Energy markets, and in particular the metering segment, have changed considerably in the past years: meter technology has changed from the electromechanical to the digital meter. Due to these changes, most meter manufacturing companies are also considering new mechanical design – and therefore new materials – for the meter housing.
In fact, housing material has become very important in the past years, particularly in relation to cost, safety and aesthetics. For these reasons, thermoplastic materials have replaced glass and thermosetting materials for most companies.
“The main requirements for meter applications concern flammability, thermal, mechanical and aesthetic properties.”
Thermoplastics are materials that soften when heated and harden when cooled. They can withstand many heating and cooling cycles and are often suitable for recycling differently from thermosetting materials; thermosetting plastics when heated will chemically decompose, so they cannot be welded or remoulded.
Most thermoplastics consist of polymers – long chains of molecules that contain smaller, repeating units called monomers and also filler materials such as powders – or fibres to provide improved strength and/or stiffness. These kinds of materials provide a variety of features. Products that are designed for electrical and electronics applications often provide protection against electrostatic discharge (ESD), electromagnetic interference (EMI), or radio frequency interference (RFI). Materials that are electrically conductive, resistive, insulating, or suitable for high voltage applications are commonly available, as flame retardant materials that reduce the spread of flames or resist ignition when exposed to high temperatures.
In this article, an overview of the main materials requirements for meter applications – and in particular their correlations with material properties – is discussed. The main requirements for meter applications concern flammability, thermal, mechanical and aesthetic properties. Some of these requirements are also very well expressed in the IEC62052-11 in terms of material properties and test standards. IEC 62052-11 covers type tests for electricity metering equipment for indoor and outdoor applications and applies to newly manufactured equipment designed to measure the electrical energy on 50 Hz or 60 Hz networks, with a voltage up to 600 V.
Concerning flammability, codes and regulations have been developed in many countries to ensure a level of public fire safety that is acceptable to society; and they are intended to provide a minimum level of safety. These criteria are based on performance in one or more fire or flammability tests. Examples are Orizzontal and Vertical UL 94 standard, Glow wire test IEC 60695-2-10, Oxigen index and Niddle flame test. The most commonly used test in meter applications are Vertical UL 94 and Glow wire test 60695-2-10. The first determines the material’s tendency either to extinguish or to spread the flame once the specimen has been ignited. The three vertical ratings, V2, V1 and V0, indicate that the material was tested in a vertical position and selfextinguished within a specified time after the ignition source was removed.
As shown in Figure 1, the flame is held in position for 10 seconds and is then removed. While applying the flame, the material may shrink or deflate. After 10 seconds, when the flame is removed, if the specimen keeps on firing the duration of the flame is noted as ‘first flame time, t1’. After the flame disappears, the flame is reapplied for an additional 10 seconds. t2, second flame time (t2), number of ignited drops, termination reason of the fire are vigorously discerned.
For meter applications, V-2 resins according with UL 94 are mainly required as polycarbonate or polyamide that belong to the engineering thermoplastic family. The glow wire test is used to simulate the effect of heat as could occur in malfunctioning electrical equipment, such as with overloaded or glowing components. The glow wire is heated via electrical resistance to a specified elevated temperature. A test specimen is held for 30 seconds against the tip of the glow wire with a force of 1 N. After the glow wire is removed, the time for the flames to extinguish is noted along with details of any burning drops.
Surrounding material of the test material, or a layer of tissue paper, is placed beneath the specimen during the test to determine the effects of burning drops. The material passes the test if there is no flame and no glowing and if flames or glowing of the specimen, or the surrounding material, extinguish within 30 seconds after removal of the glow wire. In the case of meter applications, for the terminal block glow wire temperature of 960ºC is required, while for terminal covers and meter casings, glow wire of 650ºC is required. Also in this case ignitions resistant polycarbonate or polyamide compounds are able to fulfil those requirements.
Mechanical properties are also key requirements for the application, and in particular IEC62052-11 standard refers to the vibration test (IEC 60068-2-6), hammer test (IEC 60068-2-75) and shock test (IEC 60068-2-27). All these test performances are connected to the intrinsic material properties and to the rigidity of the structure linked to the mechanical design. Relevant properties are rigidity (tensile/flexural modules and strength) and toughness (impact resistance and elongation at break). Polycarbonate based compounds offer the best compromise in rigidity and toughness.
Tests of the effect of climatic environments are also reported in the IEC standard as the dry heat test, cold test and damp heat cyclic test. These tests consist of different cycles at predetermined temperatures and times that are linked to the thermal performance of a material. Thermal properties include melting temperature, glass transition temperature and the heat deflection temperature (HDT). Heat distortion temperature is the temperature at which a material shows a predetermined distortion under a certain load. For example, for meter terminal blocks, values above 135ºC are necessary.
Protection against solar radiation – IEC 60068-2-5 – can be also considered part of the environment tests for outdoor meters, because most thermoplastic resins need to be stabilised against solar radiation. Ultraviolet stabilisers are generally used to extend the service life of plastics by the inhibition of the degradation process photo-initiated by UV light from the sun and other UV sources.
For the selection of the right material, properties other than those expressed in the standards need to be taken in consideration. Further requirements that meter manufacturing companies consider relevant include aesthetics, costs and processability. In addition to the requirements expressed above, new European directives such as WEEE (requirements for the separate handling of Br- waste) and RoHs push the polymer industry to find halogen free ignition resistant resins .
ENVIRONMENTAL CONSIDERATIONS AND WASTE MANAGEMENT
Directives 2002/95/EC RoHs on the restriction of the use of certain hazardous substances in electrical and electronic equipment and 2002/96/EC WEEE (waste electrical and electronic equipments) are designed to tackle the fast increasing waste stream of electrical and electronic equipment and complements European Union measures on landfills and incineration of waste. Increased recycling of electrical and electronic equipment will limit the total quantity of waste going to final disposal. Producers will be responsible for taking back and recycling electrical and electronic equipment. This will provide incentives to design electrical and electronic equipment in an environmentally more efficient way, which takes waste management aspects fully into account. In order to prevent the generation of hazardous waste, Directive 2002/95/EC requires the substitution of various heavy metals (lead, mercury, cadmium, and hexavalent chromium) and brominated flame retardants (polybrominated biphenyls (PBB) or polybrominated diphenyl ethers (PBDE)) in new electrical and electronic equipment put on the market from 1 July 2006.
In the meter industries, injection moulding is the most commonly used processing technique for engineering thermoplastics. Typically, the polymer pellets are melted and the melt pulled forward by means of a screw as in extrusion, so filling a mould under appropriate pressure. The shape of the mould, the number and relative location of the injection devices, and the mould cooling rate determine, together with the intrinsic properties of the material, the final quality of the moulded articles. In this way very complex article shapes can be obtained by this technique but before the construction of tools, parts and moulds, the design phase is very important. In this phase the rheologycal characteristics of the materials also need to be taken in consideration in order to get better part aesthetic and cost advantages. (Rheologycal properties describes the deformation of materials under stress and concerns their flow properties, and must be considered in all processing techniques for the fabrication of plastic articles.) Different software can be used in order to analyse the behaviour of material during processing and optimise the part and mould design, as shown in Figure 2.
Due to the number of different standards and requirements concerning materials, new compounds are frequently being developed. The Dow Specialty Plastics business has developed different blends and compounds for different industries and an example of a blend designed specifically for the metering industry is the EMERGETM PC 8701, which is an ignition resistant and halogen free polycarbonate based compound. This material has been largely used in most of the European smart meter implementation projects.