B Curve vs C Curve RCBOs – What's the Difference?

When looking at an RCBO, MCB or other circuit breaker, you will often see a letter followed by a number, such as B16, B32, C16 or C32.

The number represents the device's rated current, while the letter identifies its tripping characteristic, commonly referred to as the trip curve.

B Curve and C Curve devices are both widely used, but they are designed to respond differently to higher levels of instantaneous current.

This guide explains the difference and what you should check when identifying an existing RCBO.

What Does B32 Mean on an RCBO?

The marking can be split into two parts:

B = Type B tripping characteristic

32 = 32A rated current

Therefore:

B32 = Type B curve, 32A

The same principle applies to other ratings.

For example:

B6 = B Curve, 6A
B10 = B Curve, 10A
B16 = B Curve, 16A
B20 = B Curve, 20A
B32 = B Curve, 32A
B40 = B Curve, 40A

The current rating and trip characteristic are separate specifications.

What Does C32 Mean?

A C32 device has:

C = Type C tripping characteristic

32 = 32A rated current

Therefore:

C32 = Type C curve, 32A

You may also encounter:

depending on the manufacturer's product range.

What Is a Trip Curve?

Circuit breakers need to disconnect a circuit when certain overcurrent conditions occur.

However, electrical equipment can briefly draw current above its normal operating level without there necessarily being a fault.

The trip characteristic determines how the circuit breaker responds to these higher currents.

The characteristic is particularly relevant to the device's instantaneous magnetic operation.

What Is a B Curve RCBO?

A Type B circuit breaker is designed to operate instantaneously when the current reaches approximately 3 to 5 times its rated current, subject to the applicable product standard and operating conditions.

For example, the magnetic operating range of a B32 device relates to multiples of its 32A rated current.

B Curve protective devices are commonly encountered in domestic electrical installations.

Typical applications can include circuits supplying loads with relatively low starting or inrush currents.

However, the correct protective device must always be determined from the characteristics of the circuit rather than simply choosing B Curve because the installation is domestic.

What Is a C Curve RCBO?

A Type C circuit breaker has a higher instantaneous magnetic operating range than a Type B device.

Type C devices typically operate instantaneously at approximately 5 to 10 times their rated current, subject to the applicable standard and operating conditions.

This can make C Curve devices suitable for certain loads that experience higher starting or inrush currents during normal operation.

Examples can include some:

Motors
Pumps
Transformers
Fluorescent lighting installations
Other equipment with higher inrush characteristics

This does not mean that a C Curve device is automatically suitable whenever equipment has a high starting current. Circuit characteristics and fault protection requirements still need to be considered.

B Curve vs C Curve – The Main Difference

The fundamental difference is the instantaneous magnetic operating characteristic.

B Curve: approximately 3–5 × rated current

C Curve: approximately 5–10 × rated current

Therefore, a C Curve device generally tolerates a higher short-duration inrush current before instantaneous magnetic operation occurs.

This does not mean that a C Curve RCBO is simply a stronger or better version of a B Curve RCBO.

They have different characteristics intended for different circuit conditions.

Is a C32 the Same as a B32?

No.

Both devices have a 32A rated current, but they have different instantaneous tripping characteristics.

B32: 32A, Type B characteristic

C32: 32A, Type C characteristic

They should therefore not automatically be treated as interchangeable.

If an existing circuit is protected by a B32 device, you should not simply replace it with a C32 because both devices have 32 printed on the front.

Can I Replace a B Curve RCBO With a C Curve RCBO?

Not simply because the current rating is the same.

Changing the trip characteristic can affect the protection provided to the circuit.

Factors that may need to be considered include:

Circuit design
Cable size
Load characteristics
Prospective fault current
Earth fault loop impedance
Required disconnection times
Manufacturer requirements
Applicable wiring regulations

Changing from B Curve to C Curve should therefore be based on proper electrical design and verification.

Is B Curve the Same as Type B RCD?

No.

This is an important distinction.

B Curve refers to the overcurrent tripping characteristic of a circuit breaker.

Type B RCD refers to the types of residual current waveform an RCD is designed to detect.

They are completely different classifications.

Similarly:

Type A RCD does not mean A Curve.

An RCBO can combine specifications such as:

B32 30mA Type A

In this example:

B = overcurrent trip curve

32A = rated current

30mA = rated residual operating current

Type A = residual-current detection classification

For more information, see our Type A vs Type AC RCDs & RCBOs guide.

What Does 30mA Mean on a B32 RCBO?

30mA refers to the RCBO's residual current protection.

For example:

B32 30mA Type A

does not mean the breaker operates at 30mA of normal circuit current.

It means the device combines:

32A rated overcurrent protection
B Curve characteristic
30mA residual current protection
Type A residual-current characteristics

This is why all of the markings need to be considered when identifying an RCBO.

Which Is Better – B Curve or C Curve?

Neither is universally better.

The correct trip characteristic depends on the circuit and the equipment being supplied.

B Curve is commonly encountered on domestic circuits, while C Curve may be selected for suitable circuits where higher inrush currents are expected.

A C Curve device should not be selected simply to prevent an existing B Curve breaker from tripping.

Repeated tripping can indicate an electrical fault, overload or another issue that should be properly investigated.

Why Does the Manufacturer Matter?

Two RCBOs can both be marked:

B32 30mA Type A

and still not be interchangeable.

Manufacturers use different:

Physical dimensions
Busbar arrangements
Terminal positions
Neutral connections
Product ranges
Consumer unit systems

Even devices from the same manufacturer can belong to different generations that are not necessarily interchangeable.

This is why you should identify the manufacturer, product range and MPN as well as the electrical ratings.

How Do I Identify My Existing RCBO?

Before buying a replacement, check the markings on the existing device.

Record:

Manufacturer
Product range
Manufacturer part number
Current rating
Trip curve
Residual current rating
RCD type
Breaking capacity
Number of poles
Consumer unit model or range

For example:

Crabtree Loadstar
6FSR20/30C
20A
C Curve
30mA
Type A

Providing the exact model number is considerably more useful than simply asking for a "20A RCBO".

Don't Choose an RCBO Based Only on B32 or C32

One of the most important points when looking for a replacement is that B32 is not a model number.

There can be many different B32 RCBOs produced by different manufacturers and across different product generations.

If you need to replace an existing device, try to identify its exact manufacturer part number.

This is particularly important for older or discontinued consumer units.

Buying B Curve & C Curve RCBOs

ReCircuit Resale stocks a changing selection of new, new-other and used RCBOs from established manufacturers including Wylex, Hager, Crabtree and Eaton.

Our stock includes a range of ratings and characteristics including B Curve and C Curve devices, as well as older and discontinued models that can sometimes be difficult to source.

Where possible, our listings clearly state:

Manufacturer
Product range
MPN
Current rating
Trip curve
Residual current rating
RCD type
Condition

If you are trying to replace an existing RCBO, send us clear photographs of the device and its markings and we can help identify the model and point you towards relevant stock currently available.

Important: RCBOs are electrical safety devices. The correct current rating and trip characteristic must be determined for the individual circuit. Never change the rating or trip characteristic simply to prevent a protective device from operating. Consumer unit work should be carried out by a suitably competent person.