B Curve vs C Curve RCBOs: When Should You Choose C Curve?

What Is the Difference Between B Curve and C Curve RCBOs?

The curve describes the instantaneous magnetic operation of the overcurrent element.

Both curves still contain a thermal element for overload protection. The B or C curve mainly affects how the device responds to higher short-duration overcurrents.

A C curve is not inherently stronger, safer or higher quality. It is simply designed for a different current characteristic.

👉 Compare B curve RCBOs and C curve RCBOs.

What Is a C Curve RCBO?

A C curve RCBO combines:

• Overload protection
• Short-circuit protection
• Residual-current protection
• A C curve instantaneous magnetic characteristic

The C curve characteristic means the device is less likely than an equivalent B curve device to operate magnetically during a short, legitimate startup current.

For example, a C32 device has:

• A rated current of 32 A.
• A C curve overcurrent characteristic.
• An instantaneous magnetic range associated with the C curve.
• A separate residual-current classification, such as Type A.
• A separate residual operating current, commonly 30 mA.

The description C32 Type A 30 mA RCBO therefore defines several different characteristics. None should be omitted when specifying the device.

When Should You Choose a C Curve RCBO?

A C curve may be considered where the equipment has a documented startup current that is too high for a suitable B curve device but remains within the intended operating characteristics of a C curve device.

Typical reasons include:

• The equipment manufacturer specifies a C curve protective device.
• The manufacturer provides inrush-current data supporting C curve selection.
• A correctly rated B curve device operates magnetically during normal startup.
• The load contains a motor, transformer, compressor or power supply with verified inrush.
• The circuit can achieve the required disconnection time with the C curve device.
• The prospective fault current is sufficient for the selected protective device.
• The cable remains correctly protected against overload and fault current.

All of these points should be considered together. Inrush alone does not override fault protection, cable capacity or manufacturer requirements.

What Loads May Need a C Curve RCBO?

C curve protection is commonly considered for equipment that produces a short but significant startup current.

Potential examples include:

• Motors and motor-driven machinery
• Pumps and compressors
• Transformers
• Some large LED driver groups
• Power supplies with high capacitor-charging current
• Welding equipment
• Some heat pumps and air-conditioning units
• Some EV charging equipment
• Workshop machinery and fixed power tools
• UPS and inverter systems

These are examples of loads that may justify investigation. They do not automatically require a C curve.

The starting characteristics can vary significantly between products performing the same function. One heat pump may require C curve protection while another specifies B curve. The same applies to EV chargers, air-conditioning units, LED drivers and inverters.

Do EV Chargers Need a B Curve or C Curve RCBO?

There is no universal curve requirement for all EV chargers.

The correct curve should be determined from:

• The charger manufacturer’s installation instructions.
• The stated maximum upstream protective-device rating.
• Any specified B, C or other trip characteristic.
• The charger’s startup and normal operating current.
• The cable and installation method.
• The circuit’s measured or calculated Zs.

Many EV chargers operate without requiring a C curve device. Others may specify or benefit from C curve protection because of their internal power supplies, contactors or startup characteristics.

Do not use C curve simply because the circuit supplies an EV charger.

The residual-current element must also be selected separately. Depending on the charger’s DC fault-current protection, this may involve a Type A, Type F or Type B RCD arrangement.

Related guide: Type A vs Type B RCDs and RCBOs for EV Chargers

👉 Browse EV RCBOs and confirm the required curve against the charger documentation.

Do Heat Pumps and Air-Conditioning Units Need C Curve RCBOs?

Heat pumps and air-conditioning units may contain compressors, motors, inverters, crankcase heaters and electronic power-conversion equipment.

However, inverter-driven equipment does not automatically require C curve protection. Soft-starting and controlled compressor operation may reduce the starting current compared with a conventional direct-on-line motor.

Check:

• The manufacturer’s maximum overcurrent protective-device rating.
• The required trip curve.
• Locked-rotor or starting-current data where applicable.
• Whether an upstream fuse, MCB or RCBO is specified.
• The required residual-current type.
• The fault-loop and cable-design conditions.

If the manufacturer specifies C curve, it should be treated as part of the equipment’s supply requirements unless the designer has documented an equivalent compliant arrangement.

If no curve is specified, the starting-current data and circuit design should be used rather than assuming C curve because the equipment contains a compressor.

Should LED Lighting Use B Curve or C Curve Protection?

Large groups of LED drivers can draw high inrush current as their internal capacitors charge at switch-on.

A C curve device may be appropriate where:

• The driver manufacturer provides high inrush-current data.
• The number of drivers connected exceeds the recommended limit for a B curve device.
• The switching arrangement can energise many drivers simultaneously.
• The C curve circuit still meets all fault-disconnection requirements.

Changing the curve is not the only possible solution. The designer may also consider:

• Dividing the load across additional circuits.
• Staged switching.
• Inrush-limiting equipment.
• Contactors selected for the relevant load type.
• Drivers with lower starting current.

For a small lighting circuit with limited inrush, a B curve device may remain entirely suitable.

Why Does Zs Matter More with a C Curve RCBO?

Automatic disconnection using the overcurrent element depends on sufficient fault current flowing through the earth-fault loop.

A C curve device requires a higher current than an equivalent B curve device to guarantee instantaneous magnetic operation.

Because fault current and loop impedance are inversely related, the permitted maximum Zs for a C curve device is generally lower than for a B curve device of the same rated current.

When selecting or substituting a C curve RCBO, verify:

• The maximum Zs from the manufacturer’s data or applicable BS 7671 table.
• The measured or calculated circuit Zs.
• Temperature correction where applicable.
• The required maximum disconnection time.
• The protective device’s actual product standard.
• Whether the RCD element is being relied upon for fault protection.

Manufacturer-specific data should be used wherever available because operating characteristics and maximum Zs values can differ between protective devices.

A circuit passing the B curve maximum Zs does not automatically pass when the device is changed to C curve.

Does the 30 mA RCD Element Make Zs Irrelevant?

No. The presence of a 30 mA residual-current element does not mean the rest of the protective-device selection can be ignored.

The designer must establish which device or function is being relied upon for automatic disconnection under the relevant fault condition.

Depending on the earthing and circuit design:

• The overcurrent element may be relied upon for fault protection.
• The RCD element may provide fault protection.
• The RCD element may provide additional protection only.
• Both functions may operate under different fault conditions.

Even where the RCD element provides the required earth-fault disconnection, the C curve overcurrent element must still provide appropriate overload and short-circuit protection.

Is a Type C RCBO the Same as a Type C RCD?

The phrase “Type C RCBO” is commonly used to mean a C curve RCBO. The C identifies the overcurrent characteristic.

This should not be confused with the residual-current type.

A device described as a C32 Type A 30 mA RCBO is therefore:

• C curve for overcurrent operation.
• Rated at 32 A.
• Type A for residual-current detection.
• Rated at 30 mA residual operating current.

There is no choice between “Type A” and “Type C” because they describe different functions. A device can be both Type A and C curve.

Can You Replace a B Curve RCBO with a C Curve RCBO?

Potentially, but it should not be treated as a direct like-for-like substitution.

Before changing the device:

• Identify why the B curve device is operating.
• Measure or obtain the load’s actual inrush current.
• Rule out overload, earth leakage and equipment faults.
• Confirm the manufacturer permits C curve protection.
• Check the maximum permitted Zs.
• Verify the circuit’s measured or calculated Zs.
• Confirm cable overload and short-circuit protection.
• Check breaking capacity and prospective fault current.
• Ensure the RCBO is compatible with the consumer unit.
• Confirm the residual-current type remains suitable.

If the device is tripping because of residual current rather than magnetic inrush, changing from B curve to C curve will not address the cause.

Similarly, if the circuit is overloaded, fitting a C curve RCBO with the same rated current does not remove the thermal overload condition.

Will a C Curve RCBO Stop Nuisance Tripping?

It may prevent unwanted magnetic operation caused by legitimate startup current. It will not solve every form of unwanted tripping.

Possible causes include:

• High startup or inrush current
• Sustained overload
• Earth leakage from connected equipment
• Accumulated leakage from several loads
• A defective appliance or circuit
• Loose connections
• Incorrect RCBO type
• Supply disturbances
• Neutral-to-earth faults

The trip mechanism should be identified before changing the curve.

If the RCBO operates due to its residual-current element, a C curve overcurrent characteristic will not make that element less sensitive.

Where cumulative leakage or transient disturbances are the issue, a correctly selected high-immunity RCBO may be more relevant than changing the overcurrent curve.

How Do You Choose Between B Curve and C Curve?

1. Identify the exact equipment. Obtain the manufacturer’s electrical requirements.
2. Check the design current. Select the rated current according to the load and cable.
3. Review inrush data. Establish the magnitude and duration of startup current.
4. Check the specified curve. Follow the manufacturer’s B, C or other requirement.
5. Assess the trip mechanism. Confirm whether any existing operation is magnetic, thermal or residual-current related.
6. Verify Zs. Confirm the selected curve can achieve the required disconnection time.
7. Check prospective fault current. Confirm breaking capacity and protective-device coordination.
8. Select the RCD type. Choose Type A, F, B or another characteristic for the equipment.
9. Check residual-current rating. Confirm whether 30 mA additional protection is required.
10. Confirm board compatibility. Use devices approved or declared suitable for the consumer unit or assembly.

Browse B Curve and C Curve RCBOs

• C Curve RCBOs
• B Curve RCBOs
• Type A RCBOs
• High-Immunity RCBOs
• EV RCBOs
• 32 A RCBOs
• All RCBOs

Check the full device specification before ordering, including current rating, B or C curve, residual-current type, sensitivity, breaking capacity and pole arrangement.

B Curve vs C Curve RCBO FAQs

What is a C curve RCBO?

It is an RCBO whose instantaneous magnetic overcurrent element normally operates within the C curve range. It tolerates a higher short-duration startup current than an equivalent B curve device.

What is the difference between B curve and C curve RCBOs?

B curve devices have a lower instantaneous magnetic operating range. C curve devices tolerate greater inrush but require more fault current for instantaneous operation.

Is C curve better than B curve?

No. Each curve suits different load and fault-current conditions. C curve is appropriate only where the equipment and circuit design justify its higher magnetic threshold.

Can I replace a B curve RCBO with C curve?

Only after confirming the load’s inrush, manufacturer requirements, circuit Zs, disconnection time, cable protection, fault current and board compatibility.

Do all EV chargers need a C curve RCBO?

No. The correct curve depends on the charger manufacturer’s requirements and startup characteristics. Many chargers can use B curve protection, while others specify C curve.

What curve RCBO should be used for a heat pump?

Use the curve specified by the heat-pump manufacturer. Do not assume C curve solely because the unit contains an inverter or compressor.

What curve RCBO should be used for air conditioning?

Check the manufacturer’s stated overcurrent protection and starting-current data. B or C curve may be appropriate depending on the exact unit.

Is C curve suitable for domestic installations?

Yes, where the connected equipment requires its characteristics and the circuit meets all applicable fault-disconnection and protection requirements. It should not be used as the default for every domestic circuit.

Can C curve be used for socket circuits?

Potentially, but only where the connected loads and circuit design justify it. A general socket circuit does not automatically require higher inrush tolerance.

Can C curve be used for lighting?

Yes, where large groups of lighting drivers produce verified inrush that would operate a B curve device and the C curve circuit meets its disconnection requirements.

Does C curve stop nuisance tripping?

Only where the unwanted operation is caused by legitimate current entering the B curve magnetic region. It will not correct overload, earth leakage or equipment faults.

Does C curve have a higher current rating?

Not necessarily. B32 and C32 devices are both rated at 32 A. The difference is their instantaneous magnetic operating characteristic.

Is a Type A RCBO different from a Type C RCBO?

Type A describes residual-current waveform detection. C curve describes overcurrent operation. A device can be both Type A and C curve.

Is a Type B RCBO the same as a B curve RCBO?

Not necessarily. “Type B” may refer to residual-current detection, while “B curve” refers to the overcurrent characteristic. Check the complete device marking.

Why is the maximum Zs lower for C curve?

A C curve device requires a higher current for guaranteed instantaneous magnetic operation. A lower loop impedance is therefore generally required to produce that fault current.

Does an RCBO’s RCD element remove the need to check Zs?

No. The designer must establish how fault protection is achieved and verify every relevant function of the protective device and circuit.

What does C32 mean?

C identifies the overcurrent curve and 32 identifies the rated current in amperes. It does not identify the residual-current type or sensitivity.

When Should You Switch from B Curve to C Curve?

Switch to C curve where a verified startup current makes B curve unsuitable and the complete circuit still satisfies the protection and disconnection requirements.

Do not switch curves solely because:

• The equipment is an EV charger.
• The load contains an inverter.
• The circuit supplies a heat pump or compressor.
• The existing device has tripped.
• C curve appears more tolerant.

Confirm:

• The manufacturer’s required curve.
• The actual inrush current.
• The device’s current rating.
• Maximum and measured Zs.
• Disconnection time.
• Prospective fault current and breaking capacity.
• Cable protection.
• Residual-current type and sensitivity.

👉 Ready to spec? Compare C curve RCBOs, B curve RCBOs and high-immunity RCBOs.