B Curve vs C Curve vs D Curve MCBs – What's the Difference?

When looking at a Miniature Circuit Breaker (MCB), you will commonly see a letter before its current rating.

Examples include:

B32

C20

D16

The letter identifies the MCB's instantaneous tripping characteristic, commonly referred to as its trip curve.

The most common classifications are Type B, Type C and Type D.

Although two MCBs may have the same amp rating, different trip curves mean they can behave differently when subjected to high currents.

What Is a Type B MCB?

A Type B MCB is designed to operate instantaneously at approximately 3 to 5 times its rated current.

For example, a B32 MCB has a 32A rated current, while its instantaneous magnetic operating range is based around 3–5 times that value.

Type B MCBs are widely encountered in domestic and light commercial installations where high starting currents are not normally expected.

Common examples include circuits supplying general lighting and socket loads, although the correct protective device must always be determined from the actual circuit design.

What Is a Type C MCB?

A Type C MCB has an instantaneous operating range of approximately 5 to 10 times its rated current.

This allows it to tolerate higher short-duration inrush currents before the magnetic element operates.

Type C devices are therefore commonly encountered on circuits supplying equipment that can produce higher starting currents.

Examples may include certain:

  • Motors
  • Transformers
  • Fluorescent lighting installations
  • Commercial equipment
  • Inductive loads

This does not mean a Type C MCB can automatically replace a Type B device.

The circuit's fault current and required disconnection times need to be considered.

What Is a Type D MCB?

A Type D MCB typically has an instantaneous operating range of approximately 10 to 20 times its rated current.

It is intended for circuits where particularly high inrush currents may occur.

Applications can include certain:

  • Large motors
  • Transformers
  • Industrial equipment
  • Equipment with significant starting currents

Type D MCBs are therefore less commonly encountered in ordinary domestic consumer units.

B Curve vs C Curve vs D Curve

The basic difference can be summarised as:

Type B: Instantaneous operation at approximately 3–5 × rated current

Type C: Instantaneous operation at approximately 5–10 × rated current

Type D: Instantaneous operation at approximately 10–20 × rated current

This does not mean that one type provides better protection than another.

Each characteristic is designed for different circuit conditions.

Is a B32 the Same as a C32?

No.

Both devices have a rated current of 32A, but their instantaneous magnetic tripping characteristics are different.

A:

B32 = 32A Type B MCB

while a:

C32 = 32A Type C MCB

They should not be treated as interchangeable simply because they have the same current rating and physically fit the same consumer unit or distribution board.

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

Not without determining that the circuit is suitable.

Changing from Type B to Type C changes the current required for instantaneous magnetic operation.

This can affect whether the circuit achieves the required fault disconnection characteristics.

The circuit therefore needs to be assessed before changing the MCB characteristic.

Changing from B to C simply because an existing B-curve MCB keeps tripping is not an appropriate way to resolve a fault or overload.

Why Would an MCB Trip When Equipment Starts?

Some electrical equipment draws considerably more current during startup than during normal operation.

This is known as inrush or starting current.

If the inrush current is sufficiently high, it may cause an MCB to operate.

However, repeated tripping should be investigated rather than automatically changing the MCB type or rating.

The cause could include:

  • Excessive load
  • Equipment fault
  • Incorrect protective-device selection
  • Circuit fault
  • High starting current

The circuit and connected equipment should be assessed by a competent person.

How Do I Know Which MCB Curve I Have?

Look at the marking immediately before the current rating.

For example:

B6 = Type B, 6A

B16 = Type B, 16A

B32 = Type B, 32A

C16 = Type C, 16A

C32 = Type C, 32A

D16 = Type D, 16A

You should also record the manufacturer and full part number when looking for a replacement.

Finding the Correct Replacement MCB

Matching the trip curve and amp rating is only part of identifying a suitable replacement.

You should also check:

  • Manufacturer
  • Product range
  • Model or part number
  • Breaking capacity
  • Number of poles
  • Consumer unit or distribution board compatibility

A Wylex B32, Hager B32 and Crabtree B32 should not automatically be considered interchangeable simply because they share the same rating.

ReCircuit Resale stocks a range of new, surplus and discontinued Type B and Type C MCBs from major electrical manufacturers, including components for older consumer units and distribution boards.

When searching for an older replacement, the exact manufacturer and part number are normally the best place to start.

Electrical work within consumer units and distribution boards should be carried out by a competent person.

Next Guide: How to Identify the Correct Replacement MCB