Understanding fire safety in multi-dwelling units: best practices for internal fibre routes

As fibre networks expand into residential and commercial buildings, fire safety is integral to network design and product selection. Here's what matters for internal fibre in MDUs.

In multi-dwelling units (MDUs) - such as apartment blocks, commercial buildings and shared residential buildings - internal cabling must meet specific safety standards to protect residents, installers and the building itself.

For network builders and procurement teams, understanding how fibre infrastructure performs in the event of a fire is essential to ensuring installations are both compliant and fit for purpose.

This guide breaks down the key considerations for internal fibre cabling in MDUs, including fire performance, material choices and best practice for safe installation.

Why fire safety matters in MDU fibre installations

Unlike external network builds, fibre installed inside residential buildings operates in enclosed spaces where fire risk must be carefully managed.

Cables routed through risers, corridors and shared access areas can contribute to fire spread if not correctly specified.

This is why fire safety is not just a regulatory requirement - it's a critical part of protecting:

• Building occupants
• Installation teams
• Property infrastructure

For operators, getting this right at the specification stage helps ensure that fibre installations meet both building regulations and safety expectations.

What affects fire performance in fibre cables?

Not all fibre cables behave the same way in a fire. Fire performance is influenced by several factors, including:

• The materials used in the cable jacket
• How much smoke the cable produces
• Whether it releases harmful gases when burned

This is where terms like LSZH (Low Smoke Zero Halogen) and Construction Products Regulation (CPR) classifications come into play.

What is LSZH and why does it matter?

LSZH cables are designed to:

• Produce very little smoke in the event of a fire
• Avoid releasing toxic halogen gases

In enclosed environments like MDUs, this is particularly important. Reduced smoke improves visibility during evacuation, while low toxicity helps minimise harm to occupants and emergency responders.

For this reason, LSZH materials are commonly specified for internal fibre installations.

Understanding CPR and fire ratings (EN 50575)

Fire performance for cables installed within buildings is governed by the Construction Products Regulation (CPR).

Under CPR, cables are tested and classified based on how they react in a fire. These classifications are known as Euroclass ratings, defined under the standard EN 50575.

Typical ratings include:

• B2ca — Very limited contribution to fire
• Cca — Limited contribution
• Dca — Acceptable contribution
• Eca — Basic fire performance

In MDU environments, the required classification will depend on the building type and installation location, but higher fire performance is often expected in shared or escape areas.

For procurement teams, confirming the correct rating ensures cables meet both regulatory requirements and building safety standards.

Best practices for internal fibre routes in MDUs

Designing safe internal fibre routes is about more than just selecting the right cable. It involves thinking about how cables are installed, routed and protected throughout the building.

Key best practices include:

• Using LSZH cables for internal runs
• Selecting cables with appropriate CPR fire ratings
• Avoiding overcrowding in containment systems
• Ensuring clear separation between different services where required
• Using suitable containment systems (ducting, trays or risers)
• Following manufacturer installation guidelines

Taking a structured approach to routing helps reduce risk and supports safe, compliant installations.

Asking the right questions when sourcing cable

When specifying fibre for MDU installations, it's important to work with suppliers who can clearly demonstrate product compliance.

Procurement teams should ask:

• What is the cable's CPR classification?
• Is it LSZH compliant?
• Can a Declaration of Performance (DoP) be provided?
• Are there fire test reports available?
• Is the cable suitable for internal building use?

Having access to this information upfront helps avoid delays and ensures the correct products are used from the start.

Why supplier choice matters

Fire safety is not an area where uncertainty can be afforded.

Working with experienced suppliers ensures that products are not only available, but also supported by the correct documentation and technical guidance.

At Altnets, we work closely with network builders to supply MDU-ready fibre cable solutions that meet the necessary safety and compliance standards.

By combining product knowledge with strong supplier partnerships, we help customers specify the right materials for safe and effective installations.

Building safer fibre networks inside residential buildings

As fibre rollout continues to expand into residential environments, fire safety will remain a key consideration for every internal installation.

By selecting the right cables, understanding fire performance ratings, and following best practice routing principles, operators can ensure their networks are built with safety in mind from the outset.

Taking a proactive approach not only supports compliance - it helps protect the people and environments these networks serve.

Need support sourcing MDU-ready fibre cable?

If you're planning a residential fibre installation and want confidence in your product selection, the Altnets team is here to help. Speak to us about MDU-ready fibre cable solutions.