# Material Composition Testing Related to Measurement Instrument Enclosure Design and Safety
## 1 Introduction  
Polymeric insulating materials are widely used as enclosures in electrical and measuring instruments due to their lightweight, moldability, and electrical insulation properties. However, the fire safety of these enclosures is a critical concern because an enclosure that ignites can lead to device failure and even propagate fire to its surroundings. Ensuring the flame resistance of enclosure materials is not only a matter of product reliability but also a regulatory requirement. For example, the European Measuring Instruments Directive 2004/22/EC (MID) mandates that measuring devices \"constitute no unreasonable risk to personal safety or property,\" which includes meeting basic fire safety criteria [1, 2, 3]. Manufacturers must therefore design enclosures with materials that resist ignition and self-extinguish, in order to comply with such directives and obtain the necessary safety certifications for global markets. This has placed increased importance on selecting or formulating polymer materials that can meet flammability standards while retaining the mechanical and functional requirements of the instrument enclosure. To evaluate and classify the flammability of plastics used in enclosures, standardized small-flame tests are employed worldwide. The most common are the horizontal and vertical burning tests defined in Underwriters Laboratories UL 94 and the equivalent IEC 60695-11-10 international standard [4]. In particular, European norm EN 60695-11-10 (IEC 60695-11-10) specifies a 50 W Bunsen burner method for comparing the burningbehaviour of plastic specimens in horizontal (HB) and vertical (V) orientations [3; 4]. In the horizontal burning test (known as UL 94 horizontal burning (HB) or method A), a bar-shaped sample is held horizontally and exposed to a calibrated 50 W flame for a fixed interval. The rate and extent of burning are then measured to categorize the material's flammability. Materials that stop burning before a specified distance or burn slowly are classified in increasing order of flammability as HB40 or HB75, corresponding to the maximum allowed burning rates of 40 mm/min and 75 mm/min, respectively. These HB ratings indicate a material's ability to self-extinguish or burn at a controlled rate when ignited horizontally. Standards IEC 60695-11-4 and -11-10 define the test apparatus and procedures in detail to ensure reproducibility, including the burner specifications, flame height (20 mm), gas purity, and the conditioning of samples [5]. Using these methods, manufacturers and researchers can quantitatively compare the flammability of different polymer materials and verify their compliance with safety norms. A substantial body of research has examined how the composition of polymeric materials influences their flammability, as measured by such standardized tests. It is well known that base polymers vary widely in inherent flammability--for instance, unfilled polycarbonate or epoxy tend to char and self-extinguish, whereas materials like polyethylene or Acrylonitrile Butadiene Styrene (ABS) can burn readily [6]. To meet safety requirements, flame-retardant additives are often incorporated into polymers to reduce their ignitability and burning rate. Halogenated flame retardants, organophosphorus compounds, and metal hydroxide fillers (e.g., alumina trihydrate or magnesium hydroxide) are commonly used to impart self-extinguishing behaviour [7]. These additives act through various mechanisms (gas-phase radical quenching, char formation, endothermic cooling, etc.) and can dramatically improve a material's rating and HB in many cases. At the same time, additives and fillers can also affect the thermal stability and mechanical properties of the polymer, so their selection involves balancing flame retardancy with other performance factors. Recent studies have explored more sustainable or halogen-free flame retardants due to environmental and health concerns.