C Curve RCBO: What’s the Difference?
The main difference is the instantaneous magnetic trip characteristic.
| Curve | Instantaneous Range | Typical Direction |
|---|---|---|
| B Curve | Approximately 3–5 × rated current | Low to moderate inrush circuits |
| C Curve | Approximately 5–10 × rated current | Higher inrush loads where circuit conditions permit |
A C curve RCBO tolerates a larger short-duration starting current before entering its instantaneous magnetic trip region.
That can make C curve appropriate for some:
- Motors
- Pumps
- Compressors
- Transformers
- HVAC equipment
- High-inrush electronic loads
But this does not mean every motor, heat pump or inverter automatically requires C curve.
Likewise, nuisance tripping is not by itself enough reason to replace B curve with C curve.
The circuit still needs to meet the required fault-protection and disconnection conditions.
What Type of RCBO Is Used for Domestic Circuits?
There is no single RCBO specification that is correct for every domestic circuit.
For many conventional circuits, a B curve overcurrent characteristic is common because the connected load has relatively low starting current.
However, installers still need to select:
- Current rating
- B, C or another appropriate overcurrent curve
- RCD type
- Residual operating current
- Breaking capacity
- Number of poles
- Device format
- Consumer-unit compatibility
A common modern domestic specification might therefore be a Type A, B curve, 30mA RCBO in an appropriate current rating.
That does not mean every domestic RCBO must use that combination.
What Is a Type A B Curve RCBO?
A Type A B curve RCBO combines:
- A Type A residual-current characteristic
- A B curve overcurrent characteristic
Type A devices respond to AC residual current and pulsating DC residual current within their specified operating characteristics.
This makes Type A RCBOs suitable for many circuits supplying modern electronic equipment where Type AC would not be appropriate.
But again, the two specifications remain independent.
You could also have:
- A Type A C curve RCBO
- A Type A B curve RCBO
- A Type B RCBO with its own overcurrent characteristic
This distinction is essential when reading manufacturer datasheets or ordering protective devices.
👉 Compare Type A RCBOs.
What Do B16, B20 and B32 RCBO Mean?
The letter and number give two different pieces of information.
- B = B curve overcurrent characteristic
- 16, 20 or 32 = nominal current rating in amps
For example:
| Marking | Meaning |
|---|---|
| B6 | 6A B curve device |
| B16 | 16A B curve device |
| B20 | 20A B curve device |
| B32 | 32A B curve device |
The residual-current characteristic and sensitivity will be marked separately.
So a device marked B32 Type A 30mA is not a “Type B RCBO”. It is a Type A RCBO with a B curve 32A overcurrent characteristic and 30mA residual-current sensitivity.
Why Zs Matters When Choosing B or C Curve RCBOs
Curve selection is not purely about stopping unwanted trips.
The protective device also needs to disconnect the supply within the required time when a fault occurs.
Because B and C curve devices have different instantaneous operating characteristics, they also have different maximum earth fault loop impedance requirements.
This means changing from B curve to C curve can alter whether the circuit satisfies its required automatic-disconnection conditions.
Before changing curve, check:
- Measured or calculated Zs
- Required disconnection time
- Manufacturer’s device data
- Cable length and impedance
- Earthing arrangement
- Protective-device rating
Installer point: Do not change a B curve RCBO to C curve simply to cure nuisance tripping without first checking that the circuit still meets the required fault-protection and disconnection conditions.
RCBO vs RCD: What’s the Difference?
An RCBO and an RCD are related but do not provide exactly the same functions.
| Device | Residual-Current Protection | Overcurrent Protection |
|---|---|---|
| RCCB / RCD | Yes | No |
| RCBO | Yes | Yes |
That is why terms such as “B curve RCD” can be misleading. B and C overcurrent curves normally relate to circuit-breaker characteristics rather than a standalone RCCB.
How to Choose the Right RCBO
Do not choose an RCBO from the curve alone.
- Calculate the design current.
- Select the appropriate current rating.
- Check the load’s starting or inrush current.
- Select the appropriate B, C or other overcurrent characteristic.
- Select the correct RCD type for the load.
- Confirm the required residual-current sensitivity.
- Check maximum Zs and required disconnection time.
- Confirm breaking capacity.
- Check pole / neutral arrangement.
- Confirm consumer-unit compatibility.
- Check whether the device is directional or bidirectional where relevant.
- Follow the equipment and protective-device manufacturer’s instructions.
RCBO shorthand: Think of the specification as separate layers:
Current rating + overcurrent curve + RCD type + residual-current sensitivity.
For example: 32A + B Curve + Type A + 30mA.
RCBO Ranges
- B Curve RCBOs
- C Curve RCBOs
- Type A RCBOs
- Mini Type A RCBOs
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- 10kA RCBOs
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For wider protection equipment, browse Circuit Protection.
B Curve RCBO FAQs
What is a B curve RCBO?
A B curve RCBO combines residual-current and overcurrent protection and has an instantaneous magnetic overcurrent characteristic of approximately 3–5 times its rated current.
What is a Type B RCBO?
Type B normally refers to the residual-current detection characteristic rather than the B overcurrent curve. A Type B residual-current device is designed for a wider range of residual currents including smooth DC.
Is a Type B RCBO the same as a B curve RCBO?
No. B curve describes overcurrent behaviour, while Type B describes residual-current behaviour. They are separate specifications.
What is a Type A B curve RCBO?
It is an RCBO with Type A residual-current protection and a B curve overcurrent characteristic. This is a common combination on modern final circuits.
What does B32 mean on an RCBO?
B means the device has a B curve overcurrent characteristic, while 32 means its nominal current rating is 32A.
Where are B curve RCBOs used?
They are commonly used on circuits with relatively low starting current, including many socket, heating, lighting and general domestic final circuits.
What is the difference between B curve and C curve RCBOs?
B curve devices have a lower instantaneous magnetic operating range than C curve devices. C curve can therefore tolerate greater short-duration inrush, provided the circuit still meets the required fault-protection conditions.
Can I replace a B curve RCBO with C curve?
Potentially, but not simply to prevent unwanted tripping. Check load characteristics, Zs, required disconnection time, cable protection and manufacturer requirements first.
Can you mix B and C curve RCBOs in the same consumer unit?
Yes, where each protective device is correctly selected for its circuit and is compatible with the consumer-unit assembly.
What type of RCBO is normally used in a house?
B curve devices are common on many domestic final circuits, but the correct specification depends on the particular load, circuit design, RCD type, current rating and fault conditions.
Are all B curve RCBOs Type A?
No. B curve describes the overcurrent characteristic and does not tell you the RCD type. Check the device marking and manufacturer specification separately.
Is B curve suitable for motors?
Sometimes. Small motors with acceptable starting current may work correctly with B curve protection. Larger or higher-inrush motors may require C or another curve where the complete circuit design permits it.
Is B curve suitable for LED lighting?
It can be, but large banks of LED drivers can create significant inrush. Check the lighting manufacturer’s data and circuit design rather than assuming every lighting circuit should use B curve.
Is B curve suitable for an EV charger?
Possibly, depending on the charger manufacturer’s overcurrent requirements and starting characteristics. The RCBO curve should not be confused with whether the charger requires Type A or Type B residual-current protection.
Does a B curve RCBO provide RCD protection?
Yes. An RCBO includes residual-current protection, but you still need to check whether that residual-current characteristic is Type A, Type F, Type B or another specified type.
What are the main RCBO curve types?
B and C curves are particularly common in domestic and light-commercial applications, with other characteristics used where the load and circuit design require them.
When Should You Use a B Curve RCBO?
A B curve RCBO is commonly used where a circuit has relatively low or moderate inrush current and its fault conditions allow the B curve device to achieve the required disconnection performance.
But do not choose the RCBO from the letter B alone.
Check:
- Current rating
- Overcurrent curve
- RCD type
- Residual-current sensitivity
- Breaking capacity
- Load inrush
- Maximum Zs
- Required disconnection time
- Pole configuration
- Consumer-unit compatibility
Most importantly, remember the terminology:
B curve is not the same as Type B.
A common device can quite legitimately be a Type A B curve RCBO because one term describes its residual-current protection and the other describes its overcurrent characteristic.
👉 Compare B Curve RCBOs, C Curve RCBOs, Type A RCBOs or browse the complete RCBO range.