EICR Failed Due to Mixed MCB or RCD Brands? What It Means & How to Fix It in London

If your EICR has come back unsatisfactory and mentions mixed MCB or RCD brands, you’re not alone. This is one of the most common EICR failures we see across London, especially in older homes, rental properties, and converted flats.

Most people say the same thing:

“Everything works fine. Why is this even an issue?”

It’s a fair question. And the answer isn’t obvious unless you work with consumer units every day.

This guide explains exactly why mixed breakers can cause an EICR failure, when it’s classed as a C2, how inspectors judge it, and what the correct fix usually looks like. If you need help resolving it quickly, you can book an inspection or remedial work directly with London EICR Certificates.

Quick answer (for Google & humans)

An EICR may fail if different brands of MCBs or RCDs are installed within the same consumer unit, because those devices are not tested together as a system. This can lead to poor busbar contact, overheating, or unreliable fault protection, which is why inspectors often record it as C2 – potentially dangerous.

Now let’s go deeper.

What does “mixed MCB / RCD brands” actually mean?

A consumer unit isn’t just a metal box with breakers clipped in. It’s a tested electrical assembly, designed to operate as a complete system.

That system includes:

• the enclosure

• the busbar

• the MCBs

• the RCDs or RCBOs

• the way everything locks together mechanically and electrically

When breakers from different manufacturers are installed in the same board, it usually means a breaker was replaced in the past using “whatever fitted at the time”.

We commonly see:

• MK breakers inside Wylex consumer units

• Hager devices fitted into BG boards

• Mixed RCBOs added during partial upgrades

They may clip in. They may work.
But that doesn’t mean they are approved, compatible, or safe long term.

Why does this matter from a safety perspective?

This isn’t a paperwork issue. It’s a real-world safety concern.

1) Poor busbar contact

Different manufacturers design their breakers to grip the busbar in different ways. Mixing them can cause:

• loose connections

• heat build-up

• arcing

• melted insulation

We regularly open consumer units where the only visible issue on the EICR was “mixed breakers”, but the busbar underneath was already heat damaged.

2) Unpredictable fault performance

Protective devices are tested as part of a system. When brands are mixed:

• fault disconnection times are unknown

• short-circuit protection may not operate correctly

• RCD protection may not behave as expected

3) No manufacturer approval

If the manufacturer hasn’t approved that combination, an inspector cannot sign it off with confidence. That’s why this issue almost always leads to a fail.

Is mixed MCB / RCD always a C2 on an EICR?

This is where confusion usually starts.

In theory, if:

• the manufacturer explicitly approves the devices

• documentation is available

• the installation is in excellent condition

It might be recorded as a lesser observation.

In reality, especially in London:

• documentation is almost never available

• boards have been altered over many years

• partial repairs have been carried out

So inspectors will usually code this as C2 – potentially dangerous, making the EICR unsatisfactory.

If you’re unsure how EICR codes work, this guide explains it clearly:
👉 https://londoneicrcertificates.co.uk/how-to-read-an-eicr-report-and-ensure-electrical-safety-in-london/

Why this issue is so common in London properties

London properties are a perfect storm for this type of failure:

• older housing stock

• frequent refurbishments

• multiple electricians over decades

• quick fixes between tenancies

A breaker trips, someone swaps it, job done.
No one checks compatibility until an EICR is carried out.

We see this most often in:

• rental flats

• HMOs

• converted houses

• properties being sold or refinanced

If you manage HMOs, this page is especially relevant:
👉 https://londoneicrcertificates.co.uk/hmo-eicr-certificates-in-london/

Can I rent my property if the EICR failed for this reason?

Short answer: no, not until it’s resolved properly.

If the EICR is unsatisfactory due to a C2 observation, landlords are expected to:

• carry out remedial work

• obtain written confirmation that the issue has been fixed

This applies whether:

• you’re renewing a tenancy

• issuing a new tenancy

• dealing with council enforcement

This article explains the rental rules clearly:
👉 https://londoneicrcertificates.co.uk/blog/failed-eicr-can-i-still-rent-london/

How inspectors decide whether this needs fixing

During an EICR, an engineer will look at:

• the consumer unit make and model

• which breakers are mixed

• how they connect to the busbar

• any signs of overheating or damage

• whether manufacturer compatibility can be verified

If there’s any doubt, it will be coded as a C2. Inspectors are not allowed to “assume” compatibility.

How is a mixed-brand issue fixed properly?

The correct solution depends on the condition of the consumer unit.

Option 1: Replace incorrect breakers with compatible ones

This is suitable when:

• the consumer unit is modern

• the correct manufacturer devices are available

• there’s no heat damage

• no other C2 issues are present

This is often the fastest and most cost-effective fix.

Option 2: Upgrade the consumer unit

Recommended when:

• the board is old

• several mixed devices are installed

• there’s no RCD protection where required

• signs of overheating are present

In many cases, a consumer unit upgrade prevents repeat EICR failures in the future.

Related issue:
👉 https://londoneicrcertificates.co.uk/blog/eicr-failed-consumer-unit-cover/

How much does it usually cost in London?

Costs depend on:

• number of circuits

• accessibility

• board condition

• whether additional faults are discovered

For realistic pricing guidance:

• https://londoneicrcertificates.co.uk/eicr-certificate-cost/

• https://londoneicrcertificates.co.uk/remedial-work-for-failed-eicr-certificates/

You can also get a quick estimate here:
👉 https://londoneicrcertificates.co.uk/eicr-price-calculator/

What happens after remedial work is completed?

Once the issue is corrected:

• the work is verified

• compliance is confirmed

• documentation is issued

This is what letting agents, councils, insurers, and solicitors actually care about. Not just “it’s fixed”, but proof it’s fixed.

Why cheap fixes often fail the next EICR

This is where landlords lose money.

Common mistakes include:

• swapping a single breaker without checking compatibility

• ignoring busbar condition

• not tightening terminals correctly

• not inspecting the full consumer unit

The result? The next EICR fails again.

If you’re considering using different companies for testing and remedials, read this first:
👉 https://londoneicrcertificates.co.uk/blog/different-company-for-eicr-and-remedial-work-london/

When should you book professional help?

You should speak to a qualified electrician if:

• your EICR mentions mixed MCB or RCD brands

• you’re unsure whether a repair or upgrade is needed

• you’re a landlord under time pressure

• you’re selling or refinancing a property

We provide:

• EICR testing

• remedial works

• re-issue documentation

• fast London-wide availability

Start here:
👉 https://londoneicrcertificates.co.uk/eicr-services/

Book directly:
👉 https://londoneicrcertificates.co.uk/book-online/

Landlords:
👉 https://londoneicrcertificates.co.uk/eicr-certificates-for-landlords-in-london/

Commercial properties:
👉 https://londoneicrcertificates.co.uk/commercial-eicr-certificates-in-london/

We cover all London areas:
👉 https://londoneicrcertificates.co.uk/areas-we-cover/

Final word

A failed EICR due to mixed MCB or RCD brands doesn’t mean your electrics are about to fail tomorrow. But it does mean the installation cannot be safely signed off in its current condition.

Fix it properly once, document it correctly, and you won’t be dealing with the same problem again in five years.

That’s the difference between a pass and a repeat failure.