R24 and R25 at a Glance

In railway vehicle construction, it's not just the electrical performance of a printed circuit board that matters – its behavior in the event of a fire is equally important. EN 45545-2 defines clearly structured test procedures for this purpose, which are applied depending on the component's classification. For unpopulated printed circuit boards, the requirements sets R24 and R25 are particularly relevant. Both test concepts follow different approaches – together, they ensure the fire behavior of our material combinations of base material and solder mask complies with the standard.

R24 – Determination of the Oxygen Index (LOI)

Test Objective
R24 assesses the fundamental flammability of a material. The minimum oxygen concentration at which a material just barely burns is measured. The higher this value, the less flammable the material.

Test Procedure
According to EN ISO 4589-2, a defined sample geometry is arranged vertically in a test cylinder. A controlled oxygen-nitrogen mixture flows around the sample. By varying the oxygen content, the concentration at which stable flame propagation is possible is determined.

Measured Variable
The result is the so-called Oxygen Index (LOI) in volume percent.

Short definitions
Hazard Level: HL stands for "Hazard Level" HL1, HL2, and HL3 define the fire protection risk level of a rail vehicle. The higher the hazard level, the stricter the requirements for the materials used. OI = Oxygen Index LOI = Limiting Oxygen Index Both refer to the same measured value from the test according to EN ISO 4589-2.

Assessment
EN 45545-2 specifies minimum values ​​for different hazard levels:

• HL1 and HL2: ≥ 28%
• HL3: ≥ 32%

The OI or LOI is therefore an intrinsic material characteristic – it describes the basic flammability of the tested printed circuit board assembly. For comparison: Ambient air contains approximately 21% oxygen.

R25 – Glow Wire Test at 850 °C

Test objective
R25 simulates a realistic ignition source, such as that caused by overheated contacts or electrical faults. The focus is not on flame propagation, but on the material's behavior under direct thermal contact.

Test procedure
According to EN 60695-2-11, an electrically heated glow wire is heated to 850 °C and pressed against the sample with a defined force for 30 seconds. The burning behavior is then observed.

Evaluation criteria
The following are tested:

• Does the material ignite?
• Does any flame extinguish within 30 seconds?
• Does burning dripping occur?
• Does this ignite an underlying indicator paper?

Pass criterion
No flame formation for more than 30 seconds after removal of the glow wire and no ignited dripping. The same test temperature of 850 °C applies to HL1, HL2, and HL3.

Two test principles – one goal

R24 assesses the fundamental flammability of a material. R25 simulates a real electrical fault.

Together, these tests ensure that our bare circuit boards are not only functionally but also fire-resistant qualified for use in rail vehicles.