How to Identify the Correct Replacement MCB

If an MCB needs replacing, choosing another breaker with the same amp rating is not always enough.

Two MCBs may both be marked B32, for example, while belonging to completely different consumer unit ranges and having different specifications, dimensions or busbar connections.

The safest starting point is to identify the existing MCB as accurately as possible.

1. Identify the Manufacturer

Look for the manufacturer's name or logo on the front of the existing MCB.

Common UK manufacturers include:

  • Wylex
  • Hager
  • Crabtree
  • Schneider Electric
  • Square D
  • MEM / Eaton
  • MK
  • Contactum
  • Lewden
  • ABB

Identifying the brand narrows down the search considerably, but the manufacturer alone does not confirm compatibility.

Many manufacturers have produced several generations of MCBs.

2. Find the Model or Part Number

The model or part number is one of the most useful pieces of information when identifying a replacement.

It may be printed on:

  • Front of the MCB
  • Side of the device
  • Around the operating toggle
  • Product label

Part numbers can distinguish between MCBs that otherwise look very similar.

When searching online for an older replacement, try searching the complete part number exactly as printed.

3. Check the Amp Rating

The MCB's rated current will normally be prominently displayed.

Examples include:

6A

16A

20A

32A

40A

It may be combined with the trip characteristic, such as:

B32

or

C20

Do not increase the amp rating simply because a higher-rated MCB is available.

The protective-device rating forms part of the circuit design.

4. Check the Trip Curve

Look at the letter immediately before the current rating.

Common examples are:

B – Type B

C – Type C

D – Type D

A B32 and C32 are both rated at 32A, but they have different instantaneous operating characteristics.

The existing trip characteristic should therefore be identified when searching for a replacement.

5. Check the Breaking Capacity

The MCB should also carry information relating to its short-circuit breaking capacity.

Depending on the device, you may encounter ratings such as:

6kA

10kA

The required breaking capacity depends on the prospective fault current and characteristics of the installation.

Do not assume two MCBs are equivalent simply because their amp rating and trip curve match.

6. Identify the Consumer Unit Range

This is particularly important with older installations.

A manufacturer may have produced several consumer unit ranges over many years, and their MCBs may not be interchangeable.

For example, an older Wylex, Hager or Crabtree MCB may have a completely different connection arrangement from the manufacturer's current products.

Look for a model or range name on the consumer unit itself.

If this cannot be found, photographs of the existing MCBs, busbar arrangement and consumer unit can help with identification.

7. Check the Physical Connection

MCBs connect mechanically and electrically to the consumer unit.

Differences can include:

  • Busbar position
  • Terminal position
  • Device width
  • Device height
  • Mounting arrangement
  • Toggle position
  • Terminal design

An MCB fitting onto the DIN rail does not automatically mean it is compatible with the consumer unit.

8. Check the Number of Poles

MCBs can be supplied in different pole configurations.

Single-pole devices are commonly encountered in domestic consumer units, while multi-pole MCBs are widely used in commercial and industrial distribution equipment.

The replacement needs to match the requirements of the circuit and installation.

Can I Use a Newer Version of the Same Manufacturer's MCB?

Not automatically.

Manufacturers update their consumer unit and distribution board ranges over time.

A current Wylex MCB, for example, should not automatically be assumed to be a replacement for every previous Wylex MCB.

The same applies to Hager, Crabtree, Schneider Electric, MEM/Eaton and other manufacturers.

Check whether the manufacturer identifies the newer device as suitable for the existing assembly.

What If My MCB Has Been Discontinued?

Discontinued MCBs are common in older consumer units and distribution boards.

If the original device is no longer manufactured, you generally have three possibilities:

  • Locate an unused or surplus example of the original MCB
  • Identify a manufacturer-approved replacement or successor
  • Assess whether alteration or replacement of the existing equipment is required

Do not substitute an unrelated MCB simply because it physically fits.

What Information Do I Need to Find a Replacement?

Before searching for an MCB, collect:

  • Manufacturer
  • Full model or part number
  • Amp rating
  • Trip curve – B, C or D
  • Breaking capacity
  • Number of poles
  • Consumer unit or distribution board range
  • Clear photograph of the existing MCB
  • Photograph of the complete consumer unit

This information makes identifying older or unusual MCBs considerably easier.

Looking for a Discontinued MCB?

ReCircuit Resale stocks a range of new, surplus and discontinued MCBs from major electrical manufacturers.

This includes components from older consumer unit and distribution board ranges that may no longer be readily available through normal electrical wholesalers.

When making an enquiry, provide the part number and clear photographs of the existing breaker and consumer unit wherever possible.

Electrical work within consumer units and distribution boards should be carried out by a competent person with the appropriate knowledge and equipment.

Next Guide: Can You Replace an MCB With a Different Brand or Range?